FACTORS AFFECTING TKE EFFICIENCY OF
ENERGY CONVERSION TO BODY TISSUE*
ROY J. MARTIN, JR.
The Pennsylvania S t a t e University
The information covered under t h e t i t l e of t h i s presentation i s
probably extensive enough t o be included i n s e v e r a l books and i s indeed
a subject f o r study i n some form a t most land grant colleges i n t h e
United S t a t e s . My objectives a r e t o present an overview of t h i s subject
i n areas where t h e p r i n c i p l e s of c a l o r i c efficiency a r e f a i r l y well
understood, and t o discuss i n more d e t a i l areas where information i s
scarce o r lacking. I n t h e l a t t e r case, I hope t o include a few ideas
which have not been rigorously investigated and t o propose p o t e n t i a l
areas f o r f u r t h e r investigation.
There a r e a number of ways of looking a t c a l o r i c efficiency. F i r s t ,
l e t us consider t h e sources of energy l o s s during t h e process of t i s s u e
growth. Figure 1 gives us t h e conventional scheme of energy flow from
t h e d i e t ( g r o s s energy) t o t h e u l t h a t e conversion t o energy stored (NE
production). The quantity of energy t h a t i s not u t i l i z e d f o r maintenance
and production represents energetic i n e f f i c i e n c i e s which can be influenced
by a number of f a c t o r s . This presentation w i l l include a summary of those
procedures that are u t i l i z e d t o reduce t h e i n e f f i c i e n c i e s encountered i n
t h e conversion of energy t o body t i s s u e s .
A.
Factors involved i n t h e c a l o r i c l o s s e s of digestion, urinary
excretion and gas production.
A comparison of t h e c a l o r i c losses associated with digestion
urinary excretion, and gas production i s given i n t a b l e 1 f o r
ruminants and simple-stomach animals. There i s a considerable l o s s
of energy associated with these processes i n t h e ruminant. A
discussion of t h e f a c t o r s involved i n a l t e r i n g energy losses by
these processes w i l l follow.
1. Diaestion.
Factors which influence t h e d i g e s t i b i l i t y of a r a t i o n
have been studied extensively f o r many years. I n general, those
d i e t s which contain s i g n i f i c a n t q u a n t i t i e s of f i b e r a r e l e s s
r e a d i l y digested. E f f o r t s t o maximize t h e conversion of d i e t a r y
energy t o t i s s u e energy have resulted i n t h e p r a c t i c e of feeding
d i e t s very low i n f i b e r t o both ruminants and simple-stomached
animals. By doing t h i s t h e advantage of the ruminant f o r
digesting roughages i s n u l l i f i e d . With t h e present economics
(increased feed p r i c e s ) t h i s trend may have t o be reversed s o
* Presented
a t the 26th Annual Reciprocal Meat Conference of the American
Meat Science Association, 1973.
-
GROSS ENERGY (GE)
$.
1
t
--FECAL
ENERGY
D I G E S T I B L E ENERGY (BE)
URINARY ENERGY
GASEOUS ENERGY
METABOLZABLE ENERGY (ME)
HEAT INCREMENT
NET ENERGY (NE)
MAINTENANCE
ENERGY
Figure 1 .
PRODUCTIVE
ENERGY
Convention scheme o f energy f l o w .
that t h e u t i l i z a t i o n of high-fiber d i e t s becomes a more important
source of nutrients f o r the ruminant (Baumgardt, 1967). I n t h i s
s i t u a t i o n , t h e chemical treatment of low q u a l i t y roughages t o
increase d i g e s t i b i l i t y may be an important economic consideration
(Stone et g . , 1966).
TABLE 1. E N W Y LOSSES INCURRED DURING DIGESTION
URINARY M C I I O N AND GAS PRODUCTION
Function
1. Digestion
Average energy loss
Ruminant
(5
of gross energy)
Non-ruminant
30
I2
4
2.
Urinary excretion
5
3.
Gas production
8
--
57
84
Energy available for
maintenance and
product ion
Another f a c t o r which causes considerable energy loss i n
ruminants is t h e rumen microbial action on r e a d i l y available
sources of d i e t a r y energy (starches and sugars). This f a c t was
demonstrated by Armstrong and Blaxter (1961). When they infused
glucose d i r e c t l y i n t o t h e rumen, the efficiency with which it
was u t i l i z e d f o r gain w a s 54%. Whereas, when glucose was infused
d i r e c t l y i n t o t h e abomasum, t h e ME as glucose was used w i t h an
efficiency of 72$, which is similar t o t h e efficiency observed
i n simple-stomached animals (Melanoski, 1966). I n addition, it
has been shown t h a t protein is u t i l i z e d more e f f i c i e n t l y by
placing it d i r e c t l y in the abomasum ( L i t t l e and Mitchell, 1967).
