Epidemiologic Reviews
Copyright © 2001 by the Johns Hopkins University Bloomberg School of Public Health
All nghts reserved
Vol. 23, No. 2
Printed in U.S.A.
Participation of Lycopene and Beta-Carotene in Carcinogenesis: Defenders,
Aggressors, or Passive Bystanders?
Lenore Arab,1 Susan Steck-Scott,1 and Phyllis Bowen2
INTRODUCTION
Identification of clusters of dietary and other behaviors that
produce synergies or cancel out a possible risk-reducing
component of the diet requires new epidemiologic
approaches. The next generation of studies must incorporate
emerging strategies to address these issues if we are to contribute to selecting compounds for clinical trials and determine the most efficacious design and dietary contexts of
these trials.
This review of beta-carotene and lycopene is used as a
case study to model the directions and issues that must be
addressed when exploring other emerging classes of bioactive dietary compounds such as the phenolics (tea, red wine,
chocolate, berries), allyl sulfides (garlic), glucosinolates
(broccoli), phytoestrogens (soy, flax), and fatty acids (fish,
nuts). Each of these groups of substances has a cast of possible players; as in every play, their roles may be synergistic, adding to the forward motion of the play. Others provide
drama by being bad actors under varying circumstances.
This review takes the form of a series of questions that are
answered in discussions of these two carotenoids because
much more is known about these carotenoids than about
other carotenoids and phytochemicals.
Carotenoids, widely found in many fruits and vegetables,
have been the focus of research for over a decade, with little resolution concerning their beneficial or detrimental role
in the modulation of carcinogenesis. The findings on one,
beta-carotene, have been disappointing because published
clinical trials show an increased risk of lung cancer for current cigarette smokers; another, lycopene, is still a rising star
and is the subject of a National Cancer Institute (Bethesda,
Maryland) program to prevent prostate cancer. Can these
commonly consumed carotenoids with similar chemical
structures have different efficacies for cancer prevention? Is
the verdict still out on beta-carotene? Is lycopene worthy of
continuing scientific interest? Can continued epidemiologic
investigation shed further light on the function, risks, and
benefits of carotenoids in mammalian systems?
This review describes what is known about the similarities and differences in structure, function, metabolism, and
consumption between beta-carotene and lycopene, part of
which is summarized in table 1, to better interpret the existing epidemiologic literature and guide future explorations.
Standard phase I toxicologic studies are unlikely to discover
adverse effects of dietary substances such as beta-carotene
and lycopene because their risk reduction and/or adverse
effects are likely to be subtle and to evolve under long exposure, and these effects are impractical to detect in the
standard phase I, II, and III evaluations. This leaves epidemiology as the viable approach for sorting out the morass
of countervailing factors that provide the circumstances for
risk reduction or risk expansion. Study designs have rarely
incorporated a thorough knowledge of carotenoid bioavailability, metabolism, function, and genomic heterogeneity as
explanatory variables in the interpretation of outcomes.
SIMILARITIES AND DIFFERENCES IN CHEMISTRY,
FUNCTION, AND CONSUMPTION
What are the plant/dietary sources of the compounds
of interest?
Carotenoids are essential for sustained photosynthesis.
Beta-carotene and lycopene are only two of more than 600
carotenoids (not counting their isomers) found in the biomass. Only about 20 have been identified in human serum.
Of these, only five are found in sufficient quantities to be
measured routinely (1). Of these five, lutein and betacarotene are the most widespread in vegetables and fruits,
and their serum concentrations are relatively good markers
for fruit and vegetable intake (2). Lycopene is narrowly
distributed in foods and found predominantly in tomato
products, pink grapefruit, watermelon, and papaya (3).
Tomato and tomato products make up 80 percent of the
lycopene consumed in tomato-consuming countries (4, 5),
so lycopene does not reflect consumption of other fruits
and vegetables (2). In the United States, carrots, cantaloupe, broccoli, spinach, greens, and vegetable soups
and mixtures are the most abundant sources of betacarotene (6). Its use as a food colorant in margarine,
Received for publication April 4, 2000, and accepted for publication September 25, 2001.
