Physiol Rev 87: 1175–1213, 2007;
doi:10.1152/physrev.00047.2006.
Life and Death: Metabolic Rate, Membrane Composition,
and Life Span of Animals
A. J. HULBERT, REINALD PAMPLONA, ROCHELLE BUFFENSTEIN, AND W. A. BUTTEMER
Metabolic Research Centre, Institute for Conservation Biology, School of Biological Sciences, University of
Wollongong, Wollongong, New South Wales, Australia; Department of Basic Medical Sciences, University of
Lleida, Lleida, Spain; and Department of Biology, City College of the City University of New York,
New York, New York
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I. Introduction
II. Mechanisms: Oxidative-Stress Theory of Aging
A. Mitochondrial free radical production
B. Cellular protection: antioxidant defenses
C. Cellular protection: repair mechanisms
D. Membrane composition and lipid peroxidation
III. Mammals
A. Metabolic rate and body size of mammals
B. Maximum life span, body size, and lifetime energy expenditure of mammals
C. Mitochondrial free radical production and antioxidant defenses of mammals
D. Membrane composition, lipid peroxidation, body size, and life span of mammals
E. Insights from relatively long-living mammals (including humans)
IV. Birds
A. Metabolic rate and body size of birds
B. Maximum life span, body size, and lifetime energy expenditure of birds
C. Insights from bird-mammal comparisons
D. Insights from comparison among birds
V. Ectothermic Vertebrates and Invertebrates
VI. Treatments to Manipulate Life Span
A. Calorie restriction
B. Antioxidant defenses
C. Genetic manipulations
VII. Conclusions: So, What Determines a Species Maximum Life Span?
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Hulbert AJ, Pamplona R, Buffenstein R, Buttemer WA. Life and Death: Metabolic Rate, Membrane Composition,
and Life Span of Animals. Physiol Rev 87: 1175–1213, 2007; doi:10.1152/physrev.00047.2006.—Maximum life span
differences among animal species exceed life span variation achieved by experimental manipulation by orders of
magnitude. The differences in the characteristic maximum life span of species was initially proposed to be due to
variation in mass-specific rate of metabolism. This is called the rate-of-living theory of aging and lies at the base of
the oxidative-stress theory of aging, currently the most generally accepted explanation of aging. However, the
rate-of-living theory of aging while helpful is not completely adequate in explaining the maximum life span. Recently,
it has been discovered that the fatty acid composition of cell membranes varies systematically between species, and
this underlies the variation in their metabolic rate. When combined with the fact that 1) the products of lipid
peroxidation are powerful reactive molecular species, and 2) that fatty acids differ dramatically in their susceptibility to peroxidation, membrane fatty acid composition provides a mechanistic explanation of the variation in
maximum life span among animal species. When the connection between metabolic rate and life span was first
proposed a century ago, it was not known that membrane composition varies between species. Many of the
exceptions to the rate-of-living theory appear explicable when the particular membrane fatty acid composition is
considered for each case. Here we review the links between metabolic rate and maximum life span of mammals and
birds as well as the linking role of membrane fatty acid composition in determining the maximum life span. The more
limited information for ectothermic animals and treatments that extend life span (e.g., caloric restriction) are also
reviewed.
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I. INTRODUCTION
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Human awareness that species differ in their life span
surely predates science. Indeed, the variation in maximum life span potential (MLSP) among different species
is substantial, exceeding the variation in longevity
achieved by either genetic or physiological manipulation
by about four orders of magnitude. This variation was a
critical component of an important early analysis of the
processes involved in aging and the determination of life
span. Almost a century ago, Max Rubner (306) coupled
the observation that MLSP of mammals increases with
body size with another of his scientific interests, namely,
that the mass-specific rate of metabolism of mammals
decreases with body size (305). He combined the MLSP
and metabolic rates of five mammal species (guinea pigs,
cats, dogs, cattle, and horses) to calculate their “lifetime
energy potential” (i.e., their mass-specific lifetime resting
energy turnover) and found that it was a relatively constant value for these five mammal species. Curiously, he
did not include humans in his comparison (which will be
discussed later). Two decades later, this perspective was
elaborated and extended by Raymond Pearl (284), who
proposed that life span differences of Drosophila maintained at different temperatures could be explained by
differences in metabolic rate. Pearl (284) gave the rateof-living theory of aging its name.
Later, Harman (127) proposed the free radical theory
of aging, which provided a possible mechanistic link between metabolic rate and aging. Harman (127) proposed
that normal oxygen consumption inevitably results in the
production of oxygen free radicals which, in turn, damage
important biological molecules (nucleic acids, proteins,
and lipids, among others), resulting in the process of
“aging,” and eventually this accumulated damage results
in death of the organism. Over the last half century, the
free radical theory has developed into the oxidative-stress
theory of aging which takes into account that 1) not all
the damaging reactive oxygen molecules (or “species” in
the chemical sense hence the abbreviation; ROS) produced by normal metabolism are free radicals, and that
organisms have both 2) antioxidant defenses to minimize
ROS and 3) mechanisms to repair or remove biological
molecules damaged by ROS. Currently, the oxidativestress theory is the most widely accepted mechanistic
explanation of aging and life span, and there is much
evidence to support it. In the past, the oxidative-stress
and rate-of-living theories have been reconciled by supposing that higher levels of ROS are generated at higher
levels of metabolic activity (e.g., Refs. 22, 328). However,
this popular concept is flawed, because while it appears
true in some situations (e.g., when broadly comparing
mammals of different size), it is not true in others (e.g.,
between different states of mitochondrial respiration, fol-
lowing calorie restriction, or in bird-mammal comparisons).
Furthermore, although there is a broad correlation
between body size and MLSP, there are a number of
problems associated with presuming a linkage between
rate-of-living and MLSP. For example, 1) voluntary exercise and its associated increase in metabolic rate does not
shorten life span of rats (147) or humans (200); 2) there is
no inverse relationship between life span and mass-specific metabolic rates of individuals in populations of mice
(331) or fruit flies (157); 3) although caloric restriction
extends life span, it does not do so by reducing metabolic
rate (MR) (see later sections); 4) long-living insulin/insulin-like growth factor (IGF) mutant fruit flies do not have
a decreased metabolic rate (157); 5) although birds and
mammals have similar rates of living, birds are generally
much longer living than similar-sized mammals; and 6)
within both groups of endotherms, there are significant
species differences in MLSP that cannot be explained by
metabolic rate differences (see later sections). Collectively these problems show that part of the scientific
jigsaw puzzle that explains aging and maximum life span
is missing. In this contribution, we review the evidence
that membrane fatty acid composition is this missing
piece of the puzzle.
A century ago, mass-specific MR (and its physiological correlates, such as heart rate) were the obvious properties to be related to life span. There have been a variety
of attempted explanations for the size-dependent differences in metabolic rate over the years, and differences in
cellular composition were initially excluded (see Ref.
176). However, recently it has been shown that one aspect
of cell composition, namely, membrane fatty acid composition, does vary in a systematic manner with body size in
mammals (69, 164) and in birds (37, 162). It has also been
shown that membrane fatty acid composition is related to
MLSP (266, 271, 272, 275–277). These observations, when
combined with the long-known differences in susceptibility of different fatty acids to peroxidative damage (148),
were seminal in the development of the “homeoviscouslongevity” membrane theory (267) and the later “membrane-pacemaker” theory of aging (155). These modifications of the oxidative-stress theory of aging emphasize the
effects of membrane fatty acid composition on lipid peroxidation (and its products) on the process of aging and
how this might determine the distinctive maximum life
span of a species.
In this contribution, we concentrate on the comparison of the maximal longevity (MLSP) distinctive to different species. We first discuss the mechanisms associated with aging (based on the oxidative-stress theory) and
briefly describe the means by which the fatty acid composition of membranes might be important in the processes of aging. We then examine the links between body
mass, metabolic rate, and maximum longevity separately
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
for the two classes of endothermic vertebrates, mammals
and birds, with an emphasis on how membrane fatty acid
composition might account for variation in MLSP among
species. We briefly review the limited data on this topic
for ectothermic vertebrates and invertebrates. Finally, we
consider how changes in membrane composition might
explain how some experimental treatments can extend
longevity. We restrict ourselves largely to a physiological
context, paying limited attention to the genetic manipulations involved in much recent aging research.
II. MECHANISMS: OXIDATIVE-STRESS THEORY
OF AGING
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A. Mitochondrial Free Radical Production
That oxygen can be toxic, and consequently the beginnings of the oxidative-stress theory of aging, dates
back to the late 19th century. Paul Bert (25) described the
toxic effects of air at high pressure, as well as oxygen
alone at high concentration, and several subsequent studies showed that oxygen toxicity was particularly acute in
endotherms. In ectotherms its toxicity increased with
ambient temperature, which suggested a relationship with
metabolic rate rather than oxygen solubility. These studies demonstrated that three organ systems (lungs, retina,
and nervous system) are especially sensitive to high oxygen tensions in whole animals (195); however, a scientific
explanation of the cause of these harmful reactions was
lacking for many decades.
After Fenton (100) discovered that Fe(II) catalyzed
the oxidation of tartaric acid by hydrogen peroxide, it was
proposed that hydroxyl radicals, hydrogen peroxide, and
superoxide anions undergo a chain reaction that results in
a net conversion of hydrogen peroxide into water (119,
120). This became known as the Haber-Weiss reaction
and is now believed to be at the core of much ROS
generation in cells. However, the connection between
oxygen toxicity and ROS generation was not then understood (27). Using electron spin resonance, Commoner
et al. (67) published the first direct evidence that free
radicals were produced in living cells and found that free
radical levels were higher in tissues that were more metabolically active. The free radical hypothesis of oxygen
toxicity was first proposed by Gerschman et al. (109),
taking into account the strong similarity of the histological alterations produced by hyperoxia and by ionizing
radiation (264). Harman (127) postulated that even at
normoxia, oxygen utilization causes subtle tissue damage
due to production of free radicals. He integrated evidence
from the electron spin resonance study, post-Hiroshima
studies of radiation damage, Fenton’s findings, and contemporary theories that proposed free radical mechanisms for the oxidation of organic compounds and dismutation of hydrogen peroxide by iron salts (358). Harman
proposed that physiological iron and other metals would
cause ROS to form in the cell via Haber-Weiss chemistry
as a by-product of normal redox reactions. The ROS
would damage nearby structures like mitochondrial DNA
(129). He predicted that administering compounds that
are easily oxidized, such as cysteine, would slow down
the aging process, a suggestion recently reviewed by
Dröge (83). Harman’s proposal constitutes the basis of the
free radical theory of aging, which is supported by a large
body of scientific evidence. The discovery of the superoxide dismutase enzyme in living organisms (226) led to
serious consideration of the role of free radicals in biology. If an enzyme that decomposed oxygen radicals inside
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The biological aging process is characterized by the
progressive decline in the efficiency of physiological functions. The ability to maintain homeostasis is correspondingly attenuated with age, leading eventually to increased
risk of developing many diseases (including cancer and
neurodegenerative and cardiovascular diseases) and increasing the chance of death. For a long time aging was
believed to be restricted to multicellular animals; however, recently detailed measurements have shown it also
to occur in the common bacteria Escherichia coli (340).
The natural aging process has four characteristics: it is
progressive, endogenous, irreversible, and deleterious for
the individual (344). The progressive character of aging
suggests that causes of aging are present during an organism’s whole life span, both at young and old ages. That
aging is an endogenous process suggests exogenous factors are not causes of the intrinsic aging process, although
they may interact with endogenous causes and either
enhance or diminish their effects. The endogenous character of aging suggests that the rate of aging and MLSP of
different animal species are predominantly genotypically
determined and explain why different animal species can
age at substantially different rates in similar environments. Available evidence consistently points to the idea
that ROS, mainly those of mitochondrial origin (129, 235),
are causally related to the basic aging process (17, 22, 312,
328). Accordingly, mitochondrial ROS production fulfils
the four characteristics of aging described above: ROS are
endogenously produced by mitochondria under normal
physiological conditions, they are produced continuously
throughout life and can thus lead to progressive aging
changes, and their deleterious effects on biological macromolecules may inflict irreversible damage during aging,
especially in postmitotic tissues because cells that are
irreversibly damaged or lost cannot be replaced by mitosis. There have been many theories and explanations of
aging, and while it has not yet been “proven,” currently
the strongest candidate is the oxidative-stress theory
(also known as the oxidative-damage theory), and this is
the basis of the following description.
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1. Important reactive oxygen species and lipid
aldehydes and their common abbreviations
TABLE
Reactive Oxygen Species and Aldehydes
Radicals
Hydroxyl
Superoxide
Lipid peroxyl
Alkoxyl
Peroxyl
Nonradicals
(HO䡠)
(O2⫺䡠)
(LOO䡠)
(RO䡠)
(ROO䡠)
Hydrogen peroxide
Singlet oxygen
Lipid hydroperoxide
4-Hydroxy-2-hexenal
4-Hydroxy-2-nonenal
H2O2
O2
LOOH
HHE
HNE
1
[From Halliwell and Gutteridge (124).]
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the body was highly conserved during evolution, oxygen
free radicals were likely important in biological systems.
A free radical is any molecule capable of independent
existence that contains one or more unpaired electrons
(124). ROS is a collective term that includes oxygen radicals as well as nonradical derivatives of oxygen (124).
Reduction of univalent oxygen generates three main ROS:
superoxide and hydroxyl radicals as well as the nonradical hydrogen peroxide. Among these, the hydroxyl radical
is extremely reactive. It can be generated by the combination of superoxide radical and H2O2 in the presence of
trace amounts of iron (or copper) during the FentonHaber-Weiss reaction. Thus H2O2, although not a free
radical itself, can behave as a Trojan horse, diffusing away
from sites of its production to generate the hydroxyl and
other reactive radicals at other cellular locations, hereby
propagating oxidative damage. In many scientific papers,
use of the term ROS is restricted to what we might
describe as the three primary ROS described above (especially superoxide and hydrogen peroxide); however,
there are a number of other important ROS, both radicals
and nonradicals (see Table 1). The roles of many of these
other ROS in aging have not been fully evaluated. There is
also some overlap between ROS and reactive nitrogen
species (RNS) such as nitric oxide. In the case of mitochondria, nitric oxide production is much smaller than
superoxide production. However, nitric oxide can still be
important due to interaction with superoxide and other
radicals to produce reactive species like peroxynitrite
(76), which can modify many kinds of macromolecules
and possibly contribute to various pathologies (65).
ROS can be generated at various sites and under a
variety of conditions. These include the membrane
NADPH oxidase of polymorphonuclear leukocytes, during ischemia-reperfusion, as well as other enzymatic reactions (e.g., lipoxygenases, cyclooxygenases, peroxidases, and other heme proteins), the enzyme xanthine
oxidase, peroxisomes, or the hepatic P-450 microsomal
detoxifying system (124). In healthy tissues under normal
conditions, most ROS are generated by the mitochondrial
respiratory chain (312).
The finding that the percentage of total electron flow
in mitochondria directed to radical generation is not constant in different tissues, and under different conditions,
suggests that ROS generation is not a constant stoichiometric by-product of mitochondrial respiration (17, 19,
113, 136), as is frequently assumed. Oxygen radical generation at the respiratory chain has been classically attributed to complex III (32). However, in agreement with
early information obtained in submitochondrial particles
(347), complex I is also an important ROS generator in
intact functional heart and brain mitochondria from rats
(20, 108, 136, 137), as well as other organs and other
species (17, 184). Nowadays the concept that both complex I and complex III produce ROS is widely accepted in
the scientific literature and is already part of mainstream
biochemical knowledge (e.g., Ref. 253). There is, however, current debate concerning the identity of the ROS
generator inside complex I. Some studies have suggested
a role for flavin mononucleotide (184, 206, 380), while
others favor complex I linked ubisemiquinones (191, 256).