To counteract the inefficiencies associated w i t h microbial
fermentation various procedures have been u t i l i z e d t o protect the
concentrate ( o r g r a i n ) from microbial a t t a c k . Formaldehyde
treatment of grains r e s u l t s in a decrease i n microbialbreakdown
of soybean meal protein (Reis and Tunks, 1970) and an increase i n
nitrogen retention (Reis and Tunks, 1969). Other methods of
reducing t h e s o l u b i l i t y and decreasing microbialbreakdown of
protein include heat treatment (Sherrod and Tillman, 1962; Hudson
e t al., 1970) and tannic acid treatment (Driedger and Hatfield,
1970; Nishimuta e t al., 1973). A sumnary of some of the results
with these treatments is given in t a b l e 2.
--
TABLE 2 .
FACTORS AFFECTING THE EFFICIENCY
O F PROTEIN UTIIJ.ZATI0N
Treatment
(Reference )
Urinary
nitrogen
Nitrogen
retention
Abomasal administration
L i t t l e and Mitchell, 1967)
Sche l l i n g and H a t f i e Id, 1968)
[
decreased
NR
increased
incr eas ed
Heat Treatment
(Glimp et &., 1967)
(Nishimuta e t a l . , 1973)
NR
decreased
increased
increased
Formrrldeh e
h
k
s
, 1969)
(Nishirmrta e t a l . , 1973)
decreased
decreased
increased
Tanins
(Nish5nuta e t al., 1973)
(Dr iedger and Hat f i e Id, 1972 )
NSD
decreased
NSD
NSD
increased
NR = not reported
2.
G a s Production. Methane is one of t h e major end products of
rumen fermentation but is of no energy value t o t h e ruminant.
The energy cost of gas production i s normally insignificant in
simple-stomached animals but methane production i n ruminants can
amount t o 8%of t h e GE intake (Blaxter and Clapperton, 1965).
E f f o r t s t o reduce t h e energy l o s s of methane production by
i n h i b i t i n g methanogenic b a c t e r i a have led t o variable success.
The campound hemiacetal of c h l o r a l and s t a r c h ( c h l o r a l h y d r a t e )
i s a potent i n h i b i t o r of methane production (Thiel, 1969
When
fed t o growing Lambs t h e r e was a s i g n i f i c a n t increase i n average
d a i l y gain and feed e f f i c i e n c y (Trei, Scott and Parish, 1972).
However, i n s t e e r s c h l o r a l h y d r a t e was not e f f e c t i v e i n promoting
energetic e f f i c i e n c y (Van Leeuwen and Van Adrichem, 1968). I n
another study c h l o r a l hydrate was e f f e c t i v e i n i n h i b i t i n g methane
production from 50 t o 82$ and decreased t h e energy loss a s gases
by 2.7% of GE in Lambs (Johnson, 1972). Obviously more information
on t h i s compound w i t h a large number of animals i s needed t o
rigorously test the effectiveness of a l t e r i n g t h e energetic
e f f i c i e n c y of farm animals.
.
The energy l o s s through urinary excretion
is small i n cnmparison t o other sources of energy waste. Values
f o r ruminants average approximately 6s of GE and f o r simplestomach a~~imals
about 4% of GE (Brody, 1945). The f a c t o r s which
Fnfluence urinary energy loss include t h e l e v e l of d i e t a r y protein
(Hartsook and Hershberger, 1971), the l e v e l of food intake ( B l a x t e r
and Wainman, 1964), t h e type of roughage fed (Blaxter, 1962), and
t h e ambient t e r q e r a t u r e (Holmes, 1973). Manipulation of any of
these f a c t o r s apparently has a g r e a t e r e f f e c t on c a l o r i c e f f i c i e n c y
a t other steps i n t h e system of energy f l o w . For example, an
increase i n energy intake from 1 . 5 x maintenance t o 3 .O x maintenance
r e s u l t s i n a s i g n i f i c a n t increase i n ME e f f i c i e n c y f o r gain, but
only a slight decrease i n urinary energy l o s s (Kleiber, 1961).
3 . U r i n a r y Energy Losses.
B.
Factors a f f e c t i n g t h e u t i l i z a t i o n of metabolizable energy.
There a r e two major f a c t o r s which influence t h e e f f i c i e n c y of ME
u t i l i z a t i o n f o r growth. The f i r s t i s t h e r e l a t i o n s h i p between t h e
l e v e l of ME intake and t h e cost of maintenance. The second i s t h e
h e a t increment associated w i t h t i s s u e deposition.