Abbreviations: ATBC, Alpha-Tocopherol, Beta Carotene; CARET,
Carotene and Retinol Efficacy Trial; IGF, insulin-like growth factor;
LDL, low density lipoprotein; mRNA, messenger RNA; OR, odds
ratio; RR, relative risk.
1
Departments of Nutrition and Epidemiology, School of Public
Health, University of North Carolina, Chapel Hill, NC.
2
Department of Human Nutrition, College of Applied Health
Sciences, University of Illinois, Chicago, IL.
Reprint requests to Dr. Lenore Arab, School of Public Health,
University of North Carolina, CB 7400, McGavran-Greenberg Hall,
Chapel Hill, NC 27599-7400 (e-mail: [email protected]).
211
212
Arabetal.
TABLE 1. Similarities and differences between beta-carotene and lycopene
Beta-carotene
Characteristic
Structure/chemistry
Beta-ionone ring
Double bonds
Molecular mass
Molecular formula
Solubility
Activity
In vitro antioxidant
In vitro prooxidant
In vivo antioxidant
Smoking oxidation products
Conversion to retinoids
Immune function
Cell-to-cell communication
Cell cycle progression
Carcinogen metabolism
Animal cancer studies
Exposure sources
Diet
Dominant source, United States
Supplements
Metabolism
Bioavailability
Isomerization
Tissue accumulation
Major storage pool
Circulation half-life
Plasma concentrations
Measurement error
Cancer associations from
observational studies
Cancer-causing associations from
interventions
Lycopene
No
13
536
Yes
11
536
C
40 H 56
Highly lipophilic
Highly lipophilic
Modest
Yes, at high concentrations and partial
pressures
Circumstantial evidence for lipids
Yes
Substantial, by multiple routes
Enhanced
Enhanced
Circumstantially inhibits
Can modulate
Antitumor
Best of carotenoids
Likely, no data
Circumstantial evidence for lipids and DNA
No data
None
No data
Enhanced
Circumstantially inhibits
None
May be antitumor
Dark green, yellow, and orange fruits and
vegetables, red palm oil, food colorant
Carrots, cantaloupe, broccoli, spinach, mixed
greens
Multivitamins, single-source supplements,
food/beverage fortification
Tomatoes, pink grapefruit, watermelon, tropical
fruits, food colorant
Tomatoes
Poorly available from greens, carrots
Cooking enhances
Dietary fat enhances
Highly available from supplements
All trans- in circulation
Found in all human tissues
Supraaccumulation in testes and adrenal
glands
Adipose tissue, liver
<12 days
Multifactor dependency
Diet
Lipoprotein concentrations
Adiposity
Smoking
Biased reporting
Associated with vegetable intakes
Moderately available from tomatoes
Cooking enhances
Dietary fat enhances
Moderately available from supplements
Mostly cis- in circulation and tissues
Found in all human tissues
Supraaccumulation in testes and adrenal
glands
Adipose tissue
12-33 days
Multifactor dependency
Diet
Lipoprotein concentrations
Adiposity
Contributions from many foods and from
mixed dishes, soups
113 4-* of 281 studies (40%)
2 T* of 281 studies (1%)
Two studies showed increased lung cancer;
one study showed no effect
Supplements available
Less biased reporting
Associated with pizza, pasta, and vegetable
intakes
Few food sources, but tomatoes are used in
many mixed dishes
44 i of 121 studies (36%)
O t o f 121 studies (0%)
No intervention studies to date
i , negative association; T, positive association.
cheese, beverages, and candy is very difficult to quantify
but is thought to be relatively limited. In tropical countries, mangoes, papaya, and red palm oil are important
sources (7). Cow's milk, especially in the spring, also contains beta-carotene (8).
In recent years, beta-carotene has been widely available
as a dietary supplement and is included in a number of popular multivitamin formulations and health food products in
the United States. Most epidemiologic studies do not quantify these sources because of difficulty in dose estimation.