However, other studies localize the ROS generator in the
electron pathway inside complex I in a place between the
ferricyanide reduction site and the rotenone-binding site
(108, 139, 186). This finding discards flavin and suggests
that the source of ROS might be the complex I FeS
clusters situated in that region (139), although a role for
complex I-linked ubisemiquinones cannot be ruled out at
present. Because all FeS clusters of complex I are situated in the hydrophilic matrix domain of the complex,
ROS arising from them will damage targets situated in the
mitochondrial matrix. In contrast, complex III ROS generation seems to be directed to the cytosolic side (342).
Apart from the degree of electron flow, other possible physiological mechanisms influencing the rate of mitochondrial ROS generation include 1) a decrease in the
relative concentration of the respiratory complex(es) responsible for ROS generation; 2) the degree of electronic
reduction of these generators; the more reduced, the
higher their rate of ROS production (20, 113, 114, 208,
310); 3) uncoupling proteins (discussed in the next paragraph); 4) as well as chemical modification (e.g., glutathionylation) of mitochondrial ROS generators. The nonenzymatic glutathionylation of complex I increases its
superoxide production, and when the mixed disulfides are
reduced, superoxide production returns to basal levels
(125, 351). Oxidation of the glutathione pool within intact
mitochondria to glutathione disulfide also leads to glutathionylation of complex I, which correlates with increased superoxide formation. In this case, most of the
superoxide is converted to hydrogen peroxide, which can
then diffuse into the cytosol. This mechanism of reversible mitochondrial ROS production suggests how mitochondria might regulate redox signaling and shows that
oxidation of the mitochondrial glutathione pool could
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
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effects on cell function being elicited at the level of the
mitochondrion. One of the many cellular reactions of
nitric oxide is S-nitrosation of protein thiols, resulting in
the generation of S-nitrosothiols. There are several reasons why S-nitrosation may be an important mitochondrial regulatory mechanism. Mitochondria contain sizable
thiol pools, are abundant in transition metals, and have an
internal alkaline pH, all of which are known to modulate
S-nitrosothiol biochemistry (102). In addition, mitochondria are highly membranous, sequester lipophilic molecules such as nitric oxide, and the formation of the putative S-nitrosating intermediate N2O3 is enhanced within
membranes (42). Thus the mitochondrial respiratory
chain embedded within the inner membrane would seem
to be an ideal target for S-nitrosation. In this scenario, it
has been described that mitochondria interact with nitric
oxide at several levels, and one particularly well-characterized example is the inhibition of complex I and IV
(45, 265).
Although free radicals and derived reactive molecular species can have damaging effects, at moderate concentrations they are often important signaling molecules
involved in the regulation of several physiological functions (e.g., control of ventilation, erythropoietin production, immune responses, apoptosis). In such situations,
ROS are typically generated by tightly regulated enzymes
such as NAD(P)H oxidases (82).
B. Cellular Protection: Antioxidant Defenses
Aerobic life demands antioxidant defenses. An antioxidant is extremely difficult to define, but a broad definition is “any substance that, when present at low concentrations compared with those of an oxidizable substrate, significantly delays or prevents oxidation of that
substrate” (124). An antioxidant reacts with and neutralizes an oxidant, or regenerates other molecules capable of
reacting with the oxidant. Oxidative damage is a broad
term used to cover the attack upon biological molecules
by free radicals. Cellular protection against oxidative
damage includes both elimination of ROS and repair of
damage with antioxidants constituting the first line of this
defense.
Direct ROS scavenging antioxidant enzymes include
superoxide dismutase (SOD), glutathione peroxidase
(GPX), and catalase (CAT). SOD converts the superoxide
radical to oxygen and hydrogen peroxide. There are different types of SOD in different cellular compartments:
Mn-SOD in the mitochondrial matrix, a Cu/Zn-SOD in the
cytosol and the intermembrane space, and a different
Cu/Zn-SOD in the extracellular compartment. Although
SOD eliminates superoxide radicals, it is not a complete
antioxidant defense because it produces H2O2. CAT and
GPX are two different but kinetically complementary en-
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contribute to the pathological changes that occur to mitochondria during oxidative stress (125, 351).
Mitochondrial superoxide production is very sensitive to the proton-motive force across the inner membrane, so it can be strongly decreased by mild uncoupling.
For example, one study (239) has shown that a 10-mV
decrease in mitochondrial membrane potential is associated with 70% decrease in superoxide production. The
decrease in ROS production by isolated mitochondria
between state 4 and state 3 is likely due to the decrease in
mitochondrial membrane potential between these two
states. Similarly, it has been proposed that an ancestral
function of uncoupling proteins (UCP) is to cause mild
uncoupling and so diminish mitochondrial superoxide
production, hence protecting against oxidative damage at
the expense of a small loss of energy (38). For example,
UCP3 (found in skeletal muscle mitochondria) is activated by increased superoxide production, and the consequent increased proton conductance will cause a mild
decrease in proton-motive force and consequently a diminished superoxide production (88). The role of UCP3
activation during exercise and during the transition from
state 4 to state 3 in muscle mitochondria, and its control
on ROS production is one possible reason why increased
voluntary exercise of mammals is not associated with a
decrease in life span. It has been proposed that this
negative-feedback loop will protect cells from ROS-induced damage and might represent the ancestral function
of all UCPs (38, 249).
Other possible factors that modulate complex I activity are cardiolipin content (280) and S-nitrosation (45).
Cardiolipin, a phospholipid of unusual structure localized
almost exclusively within the inner mitochondrial membrane, is particularly rich in unsaturated fatty acids. This
phospholipid plays an important role in mitochondrial
bioenergetics by influencing the activity of key mitochondrial inner membrane proteins, including several anion
carriers and electron transport complexes I, III, and IV
(143). Mitochondrial cardiolipin molecules are possible
targets of oxygen free radical attack, due to their high
content of polyunsaturated fatty acids and because of
their location in the inner mitochondrial membrane near
the site of ROS production. In this regard, it has been
recently demonstrated that mitochondrial-mediated ROS
generation affects the activity of complex I (280), as well
as complexes III and IV (278, 279), via peroxidation of
cardiolipin following oxyradical attack to its fatty acid
constituents. These findings might explain the decline in
respiratory chain complexes observed in mitochondria
isolated from aged animals and in pathophysiological conditions that are characterized by an increase in the basal
rate of the ROS production.
In addition to cardiolipin, S-nitrosation may also
serve as another regulatory system for complex I. Nitric
oxide is a pleiotropic signaling molecule, with many of its
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of lipid peroxidation but is itself converted to a radical
during the process. Vitamin E also reduces lipid alcoxyl
radicals to lipid alcohols. Oxidized vitamin E can be recycled back to its reduced form by ascorbate or ubiquinone (coenzyme Q). Carotenoids quench singlet oxygen
and interact with other ROS at physiological tissue oxygen partial pressures. Ubiquinol, the reduced form of
coenzyme Q, is an important antioxidant. It is a hydroquinone that is synthesized and present in all cellular membranes (15), and its antioxidant activity is exhibited
through scavenging of lipids radicals or reduction of vitamin E radical. Regeneration of coenzyme Q is performed by reductases that use NADPH or NADH as cofactors. Methionine residues in proteins (see sect. IIC) can
also be considered as scavengers of oxidants.
C. Cellular Protection: Repair Mechanisms
Despite the plethora of antioxidant defense systems,
oxidative damage still occurs in vivo. The next level of
cellular protection against oxidative stress is the repair/
removal of oxidatively damaged macromolecules. As with
antioxidant defenses, space limitations restrict our coverage here. The reader is directed elsewhere (e.g., Ref. 124)
for more detailed presentation.
To avoid the gradual accumulation of oxidative DNA
base lesions and thus maintain genomic stability, DNA
damage is repaired by a number of mechanisms. In general, oxidized DNA bases are removed by base excision
mechanisms, and although these correcting mechanisms
show high fidelity, DNA lesions still accumulate with age.
Such accumulation is most dramatic in mitochondrial
DNA (312), and less is known about mitochondrial than
nuclear DNA repair (31). DNA polymerase enzymes have
proofreading and error-correcting facilities, and when this
capacity of mitochondrial DNA polymerase was removed
in mice, there was a three- to fivefold increase in mitochondrial mutations and a corresponding decreased life
span (185, 357).
Damaged proteins can either be repaired or removed.
For example, methionine residues of proteins are among
the amino acids most susceptible to oxidation by ROS,
and the sensitivity of proteins to oxidative stress increases as a function of their number of methionine residues (337). Oxidation of methionine residues generates
methionine sulfoxide in proteins, which deprives them of
their function as methyl donors, and may lead to loss of
their biological activity. Methionine sulfoxide can be repaired by the enzyme methionine sulfoxide reductase,
which uses the reducing power of thioredoxin (248). In
this context, it is very illustrative that knocking out methionine sulfoxide reductase A lowers MLSP and increases protein carbonyls and sensitivity to hyperoxia in
mice (248). The opposite manipulation, overexpression of
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zymes that can eliminate H2O2. CAT is one of the most
active enzymes known and removes H2O2 by breaking it
down directly to H2O and O2. However, it has a low
affinity for H2O2; thus it needs high H2O2 concentrations
to function effectively. Conversely, GPX enzymes (both
the selenium- and non-selenium-dependent forms) are
most functional at low hydrogen peroxide concentrations,
since they have high affinity and low rates of catalysis.
GPX(s) have the same locations as SOD, suggesting that
GPX is the main enzyme that deals with H2O2 produced by
Cu/Zn-SOD in the cytosol and Mn-SOD in the mitochondria (124).
As well as antioxidant enzymes, there are different
kinds of endogenous nonenzymatic scavenger molecules
that cooperate to limit cellular oxidative stress. After
reacting with ROS, these molecules are oxidized and thus
must be reduced to regain their antioxidant capacity.
Their low molecular mass allows them to remove ROS at
sites that larger antioxidant enzymes cannot access. Glutathione (GSH) and ascorbate are two major nonenzymatic antioxidants of the hydrophilic cellular compartments. The antioxidant activity of GSH resides in the
reduced thiol group of its cysteine residue. Glutathione
can react directly with ROS or can act as a cosubstrate of
GPX enzymes. Oxidized glutathione (GSSG) also plays an
important role in the regulation of protein function, as
mentioned above for mitochondrial complex I. Glutathione reductase is the enzyme responsible for reducing
GSSG back to GSH using NADPH (124).
Another thiol-related redox-active substance is the
protein thioredoxin (238). Thioredoxin has a redox-active
disulfide/dithiol at the active site that regulates transcription factors (such as nuclear factor ␬B and AP-1). It is
induced by various oxidative stresses and is translocated
to the nucleus. Thioredoxin is cytoprotective against oxidative stress by scavenging ROS in cooperation with
peroxiredoxin/thioredoxin-dependent peroxidase (238).
Apart from glutathione, ascorbate (vitamin C) is the
next most abundant reduced nonenzymatic antioxidant
inside cells. It is endogenously synthesized in most vertebrates (although not in human beings, fruit bats, or guinea
pigs), and it is maintained at levels as high as 1 mM in
tissues. After reacting with ROS, the oxidized form of
ascorbate must be reduced by NADPH-, GSH-, or NADHdependent reductases to regain antioxidant capacity
(14, 124).
Tocopherols and carotenoids are the main radical
scavenger antioxidants that act in lipophilic environments
of cells. The major scavenger inside membranes is D-␣tocopherol (vitamin E). Most membranes are thought to
contain approximately 1 tocopherol molecule per 1,000
lipid molecules (369). Vitamin E acts on lipid peroxyl
groups inside membrane bilayers, reducing them to hydroperoxides, and thus inhibiting the propagation of the
peroxidative chain reaction. It breaks the chain reaction
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
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FIG. 1. The relative susceptibilities of selected fatty acids to peroxidation. Values are from Homan (148), and all were empirically determined as rates of oxygen consumption. They are expressed relative to
the rate for linoleic acid 18:2 n-6 which is arbitrarily given a value of 1.
SFA, saturated fatty acids; MUFA, monounsaturated fatty acids; n-6
PUFA, omega-6 polyunsaturated fatty acids; n-3 PUFA, omega-3 polyunsaturated fatty acids. The number for each fatty acid refers to acyl chain
length:number of double bonds.
D. Membrane Composition and Lipid Peroxidation
Oxygen radicals can attack many cellular molecules.
In addition to protein and DNA modification (reviewed in
Ref. 312), damage to membrane lipids is also very relevant
for life span determination. The susceptibility of membrane lipids to oxidative damage is related to two traits.
The first is that oxygen and many radical species are
several times more soluble in lipid membrane bilayers
than in the aqueous solution (246). The second property is
related to the fact that not all fatty acid chains are equally
susceptible to damage. It is this second property that is
the key to the link between membrane composition and
oxidative damage to membranes. The carbon atoms that
are most susceptible to radical attack are the singlebonded carbons between the double-bonded carbons of
the acyl chains (124). The hydrogen atoms attached to
these carbons are called bis-allylic hydrogens. This means
that saturated and monounsaturated fatty acyl chains
(SFA and MUFA, respectively) are essentially resistant to
peroxidation while PUFA are damaged. Furthermore, the
greater the degree of polyunsaturation of PUFA, the more
prone it is to peroxidative damage. Holman (148) empirically determined (by measurement of oxygen consumption) the relative susceptibilities of the different acyl
chains (see Fig. 1). Docosahexaenoic acid (DHA), the
highly polyunsaturated omega-3 PUFA with six double
bonds, is extremely susceptible to peroxidative attack
and is eight times more prone to peroxidation than linoleic acid (LA), which has only two double bonds. DHA is
320 times more susceptible to peroxidation than the
monounsaturated oleic acid (OA) (148). With the combination of the relative susceptibilities of different fatty
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methionine sulfoxide reductase A, increases life span and
delays the aging process in Drosophila (304). The oxidized form of thioredoxin, produced during the reduction
of methionine sulfoxide, can be converted back to reduced thioredoxin by the enzyme thioredoxin reductase.
In agreement with the methionine oxidation hypothesis of
aging, it has been reported that overexpression of thioredoxin reductase increases longevity in mice (238). However, protein repair mechanisms are generally limited,
and most oxidized proteins are removed by proteolysis
either via lysosomal degradation or via the proteasome.
The proteasome complex recognizes amino acid residues that are exposed following the oxidative rearrangement of secondary and tertiary protein structures
(117).
Protection of membranes is achieved by a more complex system that involves three mechanisms: lipid repair,
lipid replacement, and scavenging of lipoperoxidationderived end products. The action of chain-breaking antioxidants can result in the production of lipid hydroperoxides. Some of these are metabolized by GPX antioxidant enzymes, which act on H2O2 and also on free fatty
acid hydroperoxides, reducing them to fatty acid alcohols. However, the peroxidized fatty acids must be first
released from the membrane lipids. A phospholipid hydroperoxide glutathione peroxidase (PHGPX) has also
been described in some mammalian tissues and is capable
of acting on peroxidized fatty acid chains still esterified to
membrane lipids and is thus a very important antioxidant
defense for membranes in situ (167). An important mechanism for removing peroxidized lipids is membrane lipid
remodeling. While the enzyme PHGPX is important in
removing peroxidized acyl chains from phospholipids,
other enzymes are also involved in the continual deacylation/reacylation of phospholipids, and this turnover of
membrane acyl chains is very rapid. Phospholipase A2 is
an important enzyme for removing acyl chains from phospholipids while acyltransferase and transacylase enzymes
are responsible for reacylation of phospholipids (98).