1. Food intake and energetic e f f i c i e n c y . The e a r l y work by Brody
(1945)demonstrated t h e f a c t that t h e higher t h e ME intake over
maintenance cost t h e g r e a t e r t h e efficiency of conversion of ME
t o t i s s u e energy ( f i g u r e 2 ) . This p r i n c i p l e of increasing energy
e f f i c i e n c y is t h e main basis f o r a very a c t i v e area of research,
t h e regulation of food i n t a k e . Several recent reviews on t h i s
subject have appeared in t h e l i t e r a t u r e (Baumgardt, 1970; Baile,
1970; Baile, 1968; Anand, 1967) therefore, I w i l l only mention
some of t h e c u r r e n t approaches used t o investigate t h i s most
f a s c i n a t i n g area of research. The study of blood borne f a c t o r s
which c o n t r o l food intake i s s t i l l an approach needed t o i d e n t i f y
This approach
important s i g n a l s t h a t axe received i n t h e CNS
has been s i g n i f i c a n t l y aided by t h e development of radio-immunoassays
f o r minute q u a n t i t i e s of hormones i n t h e blood. Much of t h e
previous work was hampered by t h e lack of a r e l i a b l e means of
obtaining blood samples from undisturbed animals. “his problem
has been circumvented by a procedure developed by Wangsness e t a l .
(1972) i n which sampling can be achieved without any disturbance
t o t h e animal.
.
Another approach is t o study t h e areas of t h e brain and t h e i r
f’unction i n controlling food intake. Investigations by Baile and
coworkers (1970) have u t i l i z e d t h e procedure of placing cannulas
i n t h e b r a i n t o study t h e s e n s i t i v i t y of c e r t a i n areas of t h e
brain t o metabolites, drugs and hormones. “his approach allows
one t o c o r r e l a t e the information obtained from t h e blood borne
f a c t o r s and some physiological response i n t h e hypothalamic feeding
center.
3000
CALORIC
INTAKE
(Kcal)
(ADAPTED FROM BRODY, 1945)
HI
2000
NE (gain)
1000
0
MAINTENANCE
0.2
0.4
0.6
0.8
1.0
PLANE OF NUTRITION
F i g u r e 2.
Plane o f n u t r i t i o n and t h e
e f f i c i e n c y o f energy u t i l i z a t i o n .
I
Short term regulation of food intake may be regulated by
i n t e s t i n a l hormones released during food e n t r y i n t o t h e duodenum
(Smith, 1973). The study of short term food intake regulation
requires a d i f f e r e n t approach. Smith (1973)used an animal
prepared with a g a s t r i c f i s t u l a as shown i n f i g u r e 3 . By removing
t h e food as it e n t e r s t h e stomach, t h e animal i s not s a t i a t e d ,
therefore, other f a c t o r s can be investigated by e i t h e r i n j e c t i o n
i n t o t h e blood stream o r being placed d i r e c t l y i n t o t h e lower
digestive t r a c t .
Other approaches a r e being used t o study food intake
regulation. Those presented above probably represent t h e most
a c t i v e areas of research and some instances represent a novel
approach t o research. The importance of food intake i n increasing
t h e efficiency of energy conversion t o body gain has been emphasized
i n s e v e r a l reviews (Baumgardt, 1967; Byerly, 1967; ReLd and
T y r r e l l , 1964).
2.
A.
Heat increment and ME u t i l i z a t i o n . The c a l o r i c e f f i c i e n c y of ME
u t i l i z a t i o n above maintenance f o r growth have been reported t o
range from 40 t o 8 6 (Milligan, 1970). This would mean a
v a r i a t i o n i n energy loss through H l from 20 t o 6% (H1 = 100-ME).
Some of t h i s v a r i a t i o n may have been due t o experimental
inaccuracies, but t h e r e i s s u f f i c i e n t evidence t o indicate that
t h e e f f i c i e n c y of ME u t i l i z a t i o n i s affected by a number of
factors.
Dietary f a c t o r s . Various d i e t a r y components have been shown t o
influence heat increment ( t a b l e 3 ) . The lower heat increment
associated w i t h feeding f a t can be explained on t h e basis t h a t t h i s
n u t r i e n t can be stored in a form i n which it i s absorbed, with only
a few energy requiring modifications ( t r i g l y c e r i d e synthesis )
Protein on t h e other hand, requires a number of biochanicalmodifications that a r e e n e r g e t i c a l l y expensive (deamination, urea synthesis,
and protein s y n t h e s i s ) . Another f a c t o r which has not been f u l l y
.