Epidemiol Rev Vol. 23, No. 2, 2001
Lycopene and Beta-Carotene in Carcinogenesis
Although lycopene supplements have recently become
available, consumption is not yet widespread. Publicity
regarding the association between tomato consumption and
prostate cancer risk might be changing US supplementation
habits, especially in older men. A tomato extract with a high
lycopene content has been approved in the United States and
other countries as a food colorant. It may be a significant
source of lycopene exposure in the future.
The quantity of both lycopene and beta-carotene consumed
from the diet only can vary widely. The Third National Health
and Nutrition Examination Survey has estimated that US
mean/median consumption levels from food alone are 1,978
|j.g/day (standard error, 44 (Ag/day) for beta-carotene and
9,401 Hg/day (standard error, 315 Hg/day) for lycopene (9).
On days that dishes containing tomato sauce, tomato paste, or
both are eaten, it is relatively easy to consume as much as
35,000 fxg carotenoids per day. Because intake depends on
access to and availability of carotenoid-rich foods, very low
mean consumption levels are found in other countries such as
China (10), India (11), and Thailand (12).
Because of the wide variety of food and supplemental
sources, accurate assessment of carotenoid intake is more
difficult than assessment of intake from many other food
components. In addition, most of the reported data are
dependent on old nutrient database values, and older epidemiologic studies need to be judged accordingly. A comparison of data analyzed by using the new database for
carotenoids versus existing data showed significant
increases in intake of both beta-carotene and lycopene (13).
213
..OH
FIGURE 1. Structures of important carotenoids. Reprinted with
permission from IARC Handbooks of Cancer Prevention, Vol 2.
Carotenoids. Lyon, France: International Agency for Research on
Cancer, 1998:26.
What are the chemical structure and general chemical
and physical properties of the compounds of interest?
There are two classes of hydrocarbons in the carotenoid
family of compounds: the carotenes and their oxygenated
derivatives, the xanthophylls. Most contain 40 carbons
arranged from eight isoprene units, with four units facing
each other. This arrangement produces an electron-rich,
alternating, double bond structure, making carotenoids susceptible to electrophilic attack (highly oxidizable) but also
giving them high electron resonance, which stabilizes the
transfer of energy from photosynthesizers such as chlorophyll and other porphyrins such as heme. This capacity
quenches both the chlorophyll triplet state and singlet oxygen and provides for life. Carotenoids also absorb light in
the blue wavelengths, producing highly pigmented yellow
(lutein), orange (beta-carotene), and red (lycopene) (14)
plant materials.
Figure 1 displays the six carotenoids that are most commonly found in human plasma and for which food table values are available for a significant number of foods. The
International Union of Pure and Applied Chemistry
(IUPAC) name for beta-carotene is p\P-carotene because of
its two beta-ionone rings. This characteristic is its distinguishing difference from lycopene (whose IUPAC name is
^'F-carotene), since these two carotenoids have identical
molecular formulas and mass. They tend to be rigid, long
chains and are very fat soluble (7). When free from chloroplasts, protein, or fiber, these carotenoids gravitate to any
Epidemiol Rev Vol. 23, No. 2, 2001
lipid layer including cell membranes, fatty food matrices, or
even plastic tubing. Their structure leads to easy stacking;
many molecules of a particular carotenoid stack together to
form microcrystals, which tend to intensify their color. This
is their condition in the chromoplasts of fruits and yellowto-red vegetables. In model phosphatidyl-choline membranes, beta-carotene increases the mobility of polar head
groups and increases water accessibility, while the xanthophylls have the opposite effect (15). Presumably, lycopene
would behave as beta-carotene does.
The carbon-carbon double bonds of the carotenoids lead
to possible cis(Z)-trans(E) isomerizations, with a possibility
of 1,056 and 272 geometric isomers for lycopene and betacarotene, respectively; only 72 are structurally favorable for
lycopene. In nature, carotenoids predominate in their alltrans form. The cw-configuration puts a bend in the chain
that increases their solubility in organic solvents and
micelles and decreases their color intensity. Because these
isomers are no longer rodlike, they tend to disorder the
domains of the cell membranes in which they reside. Small
amounts of the cw-isomers of beta-carotene and lycopene
can be found in foods and supplements, are artifactually
generated during food processing, or are the result of biologic conversions in humans after consumption. Trans-isomers of lycopene make up 90-98 percent of total lycopene
in commonly consumed tomato products (16).