When isolated rat liver cells are subjected to oxidative
stress, lipid peroxidation is increased; however, there is
no change in either the fatty acid composition of or in the
rate of acyl turnover of membrane phospholipids. There
is, however, a decrease in the polyunsaturated fatty acid
(PUFA) content of cellular triacylglycerols. It is suggested
that rapid constitutive recycling of membrane phospholipids rather than selective in situ repair is responsible for
eliminating peroxidized phospholipids, with triacylglycerols providing a dynamic pool of undamaged PUFA for
phospholipid resynthesis (110). Finally, lipid peroxidation-derived end products resulting from membrane lipid
oxidative damage can be detoxified by glutathione
S-transferases (GST; Ref. 348).
1181
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HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
acids with the fatty acid composition of membrane lipids,
it is possible to calculate a peroxidation index (a measure
of the susceptibility to peroxidation) for any particular
membrane.1 The peroxidation index of a membrane is not
the same as its unsaturation index (sometimes also called
its “double bond index”), which is a measure of the density of double bonds in the membrane. For example, a
membrane bilayer consisting solely of MUFA will have an
unsaturation index of 100 and a peroxidation index of 2.5,
while a membrane bilayer consisting of 95% SFA and 5%
DHA will have an unsaturation index of 30 and a peroxidation index of 40. This means that although the 5%
DHA-containing membrane has only 30% the density
of double bonds of the monounsaturated bilayer, it is 16
times more susceptible to peroxidative damage.
Reactive free radicals remove the bis-allylic hydrogen atoms from PUFA chains, and the carbon from which
H䡠 is abstracted now has an unpaired electron (i.e., it is
also a free radical). When such C䡠 radicals are generated
in the hydrophobic interior of membranes, a likely fate is
1
Peroxidation index ⫽ 0.025 ⫻ (% monoenoics) ⫹ 1 ⫻ (% dienoics) ⫹ 2 ⫻ (% trienoics) ⫹ 4 ⫻ (% tetraenoics) ⫹ 6 ⫻ (% pentaenoics) ⫹
8 ⫻ (% hexaenoics), while unsaturation index ⫽ 1 ⫻ (% monoenoics) ⫹
2 ⫻ (% dienoics) ⫹ 3 ⫻ (% trienoics) ⫹ 4 ⫻ (% tetraenoics) ⫹ 5 ⫻
(% pentaenoics) ⫹ 6 ⫻ (% hexaenoics).
Physiol Rev • VOL
combination with oxygen dissolved in the membrane. The
resulting peroxyl radical is highly reactive: it can attack
membrane proteins and can also oxidize adjacent PUFA
chains. Thus the initial reaction is repeated and a free
radical chain reaction is propagated. Unless quenched by
antioxidants, lipid peroxidation is a self-propagating autocatalytic process producing several potent ROS. It can
also generate lipid hydroperoxides (124, 335, 336), which
are more hydrophilic than unperoxidized fatty acyl
chains, and these can thus disrupt the membrane structure, altering fluidity and other functional properties of
membranes.
The hydroperoxides and endoperoxides, generated
by lipid peroxidation, can undergo fragmentation to produce a broad range of reactive intermediates, such as
alkanals, alkenals, hydroxyalkenals, glyoxal, and malondialdehyde (MDA; Ref. 95) (see Fig. 2). These carbonyl
compounds (collectively described as “propagators” in
Fig. 2) have unique properties contrasted with free radicals. For instance, compared with ROS or RNS, reactive
aldehydes have a much longer half-life (i.e., minutes instead of the microseconds-nanoseconds characteristic of
most free radicals). Furthermore, the noncharged structure of aldehydes allows them to migrate with relative
ease through hydrophobic membranes and hydrophilic
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FIG. 2. Schematic chemical pathway for the formation of advanced lipoxidation end products detected in tissue proteins. All of the compounds
are derivatives of lysine and arginine residues in protein. Similar compounds are formed on reaction with other nucleophilic groups in proteins, as
well as on DNA and aminophospholipids. For propagators, general structures are shown for reactive aldehydes (A) and lipid peroxidation-specific
aldehydes (B). For end products, general structures are shown for advanced lipoxidation end products (A) and advanced glyco- and lipoxidation
end products (B).
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
1183
cytosolic media, thereby extending the migration distance
far from the production site. On the basis of these features alone, these carbonyl compounds can be more destructive than free radicals and may have far-reaching
damaging effects on target sites both within and outside
membranes. These carbonyl compounds, and possibly
their peroxide precursors, react with nucleophilic groups
in proteins, resulting in their modification. The modification of amino acids in proteins by products of lipid peroxidation results in the chemical, nonenzymatic formation of a variety of adducts and cross-links collectively
named advanced lipoxidation end products (ALEs; Ref.
353). These can be useful indicators of lipoxidative stress
in vivo (Fig. 2).
Lipid peroxidation-derived end products (“enals”)
can also react at the exocyclic amino groups of deoxyguanosine, deoxyadenosine, and deoxycytosine to form
various alkylated products (218). Some common enals
that cause DNA damage (analogously to protein damage)
are MDA, acrolein, and 4-hydroxynonenal, among others.
The most common adducts arising from enals are exocyclic adducts such as etheno adducts, and malondialdehyde-deoxyguanosine (M1dG). These DNA damage markers are mutagenic and carcinogenic, with powerful effects
on signal transduction pathways (217). Furthermore, they
1) are present in the genome of healthy humans, and
other animal species, at biologically significant levels
(similar or even higher than oxidation markers sensu
stricto) (55), 2) are efficient inducers of mutations frequently detected in oncogenes or tumor suppressor genes
from human tumors (254), 3) show increased levels in
aged animals (55), 4) can be repaired by nucleotide excision repair systems and metabolized by oxidative pathways (262), 5) correlate with alterations in cell cycle
control and gene expression in cultured cells (169), and 6)
increase nearly 20-fold with a high-PUFA diet (97).
Physiol Rev • VOL
The amino group of aminophospholipids can also
react with carbonyl compounds and initiate some of the
reactions occurring in proteins, thus expanding the negative biological effects of such nonenzymatic modification (291). In this context, biological processes involving
aminophospholipids can potentially be affected by this
process. Among these, the following may be highlighted:
asymmetrical distribution of aminophospholipids in different membranes, translocation between and lateral diffusion in the membrane, membrane physical properties;
biosynthesis and turnover of membrane phospholipids,
and activity of membrane-bound proteins that require
aminophospholipids for their function (291).
Thus lipid peroxidation should not be perceived
solely in a “damage to lipids” scenario, but should also be
considered as a significant endogenous source of damage
to other cellular macromolecules, such as proteins and
DNA (including mutations). In this way, variation in membrane fatty acid composition, by influencing lipid peroxidation, can have significant effects on oxidative damage
to many and varied cellular macromolecules. For example, peroxidized cardiolipin in the mitochondrial membrane can inactivate cytochrome oxidase by mechanisms
both similar to hydrogen peroxide and also mechanisms
unique to organic hydroperoxides (251). Such an effect
could be very detrimental if it results in a greater degree
of reduction of other respiratory chain intermediates and
consequently increases production of superoxide. Figure 3
depicts the mitochondrial processes producing ROS, the
effects of ROS on lipid peroxidation, and some of the
consequent lipoxidative damage to mitochondrial DNA
and proteins.
As mentioned above, peroxidation of the PUFA
chains of phospholipids generates a complex mixture of
short-chain aldehydes. Initially, these aldehydes were believed to produce only “cytotoxic” effects associated with
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FIG. 3. Schematic diagram of mitochondrial processes that are important
for aging. The schematic shows that mitochondrial complex I and complex III
are the main free radical generators. Several physiological mechanisms influencing the rate of mitochondrial ROS generation include 1) the relative concentration of the respiratory complexes, 2) the
degree of electronic reduction of these
generators, 3) the uncoupling proteins,
4) the cardiolipin content, and 5) specific
chemical modifications. Oxygen radicals
attack lipids, carbohydrates, proteins,
and DNA. The products of lipid peroxidation include highly reactive molecules
that can cause lipoxidative damage to
mitochondrial DNA and proteins.
1184
HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
oxidative stress, but evidence is increasing that these
compounds can also have specific signaling roles during
normal function. For example, the superoxide-initiated
activation of proton conductance by UCP3, mentioned
previously, has been shown to be mediated by the lipid
peroxidation product 4-hydroxy-trans-2-nonenal (HNE)
and its homologs (88). The role of ROS and other related
molecules in normal physiological regulatory processes is
extensively reviewed elsewhere (82).
As with many biochemical processes that can be
measured in vitro, it is very difficult to assess the rate of
lipid peroxidation in vivo, and consequently to be able to
compare rates of lipid peroxidation either between species or between treatments. One method is to measure the
exhalation rate of hydrocarbons produced from lipid peroxidation. Ethane is one of the end products of n-3 PUFA
peroxidation, while pentane is produced from peroxidation of n-6 PUFA. Both are volatile and excreted from the
body via respiration, and the rate of their exhalation has
been used to assess the in vivo rate of lipid peroxidation
(178).
Unsaturated fatty acids are also responsible for the
“fluid” nature of functioning biological membranes. It has
been shown that it is the introduction of the first double
bond to an acyl chain that is primarily responsible for its
fluidity at normal physiological temperatures, and introduction of more and more double bonds to the acyl chain
has relatively little additional effect on membrane fluidity
(39). Thus substitution of fatty acids with four or six
Physiol Rev • VOL
double bonds with those having only two (or sometimes
three) double bonds will strongly decrease the susceptibility to lipid peroxidation while maintaining membrane
fluidity. This phenomenon may be helpful in longevous
animals, and in view of membrane acclimation to low
temperature in ectotherms, has been called homeoviscous longevity adaptation (267).
The importance of membrane fatty acid composition
for understanding aging (described above) and its addition to the standard scenario describing the oxidativestress theory of aging is illustrated in Figure 4. It emphasizes that membrane fatty acid composition influences 1)
metabolic rate (via effects on the physical properties of
membrane bilayers) and 2) the degree of oxidative stress
and damage to cellular molecules (via the peroxidative
susceptibility of fatty acyl chains).
In the following sections we examine the links between metabolic rate and membrane composition in the
maximum longevity of mammals and birds. It is not possible to seriously examine these relationships without
first taking into account the influence of body size on
these processes.
III. MAMMALS
As noted in section I, the relation between both maximum life span and metabolic rate and body size of different mammal species was important in the development
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FIG. 4. Schematic diagram outlining
the modification of the oxidative-stress
theory of aging emphasizing the importance of the fatty acid composition of
membranes in the determination of maximum life span. Two different examples
are presented (low membrane polyunsaturation and high membrane polyunsaturation). The thickness of the arrows
in each example represents the relative
intensity of the process.
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
1185
of the rate-of-living theory and consequently other later
insights into the mechanisms of aging. For a brief account
of the history of this subject preceding Rubner’s seminal
contribution (306), the reader is referred to Speakman
(330).
A. Metabolic Rate and Body Size of Mammals
Body size is an important species characteristic that
influences almost every aspect of organismal biology (49).
As species increase in size, they obviously require more
energy, but metabolic rate does not increase in direct
proportion to increases in body mass. This is largely
because of the constraints of geometry. For every doubling in size, the surface area of an object increases by
only 59% and not by 100% (i.e., surface area is related to
mass0.67). The basal metabolic rate (BMR) of mammals
similarly scales allometrically with body mass, and depending on the particular study, BMR has been reported
to only increase by 59 – 69% with every doubling of body
mass (i.e., it is proportional to mass0.67⫺0.76; Refs. 176,
371). The constraints imposed by such surface limitations
are manifest in the following calculation. If a mouse increased in size to that of a horse and its BMR increased in
direct proportion to the increase in body mass, the horsesized mouse would need a surface temperature of ⬃100°C
to rid itself of the heat produced by its BMR (134). Obviously if mammals, ranging in size from shrews to elephants, are to maintain essentially the same body temperature, then surface constraints (for heat loss, nutrient
uptake, waste excretion, etc.) mean BMR cannot be directly proportional to body mass (see Ref. 160 for a more
detailed discussion; the reader is also referred to the
Physiol Rev • VOL
other papers in May 2005 issue of Journal of Experimental Biology “Scaling functions to body size: theories and
facts” for a detailed coverage of the scaling of physiological functions to body size). When BMR data are expressed as mass-specific metabolic rate, the metabolic
rate of mammals is related to the ⫺0.24 to the ⫺0.33
power of body mass, depending on the particular compilation of mammalian BMR values. In other words, small
mammal species have higher mass-specific BMRs than do
larger mammals, such that for every doubling in body
mass, there is a 15–20% decrease in mass-specific BMR.
The compilation of mammalian mass-specific BMR values
presented here shows it to be proportional to mass⫺0.31
(see Fig. 5A; the data for birds in Figs. 5 and 6 will be
discussed in sect. IV, and we will only consider the mammalian data in this section).
Not all body tissues contribute equally to BMR. For
example, ⬃70% of the BMR of humans is contributed by
internal organs that constitute only ⬃7% of body mass
(317). The variation in BMR among species is thus due to
variation in both tissue size and tissue metabolic rate
(159). Although the relative importance of different cellular processes that constitute tissue metabolic rate varies
between tissue types, an interesting finding over the last
couple of decades has been that membrane-associated
activities constitute a large component of cellular metabolic activity during rest (302). For example, it has been
estimated that the maintenance of two transmembrane
ion gradients (namely, the Na⫹ gradient across the plasma
membrane and the H⫹ gradient across the mitochondrial
inner membrane) together are responsible for about half
of the energy turnover associated with the BMR of mammals (302). Another interesting finding has been that al-
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FIG. 5. The relationship between body mass of mammals and birds and mass-specific basal metabolic rate (A), maximum life span (B), and
lifetime energy expenditure (C) is shown. Metabolic rate and body mass data for mammals are from White and Seymour (371), and for birds are
from the database accumulated by Dr. C. White (personal communication). The maximum life span data are from Carey and Judge (50), and where
there was more than one value provided for a particular species, the largest value was used. For mammals, n ⫽ 267 species, while for birds, n ⫽
108 species.
1186
HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
B. Maximum Life Span, Body Size, and Lifetime
Energy Expenditure of Mammals
Although death may not always be due to the decline
in physiological function and/or homeostatic imbalance
that occur during aging, MLSP is nevertheless an important species characteristic. Reported MLSPs vary more
than 40,000-fold across the animal kingdom, and within
mammals by at least 2 orders of magnitude. Generally,
MLSP of mammals increases with body size, such that the
smallest mammals (shrews) live ⬃1 yr while the reported
maximum life span for elephants is ⬃80 yr (50). Thus
maximum life span is positively related to body mass in
mammals, such that for every doubling of body mass in
mammals, there is on average a 16% increase in MLSP
(i.e., MLSP is proportional to mass0.22, see Fig. 5B and Ref.
330). As has long been known, both BMR and MLSP are
correlated with body mass in mammals, but the MLSP of
mammals is not as strongly correlated with body mass as
is mammalian BMR. For example, whereas body mass
variation can statistically explain 64% of BMR variation, it
only explains 35% of the variation in MLSP of mammals
(see Fig. 5, A and B).