TABLE 3 .
EFFECT OF DIET ON HlUT INCREMJ3NT
Diet
Fat
CHO
Protein
(From Blaxter, 1971)
IN SWINE
H1
(Kcal/l00 Kcal ME)
14
20
26
[ ,
,I
*
,
Figure 3 .
I
, , .
I
.
A study o f f a c t o r s r e g u l a t i n g food i n t a k e
w i t h r a t s prepared w i t h a g a s t r i c canulae
(Smith, 1973).
investigated i s t h e cost of molecular and ion transport (Milligan,
1971). I n t h e case of glucose and some amino acids, a c a v e t r a n s p o r t
(energy dependent) may be required. The proportion of rumen v o l a t i l e
f a t t y acids and t h e metabolic efficiency of ruminant fed concentrates
w i l l d i f f e r g r e a t l y from ruminants fed primarily roughage ( B l a x t e r
and Wainman, 1964). Armstrong and Blaxter (1957)proposed t h a t t h e
higber efficiency of energy u t i l i z a t i o n on concentrate r a t i o n was
due t o t h e decreased a c e t a t e production. Evidence f o r t h i s proposal
is given in f i g u r e 4. B u l l e t a l . (1970) have shown t h a t when
t r i a c e t a t e was fed over a long period of time t h e efficiency of ME
u t i l i z a t i o n was not decreased. The o r i g i n a l work on a c e t a t e
u t i l i z a t i o n was done primarily i n short experimental periods which
did not permit metabolic adaptation. Other f a c t o r s are probably
involved i n t h e g r e a t e r e f f i c i e n c y of high concentrate r a t i o n over
roughage r a t i o n s . Another theory w i l l be discussed i n another
s e c t i o n of t h i s paper.
L
B.
A
Body Composition. Body corngosition may influence energetic efficiency
i n two ways. The f i r s t i s t h e extra energy i n t h e form of excess f a t
that must be trimmed from t h e carcass. Hedrick (1972) est-ted
that
t h i s trimmed f a t r e s u l t e d i n a 400 million d o l l a r loss t o t h e beef
industry from t h e production of 2 b i l l i o n pounds of excess f a t . For
t h e pork industry, Jordan (1970)estimated t h a t e x t r a f a t (2.7
b i l l i o n pounds) produced by swine resulted i n an annual loss of 270
million d o l l a r s per year. Obviously reduction of body f a t can
improve t h e e f f i c i e n c y of animal protein production. Regulation of
l i p i d deposition and t h e hormonal influence on t h i s process has been
t h e subject of s e v e r a l recent reviews (Machlin, 1972; Purchas, 1970;
Merkel, 1969; Allen, 1968, and h r s o n , 1967).
The second manner i n which body composition influences energetic
e f f i c i e n c y i s by t h e r e l a t i v e energy cost of p r o t e i n v s . f a t
deposition. Estimates have been made of t h e efficiency of energy
u t i l i z a t i o n f o r t i s s u e deposition i n s e v e r a l species. A summary of
some of these estimates i s given in t a b l e 4 . A coupcison of t h e
energetic e f f i c i e n c y of lean and f a t pigs i s given in t a b l e s 5 and
6 . The e f f i c i e n c y f o r protein and fat synthesis i s based on value
reported by Kielanowski (1966) using t h e body balance technique.
I n c a t t l e , sheep and swine estimates of t h e efficiency of ME
u t i l i z a t i o n f o r growth have been made betveen t h e young and growing
animal. Confounding factors i n t h i s type of experiment include t h e
e f f e c t s of changes i n body composition, age and p o t e n t i a l l y variable
maintenance requirements. Baldwin (1968) and Milligan (1971) have
derived maximum efficiency estimates f o r various metabolic t r a n s f o r mations; (k.e l i p i d and protein synthesis) based on known biochemical
pathways. A comparison of measured energy efficiency f o r young
growing lambs (7&) and t h a t estimated as maximal efficiency revealed
t h a t a major portion of t h e heat increment could be explained on t h e
basis of energetic losses incurred i n biosynthetic pathways (Baldwin,
1968, and Baldwin and Smith, 1970). However, t h e lower values
.
7060-
EFFICIENCY OF
50-
ME
40-
UTILIZATION
30-
2010Ow
MOLAR % RUMEN A C E T I C A C I D
Figure
4.
The e f f e c t o f rumen a c e t a t e on
t h e e f f i c i e n c y o f ME u t i 1 i z a t i o n
(Armstrong and B a x t e r ,
1957).