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Arab et al.
The antioxidant capabilities of beta-carotene and lycopene
have been studied in a variety of model systems. Lycopene
has two additional unconjugated double bonds compared
with beta-carotene, which seems to give it different properties with respect to singlet oxygen quenching, formation of
carotenoid radicals, and quenching of the biologically important superoxide-, peroxide-, and peroxynitrite- and nitrogendioxide-reactive species. One study found that lycopene was
the most efficient singlet oxygen quencher compared with a
variety of carotenoids and a-tocopherol in vitro, and it was
twice as efficient as beta-carotene (17). In model superoxidegenerating systems, Conn et al. (18) found that beta-carotene
was more likely to complex with superoxide, whereas alltrans lycopene underwent reversible transfer of electrons
with a superoxide radical, suggesting that these two
carotenoids vary significantly in radical quenching and thus
in their potential beneficial effects. Other researchers found
that lycopene was three times more efficient than betacarotene in preventing lipid peroxidation in multilamellar
liposomes (19), but neither was likely to protect against
aqueous-phase free-radical attack (20). Lycopene is well
known for its oxidative lability compared with beta-carotene,
and extra laboratory precautions are required during its handling and analysis.
Carotenoids do not have the structural features typically
associated with chain-breaking antioxidants and are more
likely to be prooxidants under many conditions. In fact, it
has been demonstrated in liquid solutions, isolated membranes, and intact cells that beta-carotene acts as a prooxidant at atmospheric oxygen pressures but as an antioxidant
at the lower oxygen partial pressures that exist in most living tissues, except lung epithelia. One should be particularly
mindful of these observations when interpreting the results
of carotenoid action in cell cultures typically grown at
atmospheric pressure, producing cells under greater oxidative stress than would be experienced by cells in animals
and humans. Whether beta-carotene acts as a prooxidant or
an antioxidant depends on its concentration, the nature of
the oxidative attack, and the presence of other antioxidants
such as the tocopherols, ascorbate, or other carotenoids (21,
22). Little is known of the circumstances that might lead to
prooxidative activity for lycopene.
What is the function of these compounds in the plants
or animals in which they are found?
Green leaves contain mostly beta-carotene and alphacarotene as well as the xanthophylls, lutein, zeaxanthin,
neoxanthin, violaxanthin, and antheraxanthin. These substances serve two major purposes: 1) light collection and
transfer to chlorophyll and 2) dissipation of the excess
energy resulting from full sun exposure (23). They also
serve a structural role. To form their active tertiary structure,
proteins of the chloroplasts require carotenoid molecules
(24).
In two situations, carotenoids have been shown to play a
similar role in humans. Beta-carotene has been found to
ameliorate the symptoms of the genetic disease erythropoetic protoporphyria, clinically characterized by a burning
sensation followed by edema and erythema resulting from
minimal sunlight exposure. In this disease, there is an abnormal buildup of photosensitizing protoporphyrins in the skin
and tissues of the afflicted person. To be effective, high
doses of beta-carotene (>180 mg/day) are required. Since
there is no decrease in the quantity of protoporphyrins with
treatment, it is assumed that beta-carotene serves the same
function as it does with chlorophyll by safely dissipating
higher-energy states of the light-energized porphyrin (25).
Lutein and zeaxanthin accumulate in the macula of the
human retina, absorbing blue light, and are thought to prevent photo-related damage to the light-harvesting rods,
cones, and retinal pigment epithelium (26).
Lycopene is a precursor in the biosynthesis of betacarotene and the xanthophylls and therefore is present in
minute quantities in all higher plants. Its presence in tomato
and other fruits is due to an increase in the synthesis of precursor carotenoids (phytoene and phytofluene) and a
decrease in the synthesis of beta-carotene, leading to a large
accumulation of lycopene as the tomato fruit matures and
chloroplasts undergo transformation to chromophores (16).
What is the bioavailability of the compounds from the
food matrices consumed most often?