The core of the rate-of-living theory ascribes species
differences in longevity to differential rates of energy
expenditure, such that a short maximum life span will be
associated with a high mass-specific metabolic rate. When
the MLSP data for the 267 mammal species in Figure 5B is
plotted against their mass-specific BMR data in Figure 5A,
it can be seen that the MLSP of mammals is indeed
negatively correlated with BMR (Fig. 6). Although the
relationship is statistically significant, it is obvious that
there is a huge amount of variation in the relationship. For
example, BMR can statistically explain only 26% of the
variation in MLSP of mammals, which is less than the
predictive power of body size for mammalian MLSP (35%,
see Fig. 5B) and also less than the predictive of body size
for the BMR of mammals (64%, see Fig. 5A). The variation
obvious in Figure 6 is a clear demonstration that the
Physiol Rev • VOL
FIG. 6. An examination of the rate-of-living theory for mammals and
birds. The relationship between basal metabolic rate and maximum life
span for mammals and birds is shown. The data are the same as those
plotted in Figure 5, A and B.
rate-of-living generalization is only a rough predictor of
how long a mammal species can maximally live. Its inability to precisely describe the maximum longevity of a
mammal suggests other factors are involved in the determination of maximum life span.
The initial version of the rate-of-living theory posited
that body mass-related variation in mass-specific BMR
and MLSP would mathematically cancel each other, such
that different-sized mammals would have a relatively constant mass-specific lifetime energy expenditure (LEE). In
other words, it suggested that because the exponents (as
a function of body mass) for MLSP and mass-specific
BMR have opposite signs and are similar in value, when
these two variables are multiplied together, their product
(LEE) would show little dependence on body mass. Regardless of the size of the species of mammal, a gram of
any animal would be expected to expend approximately
the same amount of energy before it dies at MLSP.
When lifetime resting energy expenditure (LEE) is
calculated for the mammal data illustrated in Figure 5, A
and B, and plotted against body mass (see Fig. 5C), there
is a significant negative slope such that, for every doubling of body mass, LEE of mammals decreases by 6%
(i.e., it is proportional to mass⫺0.09). A similar recent
compilation of LEE for 240 mammal species similarly
showed that there was a small (but statistically significant) negative allometric relationship of LEE with body
mass (330). Lifetime resting energy expenditure is thus
not constant (as predicted by the rate-of-living theory) but
declines with increasing body mass in mammals. Such
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though cellular metabolic rate decreases with increasing
body mass in mammals, the relative contribution of various processes to total metabolic activity does not vary.
For example, in a comparison of the respiration rate of
isolated hepatocytes from eight mammalian species, ranging in size from mice to horses, although hepatocyte
respiration rate decreased by 13% for every doubling of
species’ body mass (i.e., proportional to mass⫺0.20), the
relative contribution of mitochondrial ATP production,
mitochondrial proton leak, and nonmitochondrial processes did not change with body mass (288). When BMR
of mammals varies with body mass, all subcellular processes that constitute metabolic activity seem to vary in
unison (see Ref. 159).
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
Physiol Rev • VOL
a negative relationship between life span and body size.
For example, large dog breeds have lower mass-specific
metabolic rates than smaller dogs and also show a reduction in maximum life span. However, confounding any
definitive conclusions that may be drawn from this finding, other studies focusing within a particular dog breed
(107, 283) revealed no clear relationship between body
size and life span. The mass-specific metabolic rate of
three dog breeds that differ in maximum longevity (from
8.5 to 14 yr) varied in the opposite direction to that
predicted by the rate-of-living hypothesis, namely, the
smallest and longest-living breed had the highest rate of
living (333).
Several intraspecific studies using mice and rats (40,
146, 202, 332, 333) have not observed an inverse relationship between mass-specific metabolic rate and MLSP.
Indeed, some of these studies show the opposite of rateof-living predictions, namely, that mice with high massspecific metabolic rates tend to live longer than those
individuals with low metabolic rates. One study reported
that the individual mice with highest BMR had higher
levels of mitochondrial proton leak and higher activation
of both UCP-3 and adenine nucleotide translocase (333).
Uncoupling of mitochondrial activity to promote thermogenesis may also contribute to the combined effects of
higher metabolic rate and longevity in smaller dog breeds.
Such uncoupled mitochondrial metabolism has been suggested to result in lower generation of ROS but greater
generation of heat (36). Clearly, the simplistic concept
that metabolic rate and ROS production are directly correlated in a direct linear manner does not appear to be
true. Thus insight into the relationship between metabolism and longevity requires a better understanding of
mitochondrial function and efficiency.
C. Mitochondrial Free Radical Production and
Antioxidant Defenses of Mammals
The decrease in mass-specific BMR with increasing
body size in mammals is also associated with a decrease
in tissue density of mitochondrial inner membranes (92),
the site of mitochondrial ROS production. The in vitro
production rates of both superoxide (327) and hydrogen
peroxide (326) by isolated liver mitochondria have been
measured in mammals ranging in size from mice to cattle.
Allometric analysis of these two data sets shows there is
a 10% decrease in superoxide production (it is proportional to mass⫺0.15) and a 18% decrease in hydrogen peroxide production (i.e., proportionality to mass⫺0.29) per
milligram of mitochondrial protein with every doubling of
body mass of the mammal species. It is of interest that a
separate study of liver mitochondria from mammals ranging from mice to horses (289) reported that the amount of
mitochondrial membrane per milligram mitochondrial
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average trends, however, mask a huge amount of variation. Statistically, body mass only explains 12% of the
variation in LEE between mammals, and the average decline with body mass described above (6% for every doubling of mass) is very small compared with the 81-fold
range of values across all mammal species.
Of course, mammal species do not spend all their
lives resting in a thermoneutral environment, and thus
BMR is not a good measure of total lifetime energy metabolism. Physical activity level can account for as little as
20% of the total energy expenditure in sedentary individuals to as much as 50% in active individuals (104). As well,
the energetic cost of maintaining body temperature varies
with environmental conditions, body size, and insulation.
Similarly, foods eaten may variably influence postprandial
diet-induced thermogenesis. Field metabolic rate (FMR),
which incorporates all of these influences on energy turnover, may be a better measure of lifetime energy requirements and thus test of the rate-of-living theory. FMR has
been measured in a wide range of different-sized mammal
species by use of the doubly labeled water technique,
which can measure the rate of CO2 production in freeliving individuals. A recent compilation of such values for
79 mammal species shows that FMR of mammals is proportional to body mass0.73 (252), while another mammalian data set suggests it is proportional to mass0.62 (330).
These studies suggest that for every doubling of body size
in mammals, there is on average a 17–23% decrease in
mass-specific FMR. When lifetime mass-specific FMR was
calculated for 49 mammal species (all weighing ⬍4 kg),
there was a statistically highly significant negative
relationship with species body mass, which is also contrary to the rate-of-living prediction of a relatively constant lifetime energy turnover per gram of animal (330).
A key premise of the combination of the rate-of-living
and oxidative-stress theories is that the more energy an
organism expends, the more oxidative damage it will
accrue, and the quicker it will die. Although it does not
fully explain the differences between mammal species,
might it be an explanation within the lifetime of an individual mammal, or between individuals within a species?
If this explanation was completely adequate, couch potatoes would lead long lives, while those who follow modern trends for promoting good health by exercising regularly would die sooner. In both humans and mice, although voluntary exercise increases metabolic rate, it
does not reduce life span (147, 200). Recent studies have
revealed that this theory is fraught with exceptions and
problems. Regardless of whether data are expressed per
gram body mass, per whole animal or per lifetime, a
different pattern emerges when intraspecific variation is
examined relative to interspecific mammalian comparisons. Intraspecific studies on dogs (333), mice (234, 332),
and humans (301) reveal a positive association between
maximum life span and mass-specific metabolic rate and
1187
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HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
nous level of the antioxidant system and MLSP of mammals. This suggests that enhanced antioxidant defenses of
mammals are not the reason for the extended longevity of
some mammal species but rather are more likely a reflection of the degree of oxidative stress experienced by the
tissues of such long-living mammals. These results are
compatible with both the notion that endogenous antioxidant defenses are already at optimal levels in animals and
that their supplementation, while possibly improving general health of a population (and thus increasing mean
longevity), generally has no significant influence on MLSP
(see Tables 3 and 4).
D. Membrane Composition, Lipid Peroxidation,
Body Size, and Life Span of Mammals
Initially, attempts to understand the basis of the
body-size trends in mammalian BMR discounted differences in cell composition between species as an explanation (e.g., Ref. 176). However, nearly 30 years ago, Gudbjarnason et al. (118) published a little-noticed relationship between the DHA content of cardiac phospholipids
and resting heart rate (a direct correlate of mass-specific
BMR) of mammal species ranging from mice to whales.
To our knowledge, this is the first record of a relationship
between the fatty acid composition of cellular membranes and metabolic intensity. Later, it was demon-
2. Comparative studies of endogenous levels of tissue antioxidants in vertebrates differing in their
maximum life span potentials
TABLE
Antioxidant
Number of Species Compared
Organ
Correlation With MLSP
Reference Nos.
Ascorbate
7 Mammals
9 Mammals
7 Vertebrates
6 Vertebrates
Ames dwarf mice (df/df)†
5 or 6 Mammals
6–8 Vertebrates
Drosophila (2)
3–5 Mammals
6–8 Vertebrates
8 Vertebrates
Ames dwarf mice (df/df)†
Drosophila‡
5 Mammals ⫹ 1 bird
5 Mammals ⫹ 1 fish
6 or 7 Vertebrates
8 Vertebrates
6 or 7 Vertebrates
Drosophila‡
13 Mammals
11 Mammals
7 Vertebrates
6 or 7 Vertebrates
Ant (Lasius niger)§
Liver
Brain
Liver
Brain, lung
Liver
Liver, kidney
Brain, liver, lung
Whole
Brain, liver
Liver, lung
Brain
Liver
Whole
Liver
Brain, liver
Brain, liver, lung
Liver
Brain, liver
Whole
Liver, brain, heart
Brain
Liver
Brain, lung
Whole
⫺
⫺
71
73
210
18, 285
41
71
18, 210, 285
242
71
210, 285
18
41
242
197
78
18, 210, 285
210
18, 285
242
355
257
210
18, 285
282
Catalase
GSH
GSH-Px
GSH-Red
SOD
NS
⫺
⫺
⫺
⫺
NS or ⫺
⫺
⫺
NS
⫺
NS or ⫺
⫺
⫺
⫺
NS
⫺
NS or ⫺
NS
⫹*
NS
⫺
⫺
SOD, superoxide dismutase; GSH, glutathione; GSH-Px, GSH peroxidase; GSH-Red, GSH reductase; NS, not significant; ⫺, negative; ⫹,
positive. * r2 ⫽ 0.41, P ⬍ 0.03. Studies in vertebrates included fish, amphibians, birds, and mammals. † Ames dwarf mice (df/df) vs. normal or
transgenic GH. ‡ Drosophila: long-lived vs. short-lived lines. § Ant (Lasius niger): long-lived queen vs. short-lived workers and males.
Physiol Rev • VOL
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protein decreases by ⬃7% for every doubling of body
mass (i.e., proportionality to mass⫺0.10). Thus the link
between maximum life span and mass-specific metabolic
rate in mammals of different body size is associated with
appropriate changes in production rates of primary ROS
(i.e., superoxide and hydrogen peroxide) by isolated mitochondria, at least in the liver.
However, these studies only included mammal species that followed the rate-of-living theory so the results
obtained could also be interpreted as a correlate of that
phenomenon. That is, small mammals with short MLSP
show high mitochondrial ROS production because their
rates of mitochondrial oxygen consumption are higher.
Many biochemical reactions apart from oxygen consumption occur at an accelerated rate when the metabolic rate
is high, and some of them, unrelated to ROS production,
could also, in principle, be responsible for the accelerated
aging rate of the short-lived mammals.
It may be that it is enhanced antioxidant defenses
that are responsible for the longer MLSP of larger mammals. Thus it was initially surprising for investigators to
find that the level of endogenous antioxidant defenses
(both enzymatic and scavenger antioxidant systems) of
long-living mammal species are lower than those of shortliving mammals (2, 17, 285, 312). As can be seen from
Table 2, for a number of antioxidant defense systems in a
number of tissues, there is a significant negative relationship (or no significant correlation) between the endoge-
1189
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
3. Effect of increased antioxidant levels (by either dietary or pharmacological manipulation) on the mean
and maximum longevity of vertebrate animals
TABLE
Antioxidant Increased
Survival Curve
Effect on Mean Life Span
BHT
ETO⫹MEA
MEA
MEA
MEA
BHT
CYS, PG, DTBH, or HNCl
ETO
MET
␣-Tocopherol
Mixture*
Yes
Yes
Yes
Yes
Yes
Yes
Not
Yes
Yes
Yes
Yes
1 22%
⫽
⫽
⫽
1 12%
1 31%
⫽
1 19%
1 13%
⫽
⫽
Deprenyl
␣-Tocopherol
␣-Tocopherol
Not
Yes
Not
1 2%
⫽
2 31%
SOD, ASC, GSH, GSH-Red‡
Yes
1
Effect on MLSP
MLSP of Controls, yrs
Reference Nos.
⫽
⫽
⫽
⫽
⫽
⫽
⫽
§
1 12%
⫽
§
3
3.6
2.8
3.3
2.2
2.2
2.2
1.9
2.8
3
NS†
64
130
180
128
128
128
128
66
133
199
349
1 4%
⫽
⫽
2.7
2.8
2.8
232
287
24
6
209
Mice
Frogs
⫽
ASC, ascorbate; BHT, butylated hydroxytoluene; CYS, cysteine; DTBH, 2,6-di-tert-butyl-hydroquinone; ETO, ethoxyquin; GSH, glutathione;
GSH-Red, GSH reductase; HNCl, hydroxylamine hydrochloride; MEA, 2-mercaptoethylamine; MET, 2-mercaptoethanol; MLSP, maximum life span;
PG, propyl gallate; SOD, superoxide dismutase. * Mixture of antioxidants: ␣-tocopherol ⫹ BHT ⫹ ascorbic acid ⫹ DL-methionine ⫹ sodium selenite.
† Survival data shown only up to 1.75 yrs. ‡ 100 –1,000% tissue antioxidant inductions secondary to catalase inhibition. § Not investigated; ⫽, no
change; Yes, shown; Not, not shown; NS, not significant.
strated that this relationship was not restricted to heart
membranes, but was also manifest in the fatty acid composition of the membranes of other important tissues of
mammals (69, 164). For every doubling of body mass of
mammal species there is a 12–24% decrease in the relative
DHA content of cellular membranes of mammals (164),
which is not dissimilar to the ⬃19% decrease in their
mass-specific BMR. The cellular membranes of small
mammal species are more polyunsaturated than those of
large mammal species, and in mammals, this body sizerelated variation in membrane DHA content is the dominant influence on the body size-related variation in membrane composition. A substantial number of membraneassociated processes have also been shown to vary with
body size of mammals and have been related to this
difference in membrane composition. Indeed, it has been
proposed that it is the difference in membrane fatty acid
composition that is causal to the differences in BMR, and
this proposal has been called the membrane-pacemaker
theory of metabolism (refer to Refs. 156, 158 –160 for
detailed explanation). This theory notes that the cellular
membranes of animals with high rates of mass-specific
metabolism have high degrees of polyunsaturation and
proposes that such high membrane polyunsaturation results in physical properties of membrane bilayers that
speed up the activity of membrane-associated proteins
and consequently the metabolic rate of cells, tissues, and
the whole animal. While this theory is supported by a
large body of correlational evidence, there is also imporPhysiol Rev • VOL
tant experimental evidence (e.g., from “species crossover” studies) that substantiates the theory (93, 341, 374).