177
TABLE 4.
EFFICIENCY OF ENERGY U T I U Z A T I O N
FOR TISSUE DEPOSITION I N SEVERAL SPECIES
Species
Efficiency of ME
u t i l i z a t i o n f o r gain
(%1
Rat
54 t o 88
pig
53 t o
Cattle
and
sheep
30 t o 85
TABLE 5.
Tissue
coanponent
Fat
Protein
Total c a l o r i e output
Cost of synthesis1
Fat
Protein
Total input required
(Forbes e t al., 1946; Hartsook and
Hershberger, 1971)
1966; Nehring,
(Kielanoski, J.,
78
19691
(Forbes
et al.,
1930; Paladines
e t al., 1964; B l a x t e r and Waineman,
19% Lofgreen and Garrett, 1968;
Bull e t al.,
- 1970)
E N E X X C I C COSTS OF TISSUE SYNTEIESIS
IN LZAN AM) FAT PIGS
k
17-3
11.2
Lean
Weight
(kg)
7.1
6.6
Megacalories
E
47.6
(k
23.5
14.5
Obese
Weight
Mega(kg)
calories
9.6
5.9
92.2
i&%
115.2
Assume a c a l o r i c efficiency f o r fat synthesis of 8& and f o r protein
synthceis of 76s.
TABLE
6 . COMPARISONS OF CALORIC EFFICIENCY FOR
LEAN AND OBESE PIGS
1. Caloric efficiency of energy gain.
Efficiency of energy storage = 100 x EnerKy output
Energy i n p u t
-
Lean pig = 100 x 83.1 Mega Calories =
106.2 Mega Calories
78 ,2$
Obese Pig = 100 x 124.0 Mega Calories = 79.&
157 .O Mega Calories
2.
Caloric e f f i c i e n c y of producing animal protein (Effp)
Effp = Total c a l o r i e s f o r production
Amount of protein synthesized
a.
-
Calories
gm protein
Lean S t r a i n Pig
= 16.0 kcal/gm protein
E f f p = 106'2 Megacalories
6.64 ~ r gprotein
b.
Obese S t r a i n Pig
E f f p = 15700Mef?acalories =
5.94 kg protein
26.4 kcal/gm protein
reported f o r ME e f f i c i e n c y (see t a b l e 4) indicate t h a t t h e r e a r e
other metabolic transformations o r i n e f f i c i e n c i e s . A discussion of
t h e evidence f o r genetic control of t h e e f f i c i e n c y of energy
conversion t o t i s s u e energy and t h e p o t e n t i a l biochemical pathways
which may be involved i n v a r i a t i o n i n e f f i c i e n c y w i l l follow.
There a r e s e v e r a l types of comparisons which demonstrate genetic
control over energetic efficiency. For example, specie comparisons
(Mayer and Nelson, 1963; Preston, 1968; B l a x t e r , 1961; Bylerly, 1967)
and strain c a n p r i s o n s (Freeman, 1967; Dickinson e t a l . , 1970; Davey
e t a l . , 1969; Fowler and Ensminger, 1961) i n d i c a t e genetic c o n t r o l .
However, f a c t o r s such a s food intake l e v e l and type of d i e t a r e not
always eliminated i n these s t u d i e s . Stronger evidence f o r differences
i n ME efficiency f o r gain has been obtained with genetic models of
obesity i n rodents (Bray and Tork, 1972). These animals represent
an excellent system f o r studying metabolic lesions which may contribute
t o greater energetic e f f i c i e n c y . Table 7 presents a sulll~pary of r e s u l t s
obtained with these models and t h e magnitude of t h e difference i n
efficiency of energy u t i l i z a t i o n . These r e s u l t s dramatically demons t r a t e the e f f e c t of genetic control over energy deposition.
TABLE;
Type
ob/ob m m e
7. EFFICIENCY OF ENEilCrY STORAGE
I N GENETIC OBESE RODEPUTS
Efficiency
($ increase
over c o n t r o l )
4401
.
ob/ob mouse
I2001
Zucker f a t t y r a t
5901
Zucker f a t t y r a t
2002
Reference
(York and Bray, 1971)
(Welton,
et g . , 1971)
(York and Bray, 1971)
(Park and Hershberger , 19-73)
Efficiency i s expressed as c a l o r i e s gain as a $ of c a l o r i e i n t a k e .
2 Efficiency i s expressed as energy gain over metabolizable energy intake.
C
.
.
Biochemical transformations which may p o t e n t i a l l y lead t o a l t e r a t i o n s
i n metabolic e f f i c i e n c y
1. Electron t r a n s p o r t .
x.