The bioavailability of carotenoids varies widely and is
therefore a critical issue in estimating consumption as an
exposure variable. For example, beta-carotene from raw carrots was found to be only 5 percent bioavailable (27) and 14
percent bioavailable from a mixed vegetable diet, while
lutein was 78 percent bioavailable from the same diet (28).
Beta-carotene has been found to be twice as bioavailable
from fruits compared with green leafy vegetables (29).
Compared with carrots, supplementation of beta-carotene
suspended in oil or suspended in water from gelatin-stabilized beadlets (the form used in the major clinical trials)
raises the plasma concentration approximately sixfold
higher (30-32). One explanation for the disparity between
the risk reduction for lung cancer associated with high
plasma concentrations of beta-carotene obtained from the
diet compared with its supplementation is its higher
bioavailability from supplements, producing greater exposure than can be obtained from diet alone (33).
The diversity of response from studies of absorption point
to the many factors that affect bioavailability of each of
these carotenoids from a single food or in the context of the
total diet (34). Adding oil to tomato juice before heating
greatly enhances its bioavailability (35). Bioavailability of
lycopene from supplements compared with tomato products
has shown little difference (36, 37).
Bioavailability of both beta-carotene and lycopene can be
modified by other dietary components. Very-low-fat diets (7
percent energy) (38) and diets high in fiber such as pectin
(6) can reduce plasma beta-carotene levels. Lipid-lowering
drugs such as cholestyramine and probucol can reduce
plasma concentrations of both lycopene and beta-carotene
by 50-65 percent (39). Sucrose polyester (olestra) and margarines augmented with plant sterols, when consumed with
dietary carotenoids, reduce carotenoid plasma concentraEpidemiol Rev Vol. 23, No. 2, 2001
Lycopene and Beta-Carotene in Carcinogenesis
tions (40, 41). The nature of the food matrix for betacarotene and lycopene also matters. Cooking and steaming
of vegetables have generally resulted in greater bioavailability, with no great degradation of carotenoid content (42).
The isomers of these carotenoids differ in food and
plasma and are affected by cooking. There are large differences in the plasma concentrations of the cw-isomers of
beta-carotene and lycopene, with a remarkable accumulation of lycopene cis-isomers even when consumed in small
quantities (43), and there are negligible plasma accumulations of beta-carotene cw-isomers even when consumed in
substantial quantities (44). 9-cw-Beta-carotene may be more
subject to in vivo oxidation and degradation, thus protecting
circulating lipids (45). Fluctuations in the pattern of cisisomers found in human plasma or tissues for beta-carotene
and lycopene have led to great interest in measuring the
ratio of cis- and frans-isomers of both carotenoids in plasma
and tissues as a marker of oxidative transformations.
What lifestyle behaviors are associated with serum
levels of carotenoids?
Significantly lower circulatory levels of beta-carotene (40
percent lower) have been reported in smokers compared
with nonsmokers (46, 47). Dose-response gradients have
also been noted (48). Although the mechanism of this effect
is not known, smoke can oxidize beta-carotene in vitro to
carboxyls, epoxides, and nitro derivates (47). Alternatively,
smokers' diets are lower in fruits and vegetables that deliver
these carotenoids. It is unlikely that dietary differences
explain this finding, particularly since the effect is not as
evident for lycopene in serum. The significance of this
interaction is that it restricts the utility of serum biomarkers
of beta-carotene as indicators of prior intake (49).
The relation between alcohol consumption and serum levels of carotenoids is poorly understood. Consistent alcohol
consumption results in decreased dietary intake of
carotenoids and interferes with conversion of beta-carotene
to retinol (50). There is evidence of 39 percent lower levels
of serum beta-carotene in alcohol-drinking Japanese men
compared with nondrinkers (48). Whether this is due to
dietary differences or altered metabolism because of the
alcohol remains unknown. An alcohol interaction with
serum levels does not appear to extend to lycopene (51).
What other bioactive compounds are likely to be
consumed with high and low consumption of
carotenoids, and how are these compounds likely
to attenuate or promote the risk-reducing or riskpromoting activities of the compounds of interest?