At the same time that the fatty acid composition of
membranes from a variety of mammalian tissues was
related to variation in BMR (69, 158, 159), Pamplona and
colleagues (266, 271, 272, 275–277) also measured the
membrane fatty acid composition of a number of mammalian tissues and related it to MLSP of the particular
species. Thus the novel and unexpected finding that the
composition of membrane bilayers varies systematically
in mammals provided a link between metabolic rate and
maximum life span of mammalian species. In agreement
with this, it was demonstrated that in long-lived mammals, the low degree of membrane fatty acid unsaturation
was accompanied by a low sensitivity to in vivo and
in vitro lipid peroxidation (269, 272, 275–277) and a low
steady-state level of lipoxidation-derived adducts in both
tissue and mitochondrial proteins of skeletal muscle,
heart, liver, and brain (267, 268, 274, 277, 292, 307). These
findings were consistent with the negative correlation
between MLSP and the sensitivity to lipid autoxidation of
mammalian kidney and brain homogenates that had been
described some years earlier (72).
If the fatty acid composition of membrane phospholipids is known, it is possible to calculate a peroxidation
index (PI; see sect. IID), which is a number that expresses
the calculated susceptibility of the particular membrane
to lipid peroxidative damage. When this is done for both
liver mitochondrial phospholipids and skeletal muscle
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Rats
1190
HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
TABLE
4.
Species
Effects of modifying tissue antioxidants in vivo on MLSP: transgenic/knockout mutants
Effect on MLSP
Reference
Nos.
None
None
None
None
None
None or decrease
260
320
339
241
243
259
None or decrease
None
Increase 10%
240
151
318
Decrease
Decrease
Decrease
None
Decrease
Decrease
Decrease
Decrease
None
Decrease
None
Decrease
86
175
286
256
51
141
227
373
356
357
89
89
Mutant
Overexpression
Drosophila
Mice
Mice
Knockouts
Mn-SOD
Mn-SOD
CuZn-SOD
CAT
Homozygous extracellular SOD
Homozygous Gpx1
Homozygous Mn-SOD
Homozygous Gpx4
Heterozygous Mn-SOD
Homozygous Gpx1/heterozygous SOD2
Heterozygous CuZn-SOD
Homozygous CuZn-SOD
Studies not reporting data about effects on MLSP are not indicated. * In mitochondria.
phospholipids from a range of different-sized mammal
species, it can be seen that there is a strong inverse
relationship between PI and MLSP of mammals (see
Fig. 7). Interestingly, the relationship between liver mitochondrial phospholipid PI and MLSP has a similar slope to
that between skeletal muscle PI and MLSP. The liver
mitochondrial membrane PI of mammals is proportional
to their MLSP⫺0.40, which means that a 24% decrease in
their peroxidative susceptibility is associated with every
doubling of maximum life span. For skeletal muscle membranes, the corresponding value is that a 19% decrease in
peroxidative susceptibility is associated with every doubling of MLSP in mammals (i.e., muscle PI is proportional
to MLSP⫺0.30).
While maximum life span can differ dramatically between mammal species, there can also be significant longevity differences within a species. For example, populations of two wild-derived strains of mice display extended
longevity (both average and maximum life span) compared with genetically heterogeneous laboratory mice
when kept under identical conditions (233). The longevity
of these wild-derived mice strains in captivity exceeds
that yet observed for any laboratory-derived mutant
mouse strain. The PI of skeletal muscle phospholipids
and liver phospholipids of both wild-derived mice
strains with the extended longevity is significantly reduced compared with laboratory mice maintained under identical conditions (163). This is of interest because, as the different mice strains were fed the same
Physiol Rev • VOL
diet, it shows that the difference in membrane composition is under genetic control and not determined by
dietary differences.
We know almost nothing of the ways in which the
regulatory mechanisms controlling membrane fatty acid
composition differ between species. With the assumption
of the dietary availability of PUFAs, the control of membrane fatty acid composition will reside in the synthetic
pathways for fatty acyl chains, the pathways of de novo
synthesis of phospholipids, and also in the rapid membrane-remodeling pathways that involve enzymatic deacylation/reacylation cycles. Because PUFA cannot be synthesized de novo by higher animals, they are essential
components of the diet. They are only required in small
amounts, and in nature, because animals eat other organisms (or their products) they are almost always present in
the diet. Another source of PUFA will be their biosynthesis by gut microorganisms. It is however possible to vary
availability of different types of fats in laboratory diets.
When this has been done in rats, membrane fatty acid
composition shows a degree of homeostasis that resists
dietary alteration (see Ref. 165). For example, if fed a diet
devoid of PUFA, mammals will synthesize an unusual
PUFA, mead acid (20:3 n-9) and accumulate it, together
with more than normal amounts of MUFA in their membranes. However, with extreme manipulation of dietary
fat composition, it is possible to effect small changes in
membrane fat composition.
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Drosophila
CAT
CuZn-SOD
CuZn-SOD
Mn-SOD
GR
CuZn-SOD/CAT
Mn-SOD/thioredoxin reductase
Ectopic expression CAT*
CuZn-SOD
Catalase*
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
1191
FIG. 7. The relationship between maximum life span of mammals
and birds and the peroxidation index of skeletal muscle phospholipids
(A) and liver mitochondrial phospholipids (B) is shown. The data points
for naked mole rats are from Hulbert et al. (161) and are superimposed
on graphs from Hulbert (155).
A low PUFA content in cellular membranes (and
particularly in the inner mitochondrial membrane) will be
advantageous in decreasing the sensitivity of the membrane to lipid peroxidation and would consequently also
Physiol Rev • VOL
FIG. 8. A comparison of the rates of lipid peroxidation in mammals
of different body size relative to their metabolic rate is shown. Ethane
exhalation is a measure of the in vivo peroxidation rate of n-3 PUFA
(178), and values from the literature are for mice (79), rats (179, 245,
356), dogs and horses (375), and humans (178). Oxygen consumption is
the basal metabolic rate of each species (317).
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protect other molecules against lipoxidation-derived damage. To clarify whether the low membrane PUFA content
can protect mitochondria from lipid oxidation and lipoxidation-derived protein modification, experimental dietary
modification of in vivo membrane fatty acid composition
in rats has been performed. These studies partially circumvent the homeostatic system controlling membrane
composition and demonstrate that altering the fatty acid
composition of cellular membranes does indeed protect
postmitotic tissues against lipid peroxidation and lipoxidation-derived macromolecular damage (140,
273, 290).
The decrease in the PUFA content of mammalian
membranes as body mass increases would be expected to
result in a decreased rate of lipid peroxidation in larger
mammals. While there is considerable evidence showing
that membrane fatty acid composition affects the rate of
lipid peroxidation measured in vitro, there is less evidence that it has an effect on in vivo lipid peroxidation.
One of the products of n-3 PUFA peroxidation is ethane,
and the rate of ethane exhalation has been used as a
noninvasive method for assessing in vivo lipid peroxida-
1192
TABLE
HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
5.
Effect of aging on lipid parameters in tissues from different species
Marker/Parameter
PI/PUFA content
MDA-TBARS
Lipid hydroperoxides
Conjugated double
bonds
Expired hydrocarbons
(pentane and others)
Membrane fluidity
Whole
Liver
Liver
Liver
Liver microsomes
Liver mitochondria
Liver microsomes
and mitochondria
Liver microsomes
and mitochondria
Liver and kidney
microsomes and
mitochondria
Heart
Heart mitochondria
Heart mitochondria
Brain synaptosomes
Cerebral cortex
Species
Membrane
Drosophila
Rat
Rat
Rat
Rat
Rat
Rat
Age(s)*
Change With Aging
Reference Nos.
fatty acid composition
From 10 to 40
3 vs. 24
4 vs. 12
8 vs. 30
10 vs. 25
From 6 to 28
12 vs. 24
Increase
Increase
Increase
Increase
Unchanged
Increase
Unchanged
Pamplona, unpublished results
132
168
8
150
192
188
Rat
From 6 to 24
Increase
189
Rat
6 vs. 24
Increase
116
8 vs. 30
6 vs. 24
7 vs. 24
From 6 to 24
4 vs. 32
Membrane lipid peroxidation
Mouse
From 3 to 24
Human
From newborn to 12
Mouse
From 3 to 24
Rat
From 3 to 24
Mouse
From 3 to 24
Mouse
From 3 to 24
Rat
From 3 to 24
Mouse
From 3 to 24
Rat
From 3 to 24
Dog
From 3 to 24
Rat
From 3 to 12
Rat
From 3 to 12
Mouse
From 6 to 24
Mouse
12 vs. 24
Rat
4 vs. 12
Rat
From 0 to 24
Rat
From 6 to 24
Rat
6 vs. 24
Increase
Increase
Unchanged
Decrease
Decrease
9
201
268
60
346
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
11, 77
111
58, 77, 94
297
77, 94
237
297
237, 343, 349
297
250
297
297
77
181
168
317
57
188
Rat
From 3 to 24
Increase
382
Rat
Rat
Rat
3 vs. 24
From 6 to 24
From 5 to 31
Increase
Increase
Increase
132
12
354
Rat
Rat
4 vs. 12
6 vs. 24
Increase
Increase
168
188
Rat
3 vs. 24
Increase
132
Whole animal
Rat
From 4 to 32
Increase
308
Whole animal
Whole animal
Rat
From 6 to 30
Human
From 21 to 79
Membrane order parameters
Rat
From 4 to 32
Rat
From 3 to 24
Increase
Increase
222
384
Decrease
Decrease
346
173, 383
Rat
Rat
6 vs. 24
From 6 to 25
Decrease
Decrease
61
106
Rat
Rat
6 vs. 24
From 0 to 24
Decrease
Decrease
201
314, 315
Liver
Liver
Brain
Brain
Heart
Testis
Testis
Heart
Heart
Heart
Adrenal cortex
Kidney
Liver
Liver
Liver
Liver, brain, heart
Liver mitochondria
Liver microsomes
and mitochondria
Liver microsomes
and mitochondria
Heart mitochondria
Brain regions
Plasma, splenic
lymphocytes
Liver
Liver microsomes
and mitochondria
Heart mitochondria
Cerebral cortex
Liver mitochondria
and microsomes
Brain synaptosomes
Brain mitochondria
and
synaptosomes
Heart mitochondria
Brain, heart, liver
Rat
Rat
Rat
Rat
Rat
PI, peroxidizability index: calculated from fatty acid compositional analysis; PUFA, polyunsaturated fatty acid content. * For Drosophila, in
days; for rat and mouse, in months; for human, in years.
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Lipofuscin
Tissue
1193
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
6. Effect of aging on steady-state levels of lipoxidative protein and DNA modifications in tissues from
different species
TABLE
Compound (Units)
AMPs
CML
MOLD
GOLD
MDAL
HNE-protein
MDA-DNA
Cerebral cortex
Glomerular basement
membrane collagen
Whole
Skin collagen
Skin collagen
Articular cartilage
collagen
Lens protein
Lens protein
Urine
Liver mitochondria
Serum
Aorta
Heart
Liver mitochondria
Skin collagen
Articular cartilage
collagen
Lens protein
Liver mitochondria
Lens protein
Lens protein
Lens protein
Lens protein
Liver
Liver mitochondria
Heart
Heart
Liver
Kidneys
Testes
Brain
Liver
Brain
Species
Age(s)
Levels of Glyco- and
Lipoxidation Compounds
Reference Nos.
Human
Rat
25.6 vs. 78.6 yr
3, 10, 26, and 30 mo
8.5 vs. 441a
1.6, 1.6, 2.2, and 2.6b
213
366
Drosophila
Human
Rat
Human
10 vs. 75 days
From infancy to 80 yr
From 0 to 30 mo
From 2.5 to 80 yr
3750 vs. 6730c
Trace-1.5
0.07–0.13
Trace-5.3
263
84, 87, 316*
52
316, 367*
Human
Human
Human
Rat
Rat
Rat
Rat
Rat
Human
Human
From 1 to 100 yr
From 10 to 80 yr
Between 15 and 72 yr
10 vs. 27 mo
4 vs. 28 mo
4 vs. 28 mo
4 vs. 28 mo
6, 18, and 28 mo
From infancy to 80 yr
From 2.5 to 80 yr
⬃1–8
⬃1–5
1.6–2.5a
11.5 vs. 17.5b
0.025 vs. 0.04c
⬃13 vs. 27d
⬃19 vs. 32d
0.75, 0.46, and 0.96
Trace-5
0.1–0.8
85
1
177
13
246
246
246
192
84
367
Human
Rat
Human
Human
Human
Human
Rat
Rat
Rat
Rat
Rat
Rat
Rat
Rat
Rat
Rat
From 10 to 80 yr
6, 18, and 28 mo
10 vs. 80 yr
20 vs. 75 yr
10 vs. 80 yr
20 vs. 75 yr
8 vs. 30 mo
6, 18, and 28 mo
8 vs. 30 mo
8 vs. 24 mo
4 vs. 25 mo
4 vs. 25 mo
4 vs. 25 mo
4 vs. 25 mo
2 vs. 24 mo
From 6 to 24 mo
1.5–4
0.41, 0.21, and 0.43
15 vs. 93a
0.1 vs. 0.8b
3 vs. 8.5a
0.02 vs. 0.2b
0.22 vs. 0.31
0.14, 0.08, 0.18
0.39 vs. 0.68
Increased (WB)
9.9 vs. 25.5a
1.7 vs. 5.1a
1.7 vs. 1.1a
62 vs. not determineda
6 vs. 12b
Increase 3.5-fold
1
192
56
105
56
105
8
192
9
212
81
81
81
81
218
48
AGE, advanced glycation end products; AMPs, advanced maillard products; CML, carboxymethyl-lysine; CHML, carboxymethyl-hydroxyllysine; CEL, carboxyethyl-lysine; GOLD, glyoxal lysine dimer; HNE-protein, hydroxynonenal-protein, (cysteine, lysine, or histidine); MDAL,
malondialdehyde-lysine; MOLD, methylglyoxal lysine dimer; AU, arbitrary units; WB, Western blot. Units used: for AMPs, a AGE positive neurons
(n° cells/mm2), b AU/mM OH proline, c AU fluorescence at 440 nm; for CML, mmol/mol Lys, a ␮g/mg creatine, b pmol/␮g protein, c pmol CML-BSA
equivalent per ␮g protein, d pmol CML-BSA equivalent per ␮mol Leu equivalent; for CHML, mmol/mol OH Lys; for CEL, mmol/mol Lys; for MOLD,
a
pmol/mg protein, b mmol/mol Lys; for GOLD, a pmol/mg protein, b mmol/mol Lys; for MDAL, mmol/mol Lys; for MDA-DNA: a pmol/100 ␮g DNA,
b
⫻ 107 nucleosides. * Immunohistological staining of CML.
tion. Figure 8 shows the resting ethane exhalation rates
obtained from the literature for mice (control mice in Ref.
79), rats (179, 245, 356), dogs (375), humans (178), and
horses (375) relative to the BMR for the same five species
(317). As can be seen from Figure 8, the calculated ratio
of moles ethane exhaled:moles oxygen consumed ranges
from 0.12 in mice to 0.06 in horses (with the exception of
the human values which will be discussed in the next
section). However, measurement of the exhalation rate of
hydrocarbons is technically difficult and, because of potential interlaboratory variation, definitive conclusions
as to whether in vivo lipid peroxidation does decrease
more than BMR in larger mammals awaits measurement of different-sized mammals by the same laboratory.
Physiol Rev • VOL
The importance of membrane fatty acid composition
in the aging process is highlighted in Tables 5 and 6. The
studies summarized in Table 5 show that there are many
reports of 1) an increase in either PUFA content or PI of
membranes with age, 2) an increase in both in vitro and
in vivo membrane lipid peroxidation with age, as well as
3) age-related changes in physicochemical membrane
properties. In Table 5, there was either an increase, or no
change, in PUFA content/PI reported for most tissues,
except the brain, which showed a decrease with age in the
rat. Although it does not show the compositional changes
with age observed in other tissues, it does reveal an
elevated lipid peroxidation, and a decrease in membrane
fluidity (or increase in microviscosity) with age that are
also characteristic of the other tissues. The increased
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CHML
CEL
Tissue
1194
HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
ity quotient (LQ), or the ratio of observed MLSP to that
predicted from body mass by appropriate allometric equations (e.g., see Fig. 5B), may be a better indicator of the
relative rate of aging. Mammals with an LQ value significantly ⬎1 live significantly longer than predicted by their
body size, and most likely exhibit slow rates of aging.