I n t h e normal process of glycol s i s , a
s i n g l e oxidative reaction occurs whick: produces NADH
Since NAD
cannot cross t h e mitochondrial membrane, a l t e r n a t e means of
t r a n s p o r t a r e required. The mechanism by which these electrons
a r e transported i n t o t h e mitochondria m y involve any one of a
number of p o t e n t i a l s u b s t r a t e c a r r i e r pathway models. The t h r e e
most commonly proposed a r e t h e alpha glycerophosphate s h u t t l e
( f i g u r e 5 ), t h e acetoacetate-betahydroxybutyrate s h u t t l e ( f i g u r e
6 ) and t h e malate-aspartate s h u t t l e ( f i g u r e 7 ) . An inefficiency
i n t h e transport of electrons from t h e cytoplasm i n t o the mitochondria i s found in t h e glycerolphosphate s h u t t l e since NADH
electrons a r e transported t o FAD without any ATP formation.
Therefore, only 2 of t h e 3 p o t e n t i a l ATPs a r e formed f r o m t h e
oxidation of NADH v i a t h e glycerolphosphate cycle ( V a n Dam and
Meyer, 1971). The other two s h u t t l e mechanisms a r e not i n e f f i c i e n t
i n t h a t the t r a n s f e r of electrons i s not energy dependent
3.80
&GLYCEROPHOSPHATE CYCLE
MITOCHONDRIA
Figure 5.
CY TOPLASM
The %glycerophosphate
cycle.
THE B-HYDROXYBUTYRATE CYCLE
?I
MITOCHONDRIA
B-hydroxybutyrate
NAD
NADH
CY TOPLASM
Acetoacetate
Figure 6. The a c e t o a c e t a t e - b e t a h y d r o x y - b u t y r a t e
cycle.
Malate-Aspartate Cycle
Mitochondria
Cy t op l a m
Mal
/ I
OAA
Figure 7 .
OAA
The rnalate-aspartate cycle.
(figures 6 and 7 ) . The only indication t h a t t h e glycerophosphate
s h u t t l e may be of physiological significance i n energetic
efficiency may be found i n s t u d i e s of heat production (energy
waste) by throxine administration and i n brown adipose t i s s u e .
The t i s s u e l e v e l s of glycerophosphate dehydrogenase are elevated
i n both brown adipose t i s s u e (Ohkawa, e t a l . 1969) and thyroxine
t r e a t e d rats (Isaacs, 1969). Both s i t u a t i o n s r e s u l t in heat
production and c a l o r i c i n e f f i c i e n c i e s . There have been no reports
on t h i s enzyme i n farm animal t i s s u e s and t h e p o t e n t i a l r o l e of
t h e glycerophosphate s h u t t l e i n energy conversion e f f i c i e n c i e s .
There is a l s o no evidence in farm animals that t h e r e a r e
variations i n oxidative phosphorylation and t h e e f f i c i e n c y of
trapping energy in t h e form of ATP.
2.
Triglyceride turnover. Components of body t i s s u e a r e i n a
dynamic s t a t e , constantly being synthesized and degraded. An
excellent example of how efficiency of f a t t e n i n g may d i f f e r
from one animal t o another may be found in t h e turnover of t i s s u e
l i p i d s . The r e l a t i v e contribution of t h e energy cost f o r t u r n over of t i s s u e l i p i d s may be a s high a s 15% of b a s a l metabolic
r a t e (Baldwin, 1971). Evidence f o r a change in l i p i d turnover
e f f e c t i n g energetic e f f i c i e n c y may be found i n s t u d i e s of genetic
obese p i g s . Adipose t i s s u e r a t e s of l i p o l y s i s i n t h e obese p i g
have been shown t o be decreased (Trygstad e t al., 1972; and
Standel and Vold, 1973). This may decrease t h e net cost of
energy gain thus increasing t h e efficiency of f a t t e n i n g .
I n addition t o t h e absolute l e v e l of l i p i d turnover, t h e
s p e c i f i c pathways of turnover used a r e p o t e n t i a l sources of
v a r i a t i o n i n energetic efficiency. Figure 8 s h m s t h e d i f f e r e n t
w a y s in which l i p i d may be recycled i n animal t i s s u e s . The energy
cost of each pathway is presented in t a b l e 8 . Information on
t h e absolute l e v e l s of metabolite flow through each of these
pathway a r e not a v a i l a b l e a t present. However, it has been well
documented t h a t t h e adipose t i s s u e glycerokinase l e v e l s a r e
elevated s e v e r a l f o l d during phases of increased lipogesis
(Welton e t a1 1973).