More limited conclusions might be drawn from those
species with an LQ of ⬍1. While an LQ of ⬍1 may reflect
accelerated aging processes, it might also indicate an
inaccurate MLSP value for the particular species due to
inappropriate housing conditions and poor nutrition in
captive animals, or high predation rates and other ecological constraints on species in the wild. Relatively few
mammals have LQ values ⬎2, but there are some notable
exceptions. In Figure 9 we have presented the LQ values
for three such mammals: bats, naked mole rats, and humans.
Considerable data based on millions of records confirm that humans are extremely long-living mammals,
both for our size and in absolute terms (see Fig. 10). We
have an MLSP that is four to five times longer than pre-
E. Insights From Relatively Long-Living Mammals
(Including Humans)
As noted earlier, there is considerable variation in
maximum life span among mammals, and MLSP may be a
poor indicator of the relative rate of aging in mammals.
For most nondomesticated mammalian species, MLSP
values are based on a limited number of captive longevity
records maintained by zoological collections. The longevPhysiol Rev • VOL
FIG. 9. The longevity quotients of select mammals and birds are
shown. Longevity quotient is the ratio of a species actual maximum
longevity to the predicted maximum life span for its body mass. The
predicted maximum life span for each species was obtained using the
appropriate equation from Figure 5B. The value for bats is mean (⫾SE)
for 11 species of Chiroptera, that for albatrosses is mean (⫾SE) of 11
species of Procellariiformes, and that for fowl is mean (⫾SE) of 8
species of Galliformes.
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lipid peroxidation with age is also associated with much
greater degrees of lipoxidative damage, to both proteins
and DNA (Table 6).
PUFAs have lower melting points than saturated fatty
acids, and consequently, a relative increase in the PUFA
content of a membrane would be expected to render the
membrane more fluid. Evidence shows that during aging,
the membrane fatty acid profile tends to increase toward
peroxidizable PUFAs in many cases (e.g., see Table 5).
Based solely on the fatty acid composition, membranes
from older animals should exhibit greater fluidity than
those from younger individuals. Paradoxically, the opposite is often reported; membrane fluidity decreases with
age (see Table 5). The reasons for the changes of membrane fluidity can be explained in two ways: 1) a change
caused by fatty acid chain composition and 2) a change in
the cholesterol content of the membrane. The paradox
described above is potentially resolved by considering
that cellular components experience increased oxidative
stress with the passage of time. Because the PUFAs are
more vulnerable to free radical attack, they experience
greater lipid oxidative damage, and lipid peroxidation
products have been shown to contribute significantly to
membrane rigidity. Lipid peroxidation products and peroxidized lipids are likely dominant factors in creating
age-related membrane rigidity. The modification of the
physical state of the membrane correlates with the loss of
its functioning. Lipid peroxidation products and peroxidized lipid, compared with cholesterol, are greater inducers of membrane rigidity. Furthermore, and interestingly,
both mitochondrial structure and function are very sensitive to reactive aldehydic compounds from lipid peroxidation (188, 189, 291, 382, 383).
In view of these widespread changes in membrane
composition and lipid peroxidation with age, it is of interest that in the senescence-accelerated mouse (SAM)
strain, those mice that are SAM-prone (SAM-P mice) have
greater levels of the highly polyunsaturated peroxidationprone fatty acids (both 22:6 n-3 and 20:4 n-6) and lower
levels of the less peroxidation-prone PUFA (18:2 n-6) in
their membranes, and consequently a greater PI, than
SAM-resistant mice (59, 281). SAM-prone mice also show
greater degrees of lipid peroxides in their tissues than do
SAM-resistant mice (221).
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
dicted from the allometric relationships (e.g., Fig. 5B; see
also Refs. 7, 294). For example, the similar-sized pig is
recorded as having a maximum life span of only 27 yr and
sheep only 24 yr, compared with ⬎100 years for Homo
sapiens (50). Relatively few other mammals attain similarly high longevity quotients, and there have been numerous theories (some involving superior intelligence) to
explain our extraordinary longevity. Indeed, Rubner (306)
Physiol Rev • VOL
calculated the lifetime energy potential (LEP) of his five
nonhuman mammalian species to range from 590 to 1,110
MJ/g, while if he had included his data for humans
(⬃3,000 MJ/g) in his comparison, the consequent fivefold
range in the LEP of mammals would have made his proposal less convincing.
It has been suggested that the bowhead whale may
live to more than 200 yr; however, such an estimated
MLSP for this species is not based on captive or freeranging records, but rather has been estimated from bone
growth patterns (300). The longevity record for the larger
blue whale is only 110 yr (50). The most reliable record of
the longest-living human is for the French woman Jeanne
Louise Calment, who lived for 122 years and 164 days
(February 21, 1875 to August 4, 1997). Analyses of mortality and census data from the United States suggest that
human maximum life span may be ⬃130 yr. Mean life
expectancy of humans has changed dramatically with
improved public health conditions in recent times; however, maximum life span of humans is unchanged (80).
Mammals with a similar extended longevity as humans (i.e., a similar LQ; see Fig. 9) include bats (43) and
the subterranean naked mole rat (44). It is tantalizing to
assess common features that may contribute to their exceptional longevity. Shared physiological, biochemical,
and cellular traits in these species may reveal important
components of slow aging and therefore give insight into
human aging and our maximum life span.
Naked mole rats are mouse sized and are the longest
living rodents known, with a recorded MLSP exceeding 28
yr (44). Naked mole rats have a low BMR for their size
(255), but the 30% reduction in BMR is not great enough to
explain their fivefold extended maximum longevity. In a
series of recent studies, several variables that are considered key tenets of the oxidative stress theory of aging
have been compared between naked mole rats and similar-sized mice. Surprisingly, a large amount of the data
obtained to date provide little support for a diminished
level of oxidative damage in these long-living rodents.
Although naked mole rats have low BMR, rates of hydrogen peroxide production are similar in heart mitochondria from both species (190a) and in vascular endothelial
cells (187); no differences in overall antioxidant activities
are evident (3), and levels of accrued oxidative damage to
proteins, DNA, and lipids are greater in the longer-living
species (4). One important difference between these two
similar-sized rodent species appeared to be the resistance
of vascular cells and fibroblasts to oxidative insults and
heat stress (187). While mice cells readily underwent
apoptotic cell death, in naked mole rat blood vessels only
the highest doses of H2O2 induced apoptotic cell death.
These data suggest increased life span potential is associated with an increased resistance to proapoptotic stimuli and oxidative insults. Cell membrane properties may
play an important role in this regard.
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FIG. 10. A comparison of the distribution of maximum life span
potential (MLSP) in 5 vertebrate classes. Values are means ⫾ SD; range
and number (n) of MLSP values are also presented for each vertebrate
class. All data are from Carey and Judge (50), and only the maximum
value given for each species was used in the analysis. In each graph,
values on the y-axis represent number of species.
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HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
IV. BIRDS
A. Metabolic Rate and Body Size of Birds
It was 20 years after the publication of the famous
“mouse to elephant” curve by Benedict (23), which
showed that mammalian BMR was allometrically related
to body mass, before the comparable “hummingbird to
ostrich” curve was produced (196). The latter study,
Physiol Rev • VOL
which demonstrated that BMR of birds was also allometrically related to body mass, responded to the opinion (174)
that small birds, mainly songbirds, had relatively higher
metabolic rates than predicted from existing allometric
relations and, accordingly, produced two separate allometric relations: an elevated relationship for perching
birds (passerines) and a lower one for nonpasserine birds.
This separate treatment persisted in later allometric comparisons (e.g., Ref. 6), but was challenged because it was
biased by overrepresentation of closely related groups.
When an updated data set of avian BMR values was
corrected for phylogenetic bias (298), there was no statistically significant difference between passerine and
nonpasserine BMR values, a result later confirmed (228).
There are a number of allometric comparisons of mammalian and avian metabolic rates, but a robust interpretation of these is that the scaling of mass-specific BMR in
relation to body mass is indistinguishable between birds
and mammals (e.g., in Fig. 5A, mass-specific BMR is proportional to mass⫺0.34 for birds and to mass⫺0.31 for mammals), but that birds have BMRs that average ⬃1.5 times
greater than those of comparably sized mammals (Fig. 5A
and see Refs. 228, 330, 372). Therefore, if the rate-of-living
theory accurately described vertebrate life span potential,
birds would be expected to live only about two-thirds as
long as comparably sized mammals.
Birds have evolved endothermy, and its associated
high level of BMR, independently to mammals and, as a
test of the membrane-pacemaker theory of metabolism,
the body-size variation in metabolic rate has been recently
examined in bird species ranging in size from finches to
emus. As for mammals, membrane-associated processes
are significant components of respiration rate of avian
hepatocytes and, while the total respiration rate decreases with body mass, the relative contribution of mitochondrial ATP production, mitochondrial proton leak,
and nonmitochondrial processes is unchanged (91). As it
is in mammals, the proton leak of isolated liver mitochondria is allometrically related to body mass in birds (37).
The PUFA content of avian liver mitochondrial membranes is negatively related to body mass, and this was
predominantly due to decreased levels of n-6 PUFA as
birds increased in body mass (37). This differs from the
situation in mammals where it is n-3 PUFA variation that
is primarily responsible for body size trends in liver mitochondrial membrane composition (289). The functional
significance of this mammal-bird difference will be discussed later. The general conclusion from these studies is
that membrane-associated processes are similarly important in metabolic activity of birds, as they are in mammals,
and that the high mass-specific BMR of small avian species is associated with more polyunsaturated membranes,
as it is for mammals.
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When membrane fatty acid composition was measured in tissues from naked mole rats, they were found to
have very low levels of DHA in their tissue phospholipids
for their body size. Although both mice and naked mole
rats have similar levels of total unsaturated fatty acids in
their tissue phospholipids, the low DHA levels in naked
mole rats result in more peroxidation-resistant membranes. The PI values calculated for both skeletal muscle
and liver mitochondria show that the peroxidation susceptibility of the membranes of naked mole rats is what
one would predict for such a long-living rodent species
(161; see Fig. 7).
Similarly, the low PI of human membrane phospholipids may be an important trait for the relatively long
MLSP of Homo sapiens (275). The relationship between
human longevity, membrane composition, and dietary
DHA in humans may be complex and is still to be fully
elucidated. For example, humans with severe coronary
atherosclerosis have erythrocyte membranes with more
PUFAs and particularly more DHA (28). Centenarians
show a reduced susceptibility to lipid peroxidation than
other human subjects, yet have higher levels of DHA (295,
329), which is associated with a decreased incidence of
metabolic syndrome and cardiovascular disease and superior insulin sensitivity (21). These data suggest that
membrane composition may possibly be involved in the
extended longevity of humans, yet the precise picture is
still emerging. In this respect it may be that the DHA
content of different subcellular membranes is important.
For example, it may be excluded from mitochondrial
membranes of cells to minimize lipid peroxidation, but be
necessary in plasma membranes for normal insulin sensitivity. In this respect it is of interest that the molar ratio of
ethane exhalation to oxygen consumption for humans is
very low compared with the other mammals for which
there are data (see Fig. 8). As ethane is one of the end
products of the peroxidation of n-3 PUFA (124) and the
rate of ethane exhalation is used as an indicator of the
in vivo rate of lipid peroxidation (178), this may indicate
that relative to our metabolic rate that humans have a low
rate of lipid peroxidation. This is also consistent with the
low peroxidation susceptibility calculated for our membranes (see Fig. 7) and may be an important factor in why
we are such exceptionally long-living mammals.
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
B. Maximum Life Span, Body Size, and Lifetime
Energy Expenditure of Birds
Physiol Rev • VOL
power concerning the maximum longevity of bird species, this power is limited and suggests that other factor(s) are involved in the determination of MLSP of
avian species.
As in mammals, the steeper relationship between
BMR and body mass of birds than between their MLSP
and body mass (Fig. 5, A and B) means there is a negative
relationship between calculated LEE of birds and body
mass (see Fig. 5C). The slope of the relationship between
LEE and body mass means that for every doubling of
mass there is, on average, a 9% decrease in LEE. This also
is contradictory to a strict interpretation of the rate-ofliving theory. The combination of a relatively higher metabolic rate and a longer life span means that birds, on
average, will have about a threefold greater LEE than
mammals.
As in mammals, there is 1) a very large (32-fold)
amount of variation in the calculated LEE among birds
and 2) when field metabolic rate instead of BMR was used
to calculate LEE, there was a slight negative relationship
with body size in birds, but unlike the situation in mammals, it failed to reach statistical significance (330).
C. Insights From Bird-Mammal Comparisons
About 15 years after the disparity in MLSP between
birds and mammals was initially highlighted (204), two
separate research groups (the Sohal group in the United
States and the Barja group in Spain) compared mitochondrial ROS production and antioxidant levels in short-lived
rats (MLSP ⬃3 yr) and long-lived pigeons (MLSP ⬃35 yr).
Ironically, another laboratory had examined H2O2 production in rat and pigeon mitochondrial preparations ⬃20 yr
before this time, but not in a comparative context (33,
211). The 10-fold greater longevity of pigeons was suggested to be mostly due to a lower rate of mitochondrial
H2O2 and superoxide formation compared with rats (19,
183). One group suggested a slightly greater level of antioxidant defense (183) while the other found antioxidants
to either not differ significantly between these animals or
to be inversely related to MLSP (19, 210; Table 3). Importantly for reconciling the ability of birds to have both a
higher MLSP and a higher BMR than mammals, the rate
of mitochondrial ROS production per unit of oxygen
consumed was considerably lower in pigeons than in
rats (19). The notion that MLSP is inversely related to
rate of mitochondrial ROS production gained further
support from the finding that heart mitochondrial H2O2
production in canaries (MLSP ⬃24 yr) and budgerigars
(MLSP ⬃21 yr) was also significantly lower than in mice
(MLSP ⬃3.5 yr) (137).
In addition to low rates of mitochondrial ROS formation, these bird species were shown to also have a membrane fatty acid composition that is considerably more
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Estimates of MLSP for many bird species suffer similar constraints to those for mammals. There is a scarcity
of longevity data for most free-living species, and even
when available, the extent to which extrinsic mortality
factors might disguise the actual MLSP for the species is
often unknown because in the wild many individuals die
from external causes and not from endogenous aging
processes. This is sometimes overcome by basing MLSP
on survivorship data for captive animals, but this assumes
that diets and caging conditions are conducive to realization of that species’ maximum life span. Although it is
very likely that zookeepers were aware of the relative
differences in longevities of different types and sizes of
animals (e.g., Ref. 101), the first formal characterization of
avian MLSP in relation to size was performed by Lindstedt
and Calder (204). Their analysis was prompted by the
concept of “physiological time” (141), which noted that
because metabolic rate and longevity covaried with body
size, physiological time itself might be similarly scaled
with body size. Based on the limited data then available,
these authors showed that longevity of birds and mammals increased similarly with increased body size, but
that birds lived up to four times longer than comparably
sized mammals (204). A more recent characterization of
bird longevity using a larger set of data (see Fig. 5B)
confirms the common scaling of longevity with body size
in birds and mammals (i.e., MLSP is proportional to
mass0.20 in birds and mass0.22 in mammals), but reveals
that birds live, on average, twice as long as similar-sized
mammals (Fig. 5B and Ref. 330). Body mass is a better
predictor of both BMR and MLSP in birds than it is in
mammals. For example, body mass can explain 84% of the
variation in BMR in birds, compared with 64% in mammals, and it can explain 48% of MLSP variation in birds,
compared with 35% in mammals (see Fig. 5, A and B).
An important prediction of the rate-of-living theory is
that the maximum longevity of an organism should be
predicted by its mass-specific metabolic rate. Thus when
the MLSP values for 108 species of birds (Fig. 5B) are
plotted against their respective BMR (Fig. 5A), there is
indeed an inverse relationship between the MLSP and
BMR of birds (Fig. 6). Although this relationship is statistically significant, the BMR of a bird species is a less
reliable predictor of its MLSP than is its body mass. That
is, whereas variation in BMR can explain 41% of variation
in MLSP of birds (see Fig. 6), body mass can explain 48%
of variation in MLSP and 84% of BMR variation (see Fig. 5,
B and A). As well, the relationship between BMR and
MLSP of birds is different from the relationship for mammals. This supports the conclusion reached following the
analysis of the equivalent data set for mammals, namely,
while the rate of living does have some explanatory
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HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
D. Insights From Comparison Among Birds
As with mammals, considerable insight into the
mechanisms of aging will likely emerge from comparison
of similar-sized birds that differ dramatically in maximum
life span. The variation in MLSP within birds of similar
size dwarfs the average twofold difference in the MLSP
between birds and mammals (see Fig. 5B). As with mammals, there are particular groups among birds that are
either longer living or shorter living than the MLSP predicted from the general avian equation for their mass. As
in mammals, these differences can be expressed by their
LQ values (i.e., ratio of actual MLSP to predicted MLSP
from body mass). Two such families of birds, the Procellariiformes (petrels and albatrosses) and the Galliformes
(pheasants and fowl), are highlighted in Figure 9 as examples. Species from both families cover a substantial
and overlapping size range. The selected Procellariiformes range from the 45-g Leach’s petrel to the ⬃12-kg
wandering albatross, while the Galliformes range from
Physiol Rev • VOL
the 45-g little button quail to the ⬃11-kg wild turkey. The
average LQ for the Procellariformes (labeled albatrosses
in Fig. 9) are 1.6 ⫾ 0.2 (SE, n ⫽ 11) while the average LQ
for Galliformes (labeled as fowl in Fig. 9) is 0.5 ⫾ 0.12
(SE, n ⫽ 8). The two families have an average threefold
difference in maximum life spans, but have indistinguishable BMRs. Thus mass-specific BMR does not explain
these longevity differences.
The substantial difference in MLSP between these
two families might be related to their very different lifehistory traits. The Galliformes breed early and produce
large clutches with a short incubation period, whereas
the Procellariiformes commence breeding much later and
have a very small clutch size with an extended incubation
period (361). Experimental studies have demonstrated
that both early breeding and high fecundity reduce longevity of kestrels and goshawks (74, 182), but these traits
foster higher rates of annual reproduction and are, therefore, beneficial to species such as Galliformes that have
high extrinsic rates of mortality. The tendency of MLSP to
increase directly with size in mammals and birds may also
be influenced by size-related differences in extrinsic mortality rates (299), due in part to larger animals being less
prone to predation and coping better with periods of food
deprivation than smaller forms (7, 46).
Regardless of the factors ultimately responsible for
MLSP variation, there are two traits that are often associated with long-lived species: reduced rates of mitochondrial free radical production and reduced susceptibility of
membranes to lipoxidation. The differential rates of mitochondrial H2O2 formation in pigeons compared with
rats are especially pronounced when fatty acids are used
as a metabolic substrate (342), which, in turn, may be due
to a greater extent of upregulation of uncoupling proteins
(UCPs) in the pigeon mitochondria. This interpretation is
consistent with observations that fatty acids stimulate
UCP2 and UCP3 expression (47, 309) and that avian muscle mitochondria show increased uncoupled respiration
following exposure to exogenous superoxides (348). If
the different MLSP of birds with different life histories is
partly due to variation in mitochondrial free radical production, then comparative studies focusing on UCP expression and function in birds, particularly the potential
for superoxides to stimulate UCP2 and UCP3 expression
(38), might yield significant insights (70).
The other factor coinciding with an extended MLSP
is having peroxidation-resistant membranes (16, 154,
162). This attribute would complement reductions in mitochondrial free radical production, but is essential for
protecting against all sources of oxidative stress. In this
regard, it is important to recognize that the intensity of
immune reactions, which are a potent source of free
radicals, increases directly with MLSP in birds (352). Consequently, one would predict that membrane fatty acid
composition favoring peroxidation resistance to be a
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resistant to peroxidation than equivalent membranes
from the mammals and also to have low levels of lipid
peroxidation products in their tissues compared with
mammals (275–277). That the mammal-bird difference in
membrane fatty acid composition was an important determinant of this low level of lipid peroxidation in bird
tissues was demonstrated by the dramatic reduction in
MDA produced by avian mitochondria compared with
mammalian mitochondria when both were subjected to
an in vitro free radical generation system (276, 277). Measurement of the fatty acid composition of tissues from
eight bird species, ranging from 14 g zebra finches to 34 kg
emus, shows that, as in mammals, small birds have more
polyunsaturated membranes than larger species (37, 162)
and also confirms a lower level of membrane polyunsaturation in birds compared with mammals. A common finding is that bird phospholipids (and thus membranes),
compared with mammal phospholipids, have a PUFA balance that generally favors n-6 PUFA over n-3 PUFA. This
difference results in bird membranes having a lower PI
than mammalian membranes. They are thus less susceptible to peroxidative damage.
When the PI of skeletal muscle and liver mitochondrial membranes of bird species are plotted against their
MLSP (see Fig. 7), they follow the same relationship as
observed for mammals. This is an intriguing finding and
suggests that membrane fatty acid composition differences between birds and mammals are part of the explanation for the relatively greater longevity of birds compared with similar-sized mammals. Such bird-mammal
differences in membrane composition have also been related to differences in lipoxidative damage to proteins in
the tissues of birds and mammals (138, 292).
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
V. ECTOTHERMIC VERTEBRATES AND
INVERTEBRATES
In this section we consider the relatively limited information available for nonendothermic species, first the
ectothermic vertebrates and then the invertebrates. Maximum longevity is positively correlated with body size in
reptiles, amphibians, and fish (29) as it is for mammals
and birds. The equation relating the maximum life span of
reptiles to their body mass (kg) is very similar to the
equation for mammals (for reptiles MLSP ⫽ 14.6mass0.23
compared with 11.6mass0.20 for mammals, see Table 6.1 of
Ref. 49). In addition, the mass-specific rate of metabolism
is also negatively related to body mass in reptiles, amphibians, and fish, with the slope of this allometric relationship being essentially the same as for mammals and
birds (134). However, while mammals and birds are endotherms and therefore maintain a relatively constant
body temperature throughout their individual lives, this is
not the case for reptiles, amphibians, and fish. In the wild,
ectothermic vertebrates will have variable, environmentally determined body temperatures throughout their individual lives.
This complication highlights the different manners
by which species-specific data for 1) maximum longevity
and 2) metabolic rates are obtained. It thus precludes the
calculation of lifetime mass-specific metabolism for reptilian, amphibian, or fish species in the same manner as it
has been done for mammals and birds. Mass-specific metabolic rate of ectotherms is a laboratory measurement
obtained at a body temperature determined by the experimenter. Values for maximum longevity, although relatively species specific, are a more heterogeneous data set,
generally obtained under a wide range of conditions and
in the case of ectothermic vertebrates under unknown
Physiol Rev • VOL
average body temperatures. Maximum longevity values
for a particular species are often obtained opportunistically from captive animals or from marked individuals
recaptured in the wild and may not always be reliable
maximum values.
The distribution of MLSP values for ectothermic vertebrates (reptiles, amphibians, and fish) and endothermic
vertebrates (mammals and birds) together with mean and
standard deviation of MLSP values for each of the vertebrate classes are presented in Figure 10. This figure does
not take into account size differences between the different species but does indicate that there is considerable
overlap between the different vertebrate classes in their
MLSP. The mean MLSP values range from 8.3 yr for
amphibians to 17.5 yr for mammals. While humans have
the highest MLSP record for mammals, there are some
ectothermic vertebrates (fish and reptile) with longer
MLSP values.
The level of resting metabolism in mammals and
birds is 5–10 times that of similar-sized ectothermic vertebrates at the same body temperature (see Ref. 152).
Thus, if the rate of living is a significant determinant of a
species life span, we would expect that the maximum
longevities of ectothermic vertebrates should, on average,
be an order of magnitude greater than that of similar-sized
endothermic vertebrates at the same body temperature,
and even greater if their lifetime average body temperature was lower than the body temperatures typical of
mammals and birds (which is almost always the case). As
can be seen from Figure 10, this is not the case and is thus
an additional argument against a determinate role for the
rate-of-living theory on maximum life span of vertebrates.
Comparative studies of tissue antioxidant defenses that
include values for amphibians and fish show them to be
similar to those of mammal and bird species (18, 285).
The effect of body temperature on aging of vertebrates was examined by Walford and Liu in a series of
elegant studies on fish (see Ref. 378). These studies questioned whether the temperature effect is due to the simple
“slowing down” or “speeding up” of general metabolic
activity. For example, lowered temperature increased life
span but at the same time increased growth rate of fish
(368). More intriguingly, these authors later showed that
fish kept at a low temperature during the last half of their
life lived longer than fish kept at the same low temperature throughout their whole life span and also lived longer
than fish kept at the low temperature for only the first half
of their life (205). These observations raise questions
concerning the mechanisms by which low temperature
extends life span in fish and argue against their mediation
simply being due to decreased rate of living.
Invertebrate species are also ectothermic. However,
unlike most ectothermic vertebrates, their maximum longevity values have generally been determined in the laboratory under conditions of constant specified body tem-
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strongly selected attribute in long-lived species. Comparison of the membrane acyl composition in birds with
particularly extended MLSP (e.g., petrels, hornbills, parrots) to those of birds with relatively short MLSP (e.g.,
gallinaceous species) is an obvious test. Preliminary measurements (W. A. Buttemer, H. Battam, and A. J. Hulbert,
unpublished results) of skeletal muscles of petrels and
albatrosses (taken as bycatch during long-line fishing operations) show them to have membranes with ⬃30% more
n-6 than n-3 PUFA, and PI values indicative of animals
with MLSP ⬎30 yr. In contrast, their adipose tissue triglycerides were dominated by n-3 fatty acids (5 times
more n-3 than n-6 PUFA). This dominance of n-3 PUFA in
storage fats is not surprising in view of their fish diet, but
the relative absence of n-3 PUFA in their membranes
highlights the control of membrane composition as a
potentially important mechanism in the determination of
a species maximum longevity.
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Physiol Rev • VOL
rate and individual life span (157). Within a population of
blowflies, there was also no correlation between individual rates of food consumption and individual life
span (166).
The role of metabolic rate in the determination of
adult life span of insects has also been examined by
manipulation of flight activity. Unlike healthy rats and
humans, where the increased metabolism associated with
exercise does not shorten life span (e.g., Refs. 147, 200)
but often has beneficial effects sometimes even increasing
average longevity (e.g., Ref. 145), a number of studies
have shown that this is not the case for insects. Both
houseflies and fruit flies allowed to fly have much shorter
longevities than those prevented from flight (215, 322). In
both species, flight activity is associated with greater
oxidative damage to some proteins. Interestingly,
whereas flight activity is associated with greater mitochondrial H2O2 production in houseflies (376), there is no
effect of activity on H2O2 production in fruit flies (215). In
the Drosophila, there were changes in fatty acid composition such that the peroxidation index of lipids increased
with age and flight activity and was associated with an
increased susceptibility to lipid peroxidation (215). The
fatty acid composition of phospholipids from the heads of
wild-type Drosophila shows them to have short-chain
PUFA but lack long-chain PUFA (338, 379). Although
there are considerable data for fatty acid composition of
invertebrates in general, they are almost never presented
in relationship to aging or maximum life span of the
particular species.
The social insects have been suggested as particularly good model organisms to investigate the mechanisms of aging for two reasons: 1) “queens” can be extraordinarily long living, and 2) there is sometimes tremendous variation in life span between genetically
identical “queens” and “workers” (171). In some ant species, queens can live up to 30 yr and frequently live 10
times longer than workers (144). Similarly, queen honeybees are reported to have a maximum longevity an order
of magnitude greater than worker (i.e., nonreproductive
female) honeybees (373). Enhanced antioxidant defenses
in queens are not a likely explanation, as one study has
shown queen ants have a reduced expression of Cu/ZnSOD compared with shorter-living workers (282) while
others have shown that queen honeybees generally have
the same (or, in some cases, lower) levels of antioxidant
defenses than worker honeybees (68, 370). The massspecific metabolic rates of worker and queen honeybees
are essentially the same (96). Recent measurements show
that queen and worker honeybees have different membrane fatty acid composition and that the peroxidation
index of phospholipids of from queen honeybees is ⬃33%
of that of workers (122). If the slope of the relationship
between PI and MLSP is mathematically similar in honeybees to that described for mammals and birds (155, see
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perature. Two invertebrates have been especially important in elucidating the mechanisms determining life span.
They are the fruit fly Drosophila melanogaster and the
nematode Caenorhabditis elegans. Loeb and Northrop
(207) first demonstrated that reducing the temperature
increased the life span of Drosophila. This finding was
important evidence in the development of the rate-ofliving theory of aging when Pearl (284) argued that it was
mediated by the general influence of temperature on metabolic rate.
Lowered temperature has been shown to extend life
span of the fruit fly (236), the housefly (323), and the
milkweed bug (223). All three studies also showed that
lowered temperature decreased the rate of oxygen consumption by these insect species, and the calculated lifetime oxygen consumption was relatively similar at the
different experimental temperatures. Although this is
compatible with the rate-of-living theory, it is not proof of
the theory as temperature is known to affect the rate of
many processes (as well as other variables such as cellular composition), and it may be that longevity extension is
due to one (or more) of these other effects of temperature. Interestingly, repeated short periods of mild heat
stress have also been shown to extend life span of Drosophila (135).
In the housefly, the in vivo rate of lipid peroxidation
of n-6 PUFA has been shown to increase with age when
houseflies are kept at 25°C and to be reduced when
houseflies are kept at 18°C (325). Whether this lower n-6
PUFA peroxidation is due to a decrease in the rate of
oxygen consumption or change in membrane composition (or both) is unknown. It is of interest that the relative
n-6 PUFA content increases with age in houseflies at 25°C
and decreases when flies are maintained at 18°C. In both
cases, these n-6 PUFA changes are compensated by opposite changes in the content of peroxidation-resistant
MUFA (323). In Drosophila, there is no increase in metabolic rate with age (5, 157, 242, 303), and similarly in the
blowfly Calliphora stygia, there was no increase in food
consumption with age (166). Rather than an increase,
some of these studies actually report a drop in metabolic
rate at the end of the adult life span of these insects.
Fruit flies that differ in life span have also been used
to examine the role of metabolic rate in the determination
of life span. There is no association between metabolic
rate and life span in five species of Drosophila (293).