.,
TABU
8 . COMPARISON OF TRIGLYCERIDE TURNOVER
Pathway
1. Monoglyceride pathway
2 . Glycerokinase (adipose )
3.
Glycerokinase ( l i v e r )
ATP per
f a t t y acid cycle
2 .o
Efficiency
$
100
87
74
TRIGLYCERIDE TURNOWR
TG
MG
___jc_
Fatty acids
C
Glycerol
fatty CoA
CoASH
Figure 8.
Triglyceride cycles.
Protein turnover i s another f a c t o r i n metabolic efficiency
( B a l d w i n , 1968). However, because of t h e heterogeneous n a t u r e
of protein turnover and t h e l a c k of s p e c i f i c information on
metabolite f l u x r a t e and energy cost of c e r t a i n trmsformations,
t h i s f a c t o r w i l l not be discussed.
.
3 . Malate vs Pentose Phosphate Pathways.
Reducing equivalents i n
t h e form of reduced triphos*opyridine nuleotides (NADPII) a r e
required f o r f a t t y acid synthesis. These requirements can be
met by a t l e a s t two d i f f e r e n t metabolic pathwa 6 , t h e malate
cycle and the pentose phosphate pathway (shunt
“he energetic
e f f i c i e n c y of t h e shunt is approximately 26$ more e f f i c i e n t than
t h e laalate cycle (see t a b l e 9 and figures 9 and 10). In t h e
process of going from r e l a t i v e l y slow r a t e s of lipogenesis t o
high r a t e s of lipogenesis t h e contribution of reducing equivalents
from both t h e malate cycle and the shunt i s a l t e r e d ( F l a t t , 1970).
Because the malate cycle i s saturated f i r s t , i t s contribution i s
g r e a t e s t during periods of slow lipogenic r a t e s ( F l a t t , 1970).
As t h e lipogenic r a t e increases, t h e contribution of t h e shunt
increases ( f i g u r e 11). m e r e f o r e , increased energetic e f f i c i e n c y
should be found i n s i t u a t i o n s of hyperlipogenesis. Evidence f o r
t h i s theory is found in s e v e r a l d i f f e r e n t experimental s t u d i e s
where enhanced shunt a c t i v i t y and energetic e f f i c i e n c y is observed
( t a b l e 10). Thyroxine administration which increase h e a t
production causes a 6 t o 10 f o l d increase i n malic enzyne w i t h
l i t t l e or no influence on shunt enzymes (Young, 1968). Enhanced
lipogenesis i s found in meal-fed animSls and an increased e f f i c i e n c y
of t i s s u e deposition is observed but malic enzyme i s increased a s
well as shunt enzymes (Cobn and Joseph, 1959; Leveille and Hanson,
1966; Leveille, 1970). More research directed a t quantitating the
e f f i c i e n c y of t i s s u e growth with metabolic sequences i s required
t o determine t h e v a l i d i t y of this proposal. Fyruvate carboxylase
is probably t h e rate l i m i t i n g enzymatic s t e p i n the malate cycle
( F l a t t , 1970). Compasisons of pyruvate carboxylase and shunt
enzymes may be more representative of t h e two NADPH producing
pathways. It w o u l d be b e t t e r t o obtain d i r e c t measurements of
both t h e shunt (Katz e t al., 1966) and t h e malate cycle ( F l a t t ,
1971) by t r a c e r s t u d i e s in t i s s u e s from animals with varying
e f f i c i e n c i e s f o r metabolizable energy conversion t o t i s s u e energy.
3.
4.
F u t i l e C c l e s . Cycles which tend t o operate w i t h a production of
h e r o l y s i s ) with no net flux of metabolite a r e c a l l e d
f u t i l e cycles. An example of f u t i l e cycles i s shown in f i g u r e 12.
It is unlikely that these cycles a r e important i n normal
s i t u a t i o n s where metabolic control is exerted. However, no
information i s a v a i l a b l e of a d i r e c t nature that would eliminate
t h e p o s s i b i l i t y t h a t these cycles are operative in circumstances
of energetic inefficiency.
186
TABLE 9.
MALATE CYCIE VS PENTOSE pHOSPE?A’lS PATHWAY
COMPARE:
1. Pentose phosphate pathway
38
2.
ATP equivalents =
I2 NADPH
= 3ATP (equivalents)
Cycle cost =
E
4ATPINADPH
Total
.
ATp/Nmw
Malate cycle
NADH
..
.17
Pentose Phosphate Pathway i s 2646 more e f f i c i e n t
t h a n t h e malate cycle.