Similarly, laboratory-produced strains of Drosophila
melanogaster that differ substantially in life span do not
differ in metabolic rate (5, 242, 364). Furthermore, the
long-living lines of Drosophila had essentially the same
antioxidant enzyme activities as the short-living fruit-fly
lines (242). Thus neither the rate-of-living nor antioxidant
defenses appear to explain longevity differences in these
Drosophila lines. Similarly, within a Drosophila population, there is no correlation between individual metabolic
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
VI. TREATMENTS TO MANIPULATE LIFE SPAN
A. Calorie Restriction
Dietary calorie restriction (CR) is a long-known treatment that can extend life span and was first described for
rats (224). It has been shown to extend longevity in a wide
range of other animal species and is probably the most
investigated treatment in the study of aging (for an excellent review, see Ref. 219). In rats and mice, the degree of
life span extension is linearly related to both the degree of
CR and the time for which CR is imposed (231). CR is
proposed to extend life span by both affecting the intrinsic rate of aging as well as suppressing pathogenesis and
thus reducing the short-term risk of death (381). There is
some argument as to whether CR slows the rate of aging
Physiol Rev • VOL
or whether it delays the start of aging but does not slow
its rate (see Ref. 220).
In line with the rate-of-living theory of aging, many
investigators initially thought that CR extended life span
by lowering metabolic rate. While rats subjected to CR
had a low BMR per whole animal compared with controls,
CR also results in a reduced body size, and it was shown
over twenty years ago that the CR in rats does not reduce
their mass-specific metabolic rate, expressed relative to
either total body mass or lean body mass (225). Indeed, it
has recently been shown that CR in rats results in a
metabolic rate higher than would be predicted from the
altered body composition (319). Similarly CR does not
reduce metabolic rate in either mice (99) or Drosophila
(157).
A reduction in oxidative damage is a consistent finding following CR and, although the reason for this reduced oxidative damage is not completely understood, it
is thought to be the cause of its ability to extend longevity
(112, 219, 231). Measurement of mitochondrial in vitro
production of superoxide and hydrogen peroxide (generally referred to as ROS production) shows it to be lowered
following long-term CR (112), but the results are more
equivocal following short-term CR. In mice there is a
small effect on isolated liver and muscle mitochondria
following 6 mo CR but none after 3 mo (99). In vitro ROS
production is reduced in rat heart mitochondria following
1-yr CR but not after 6-wk CR (113). It is reduced in rat
liver mitochondria after 6-wk CR (114), while others report no effect after 1-, 6-, or 12-mo CR (123, 296). It is
reduced in rat muscle mitochondria after 2-wk and 2-mo
CR (26); however, another study reports no change after
2-mo CR (115). All of these studies have measured in vitro
ROS production by isolated mitochondria, and it is of
interest that a recent report shows no effect of 4-mo CR
on cellular ROS production by rat hepatocytes (194).
A reduction in mitochondrial production of superoxide or hydrogen peroxide is not the only means to limit
oxidative damage. Two other means are 1) enhanced
antioxidant defenses and 2) diminished lipid peroxidation. The effect of CR on antioxidant defenses is mixed.
Some studies report that CR enhances antioxidant defenses, whilst others report either no changes or diminished defenses. There does not appear to be a consistent
effect of CR on antioxidants, and the reader is referred
elsewhere for details (219). As pointed out earlier, unlike
oxidative damage to nucleic acids and proteins, oxidative
damage to polyunsaturated lipids produces many powerful reactive molecules that can further damage other important biological molecules. Thus lipid peroxidation can
be both the manifestation of, as well as the cause of,
oxidative damage. In vivo lipid peroxidation is decreased
during CR in rats, with one study reporting a decreased
pentane exhalation (indicator of n-6 PUFA peroxidation)
but no decrease in ethane exhalation (indicator of n-3
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Fig. 7), then this difference is capable of explaining the
order of magnitude difference in longevity between queen
and worker bees. It may be that the dramatic longevity
differences between castes of social insects are mediated
by different fatty acid composition of cellular membranes
of these insects.
The nematode C. elegans has been a popular model
organism in the study of aging. Both its short adult life
span and its relatively simple genome have been important aspects of this experimental popularity. However,
while C. elegans has given much insight into the genes
that influence aging and the determination of life span, its
small size has limited its ability to give insight into the role
of metabolic rate in aging. Metabolic rate has been measured both by respirometry and calorimetry in C. elegans,
and both methods show an unusual pattern of changing
metabolism during the nematode’s lifetime. Unlike the
situation in higher multicellular animals, where metabolic
rate is relatively constant throughout adulthood, there is a
dramatic and substantial continual decline in metabolic
rate throughout the adult life span of C. elegans (34). In
addition, the metabolic rate of C. elegans is very labile
and dependent on environmental conditions (362). There
is no agreement as to whether metabolic rate of control
and long-living mutant strains of C. elegans differ (35, 363,
365). To our knowledge, there are no examinations as to
whether membrane fatty acid composition differs between strains of C. elegans that differ in longevity. One
interesting recent study reports that genetic manipulation
to increase the expression of glutathione transferases
(enzymes that metabolize 4-HNE, an important and damaging product from peroxidation of n-6 PUFA) increase
both the resistance to stress and the longevity of C.
elegans(10). This study showed a fascinating and strong
positive correlation between glutathione transferase
activity for 4-HNE and median life span of lines of
C. elegans.
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creases the degree of polyunsaturation of n-6 PUFA in
membranes (153), which is similar to the changes observed during CR.
B. Antioxidant Defenses
A number of techniques have been used in attempts
to influence the maximum life span of animals by manipulation of antioxidant defenses. These include dietary
supplementation, pharmacological intervention, and
more recently transgenic techniques. The antioxidants
examined include both those naturally produced by animals as well as novel antioxidant defenses. The effects of
increased antioxidant levels, either by dietary or pharmacological means, on both the mean and maximum longevity of various vertebrate animals are summarized in Table
3. As can be seen from Table 3, there was generally no
effect on maximal life span, although in some studies
there was an increase in average longevity. This is also
consistent with studies in invertebrates (198, 261). The
extension of maximum life span by SOD mimetic drugs
described a few years ago for C. elegans (230) has been
unable to be repeated by others either for C. elegans (170)
or Drosophila (214).
Antioxidant defenses have also been modified by
transgenic means in a number of studies (see Table 4).
While maximum life span has been decreased in a number
of studies following the complete removal of particular
antioxidant defense (e.g., in homozygous knockouts), the
overexpression of antioxidant defenses has essentially
had no effect on maximum longevity. It is of interest that
heterozygous knockouts (i.e., resulting in a 50% reduction
in antioxidant levels) have often had no influence on
maximum life span. The results from the transgenic studies (Table 4) are in agreement with the other studies
(Table 3). They indicate that 1) while the complete ablation of specific antioxidant defenses can be detrimental
and 2) enhanced antioxidant defenses might improve the
health of some individuals in a population (and thus increase average longevity), that antioxidant defenses do
not appear to be limiting and do not appear to slow down
the endogenous aging process. Consequently, they do not
appear to be responsible for determining the maximum
life span distinctive for individual species.
Overall, studies that experimentally increase tissue
antioxidant levels do not increase maximum life span in
vertebrates (312). While for most dietary studies the additional antioxidants do not change MLSP, a few show
marginal increases in longevity. However, these increases
have only been observed in investigations in which the
MLSP of the control animals was less than 3 yr, which is
a short life span for the examined rodents. In studies in
which the maximum life span of the control rodents was
greater than 3 yr, there was no effect on MLSP. The most
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PUFA peroxidation) following 12-mo CR (222) while another study reports decreased ethane exhalation following 2-wk CR in rats (121).
A decreased rate of lipid peroxidation can come
about via a decrease in the availability of peroxidizable
fatty acids, and it has been proposed that such membrane
alteration is the basis of the protective effect of CR (382).
Laganiere and Yu (188) first showed CR altered the fatty
acid composition of liver mitochondrial and microsomal
membranes in rats such that they became less susceptible
to peroxidative damage. Since this seminal observation,
others have reported similar CR-induced changes for
other tissues (53, 190, 201, 268, 270, 346) as well as for
different classes of liver phospholipids (54, 168). All of
these studies involved long-term CR in rats with the shortest period examined being 10-wk CR (54). A study of CR
in mice (99) reports similar changes in the fatty acid
composition of phospholipids from liver, heart, kidney,
and brain, as well as liver and muscle mitochondria, with
the changes in this study being manifest following 1 mo of
CR (the earliest period sampled). Recently, it has been
found that CR treatments as low as 8.5 and 25% result in
a decreased PI of membrane lipids and lipoxidative damage to proteins (Pamplona, unpublished results). The effect was especially pronounced when specific components of the diet (protein and methionine) were restricted
(9, 313).
A few studies have failed to show significant CRinduced changes in mitochondrial fatty acid composition
(192, 270, 296); however, these studies have measured
total lipids rather than phospholipids, and thus their values include the fatty acid composition of triglycerides
associated with the mitochondrial pellet. In view of the
fact that CR will have significant influence on the triglyceride content of cells, and triglycerides generally differ in
their fatty acid composition compared with phospholipids, it has been suggested (99) that CR effects on membrane composition may have been masked in these
studies.
Rapid and reversible effects of CR have been reported for mice (75, 334) and fruit flies (216). The most
rapid responses to changes in energy intake will be hormonal systems (e.g., insulin). In mice, CR-induced
changes in membrane fatty acid composition are early
effects and are likely mediated by hormones. Insulin,
triiodothyronine, and growth hormone are candidates as
they are all rapidly and significantly lowered by CR (219).
For example, low insulin levels (due to diabetes) are
associated with membrane fatty acid changes similar to
those observed following CR (126, 131, 227). Insulin and
growth hormone have also been shown to change membrane fatty acid composition, both during ad libitum feeding and during CR in rats (311). Insulin has recently been
shown to reverse some CR-induced changes in liver mitochondrial function in rats (193). Hypothyroidism de-
METABOLIC RATE, MEMBRANE COMPOSITION, AND LIFE SPAN
C. Genetic Manipulations
Other genetic manipulations (apart from the antioxidant-related genes mentioned above) have also been
shown to increase the maximum life span of some species. Discussion of these is limited in the current contribution, but one type deserves special mention. Mutations
that diminish signaling in insulin/IGF pathway have been
shown to extend life span in C. elegans (103, 172), Drosophila (62, 350), and mice (30, 149).
The role of diminished IGF in extending longevity is
interesting and likely explains the opposite relationship
between body size and maximum life span observed
within a mammalian species compared with between species. As outlined in earlier sections, a long life span is
often associated with a large body size among vertebrate
species. However, within a species, extended longevity is
often associated with a small body size. For example,
long-living mice are often small and have low IGF-I levels
(234). Similarly, the smaller dog breeds often have longer
life spans than large breeds of dog (203), and smaller dog
breeds have the same growth hormone secretory capacity
but reduced IGF compared with large breeds (89).
Physiol Rev • VOL
To our knowledge, nothing is known as to whether
membrane fatty acid composition is altered in any of
these mutant strains and thus is a good topic for further
investigation. It is known that both insulin and growth
hormone affect membrane fatty acid composition in rats
(311). The suggestion that diminished insulin secretion
may be responsible for mediating some of the effects of
CR (see sect. VIA) provides a common thread between
these two methods of extending life span. In this connection it is also of interest that the longest mean life span
achieved by CR chico1 mutant flies (with their diminished
insulin/IGF pathway) is the same as that achieved by CR
in wild-type fruit flies (63), which suggests that the longliving mutant flies are less sensitive to food concentration
and both this genetic mutation and CR may extend life
span by the same mechanism.
VII. CONCLUSIONS: SO, WHAT DETERMINES A
SPECIES MAXIMUM LIFE SPAN?
The fact that maximum life span is a species characteristic suggests that it has a genetic basis, but the specific
biochemistry that is being controlled by such genes is not
always obvious. The observation that large mammals tend
to live longer than small mammals was seminal in the
development of theories to explain aging and the determination of life span. The connection between this trend
in longevity and the slower metabolic rate of large compared with small mammals identified aerobic metabolism
as being central for understanding what determines a
species life span. This centrality was initially expressed in
the rate-of-living theory that, in turn, led to the development of the free-radical theory and consequently the oxidative-stress theory of aging, which is currently the most
accepted explanation of aging. We do not suggest that the
rate-of-living theory and the membrane-pacemaker modification of the oxidative-stress theory described here are
incompatible. Rather, we suggest that the rate-of-living
theory cannot alone explain all of the variation in longevity of animals. However, many of the exceptions that
cannot be explained by the rate-of-living theory (and are
summarized in sect. I) do appear capable of explanation
by knowledge of membrane fatty acid composition in
each particular case.
Although antioxidant defenses are an important part
of the oxidative-stress theory, it is now obvious that antioxidant defenses are not significantly involved in determining the maximum life span of a given species. Certainly diminution of antioxidant defenses can shorten life
span, but many attempts over the years to significantly
extend the maximum life span of species by manipulation
of antioxidant status have not been successful. Like most
other enzymes, enzymatic antioxidant defenses are likely
present in excess. If antioxidant defenses were limiting,
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frequent benefit observed was an increase in mean life
span. In most reports this effect was due to a decrease in
early death from a reduced incidence of disease. Antioxidant supplementation increased mean life span when
survival of the controls was suboptimal, whereas when
the survival of the control animals was more optimal, the
antioxidants were without effect. Antioxidants have not
increased maximum life span beyond the values of controls reared under optimal conditions. These results suggest that antioxidants can protect against increased oxidative stress from pathological situations or exogenous
damage but do not change aging rate. This is confirmed in
models in which transgenic mammals overexpressing different kinds of antioxidant enzymes show increased resistance to oxidative stress, but no increase in maximum
longevity.
Similarly, in the case of Drosophila, early studies
found increases in maximum longevity after combined
SOD plus catalase overexpression (but not individually);
however, this was not observed in later studies when
longer-lived control strains were used (259). It is possible
that overexpression of antioxidant enzymes is more important in short-lived very active insects than in mammals. In summary, enhanced antioxidant defenses seem
to increase life span in studies where the control group
has short life span (possibly because of suboptimal antioxidant defenses in the control animals). Antioxidant defenses seem to be optimized in vivo such that additional
expression is unable to increase protection against agingrelated oxidative damage, whereas they are useful when
oxidative damage is higher than normal.
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HULBERT, PAMPLONA, BUFFENSTEIN, AND BUTTEMER
Physiol Rev • VOL
to other cellular constituents. All these studies suggest
that variation in membrane fatty acid composition may be
an important missing piece of the jigsaw puzzle explaining
aging and the mechanisms that determine the maximum
life span specific for each species. It is a testable hypothesis that awaits further experiments. Since metabolic rate
has been called the “fire of life,” then maybe the mitochondrial production of superoxide from this fire can be
likened to the “pilot light” and lipid peroxidation (and its
reactive products) can be likened to the “furnace” of
oxidative stress.
ACKNOWLEDGMENTS
We especially thank Dr. Craig White who provided us with
access to his extensive database on the BMR of birds. We also
thank Zoe Hulbert Johnson for assistance. The editor and two
anonymous reviewers provided comments that resulted in an
improved manuscript.
This work is a combined contribution from the four speakers in the “Life and Death: Metabolic Rate and Life Span” symposium at the XXXV International Congress of Physiological
Sciences (San Diego, Mar-Apr 2005).
Address for reprint requests and other correspondence: A. J.
Hulbert, School of Biological Sciences, Univ. of Wollongong,
Wollongong, NSW 2522, Australia (e-mail: [email protected]).
GRANTS
The investigations from our respective laboratories described here have been supported by the following sources:
Australian Research Council grants (to A. J. Hulbert and W. A.
Buttemer); Grants FIS (98/0752, 00/0753), MEC (BFI200301287, BFU2006-14495/BFI), and Generalitat of Catalonia
(2001SGR00311, 2005SGR00101) (to R. Pamplona); and National
Institutes of Health Grants GM-08168-25 and AG-22891 (to R.
Buffenstein).
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