TABU 10. THE RELATIONSHIP BETWEEN PENTOSE PHOS€’H&I%
AND MALATE CYCLE ENZYMES AND ENERGETIC EFF1CIE;NCY
Glucose-6-PO4
dehydrogenase
Experimental
model
Mouse, ob/ob
Zucker r a t
Osabau p i g
Meal fed p i g
Ruminants
obese’
lean
103%
obese2
lean
obese3
lean
19746
,
fed4
nibbler
concentrat e 3
roughage
(Martin e t a l . , 1972)
(-in,
1973)
3 (Martin e t a l . , 1973b)
4 (Allee e t a l . , 1972)
5 (Martin e t a l . , 1973)
L-
Malic
enzyme
236%
71%
342%
PENTOSE CYCLE
Glucose 6-PO
4
+
+
12 NADP .--6co2
I2NADPH
(6 t u r n s o f t h e c y c l e )
NET:
Glucose o x i d a t i o n :
OR
F i g u r e 9.
38ATP
12NADPH
-
38 ATP e q u i v a l e n t s = I2 NADPH
3.17
ATP
The e n e r g e t i c e f f i c i e n c y o f t h e pentose c y c l e .
MALATE CYCLE
Malate
+
Pyruvate
ME
NADP
+
PC
C02 + ATP
Oxalacetate
+
NET:
+
NADP
Pyruvate
MDH
NADH
F i g u r e 10.
NADH
+
ATP -NADPH
+
NADPH
+
Oxalacetate
Malate
+
C02
+
ADP
NAD
+
NAD
+
The e n e r g e t i c c o s t of t h e m a l a t e c y c l e .
ADP
25J
(From Kather et a1,1972)
20-
%Contribution
of the
Pentose PO4
Pathway
-
15105-
0% Glucose in Fatty A c i d s
Figure 1 1 .
The change i n % c o n t r i b u t i o n of t h e
pentose pathway w i t h changes i n f a t t y a c i d s y n t h e s i s
et a i , 1972).
(Kather -
"FUTILE
1.
CYCLES"
FRUCTOSE-6-POb
-
FRUCTOSE DIPHOSPHATE
2.
ADP
P E P
ATP
PXK6VATE ,--OXALOACETATE
ATP
F i g u r e 12.
ADP
"Futile cyclesll.
191
CONC WSIONS
The energetic c o s t s of b i o l o g i c a l functions a r e influenced by a
number of f a c t o r s . Those f a c t o r s discussed in t h i s paper represent the
major contributors t o the v a r i a t i o n in energetic e f f i c i e n c y . The
q u a n t i t a t i v e significance of t h e metabolic transformations discussed i n
t h e l a t t e r p a r t of t h i s paper is s t i U unhown. Because of t h e developments
e f f i c i e n c y of only a few $ can
in 8-1
industry, t h e improvement
make the difference between p r o f i t and loss. Therefore, techniques must
be developed t o take advantage of every possible means of improving the
energy conversion t o body t i s s u e s .
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D. G . TOPEL: We're g e t t i n g a l i t t l e l a t e i n our schedule. We'll
take time f o r one o r two questions and then w e ' l l h o l d t h e other questions
f o r t h e general discussion period. Are t h e r e one o r two questions f o r
D r . M a r t i n a t t h i s time? Okay, f i n e , w e ' l l catch them l a t e r . Our next
speaker, D r . Dikeman f r o m Kansas S t a t e University, i s a native of Kansas
and received h i s Master's from Michigan S t a t e University and Ph .D from
Kansas S t a t e University. We asked M i k e t o give t h e f i n a l presentation
on t h i s session and t h e t i t l e , "Relation Between Efficiency of Live
Weight G a i n and Body Composition During Growth of Domestic Animals." We
asked him t o be s p e c i f i c i n t h i s presentation i f he could and r e l a t e
composition of gain t o production c o s t . Yesterday i n our welcome here
a t The Pennsylvania S t a t e University t h e President of t h i s i n s t i t u t i o n
wanted t o know when we were going t o talk about d o l l a r s of production
cost and e f f i c i e n c y and t h i s t o p i c i s going t o be discussed h e r e . I t ' s
a d i f f i c u l t one t o prepare because I am sure t h e r e ' s a limited amount of
information t h a t relates production e f f i c i e n c y t o carcass composition and
i t ' s r e a l l y why we stimulated our i n t e r e s t s i n s e t t i n g up t h i s p a r t i c u l a r
session. So, Mike, I'll t u r n it Over t o you a t t h i s time.
.