Beta-carotene is so widespread throughout fruits and vegetables that it may be a marker for consumption of a variety
of other bioactive compounds or their combinations that
reduce the risk of various cancers. Similarly, many other
components of tomatoes might be responsible for the effect
linked to lycopene. Tomatoes belong to a family marked by
their glycoalkaloid content, which is thought to protect them
from disease and insect predation. The cultivated tomato
(Lycopersicon esculentum) contains one identified glycoalEpidemiol Rev Vol. 23, No. 2, 2001
215
kaloid, tomatine. Tomatine and similar glycoalkaloids form
complexes with cholesterol in membranes, disrupting
intestinal epithelial cells (52). Tomatoes are also rich in a
variety of phenolic compounds; total phenolics range from
50 to 100 percent of the weight of lycopene in various
tomato products. In a variety of in vitro antioxidant detection systems, the water-soluble portion of tomato products
containing these phenolics and ascorbate was found to have
more antioxidant activity than the lipid-soluble portions
containing lycopene (53). Some investigators have suggested that the bioactivity of lycopene in tissue culture
might be due to lycopene oxidation products. To our knowledge, these products have not been evaluated in epidemiologic studies.
How are these compounds metabolized, and what is
their tissue distribution?
Primates and a few other mammalian species are unique
in their accumulation of intact carotenoids from the low
quantities consumed in their average diets (8, 54, 55). In
humans, carotenoids are absorbed and transported with
triglycerides via chylomicrons to all parts of the body, and
they can be found in all circulating lipid fractions. Carotenes
are slightly more concentrated in low density lipoprotein
(LDL) cholesterol and very low density lipoprotein cholesterol, whereas xanthophylls are more concentrated in high
density lipoprotein cholesterol (56). Both beta-carotene and
lycopene reach their maximum concentrations 24 hours
after dosing. A careful evaluation of beta-carotene metabolism indicates at least a biphasic clearance, with half of it
disappearing within 2 weeks of a depletion diet. A much
longer period is required to produce negligible plasma concentrations, probably because of the preexisting adipose tissue pool (57). The biphasic nature of the metabolism of
these carotenes has led to widely varying estimates of their
biologic half-lives, ranging from a mean of <12 days for
beta-carotene and between 12 and 33 days for lycopene (6).
This variation indicates that epidemiologic studies in which
plasma or serum are used as biomarkers of carotenoid intake
are measuring short-term intake as opposed to studies based
on adipose tissue that represents more long-term storage—
the time frame of interest.
All carotenoids are thought to be distributed in tissue
throughout the human body via LDL cholesterol transport
(58). In addition to circulating levels in plasma or serum,
carotenoid concentrations have been measured in adipose
tissue, buccal cell, lung, liver, kidney, pancreas, adrenal,
spleen, heart, prostate, ovary, cervix, thyroid, macula, and
retina samples.
Although adipose tissue, because of its mass, does not
contain especially high concentrations of carotenoids compared with other tissues, approximately 85 percent of body
carotenoids are stored there (59). Parker sampled adipose
tissue from 19 adults in the United States and found mean
concentrations of 0.62 ng/g for beta-carotene and 0.58 u.g/g
for lycopene (31).
We found mean concentrations of 0.7 and 0.3 ug/g for
beta-carotene and lycopene, respectively, in the adipose tis-
216
Arabetal.
sue of European males (49). While beta-carotene was twice
as concentrated as lycopene in the adipose tissue of men,
their mean plasma lycopene concentration was higher than
their beta-carotene concentration (49).
Lycopene has been found to be the most abundant of nine
carotenoids in human prostate taken from men in the United
States (4); c/s-isomers of lycopene accounted for 79-88 percent of total prostate lycopene, pointing to accumulation of
these cw-isomers in tissue as compared with the lower cisisomer content of food (5—20 percent) and plasma (50-60
percent). Lycopene made up approximately 80 percent of
the total carotenoids found in testes and over 60 percent of
the total in adrenal tissue. These tissues (adrenal, testes, and
liver) also have the greatest number of LDL cholesterol
receptors and have maximal rates of lipoprotein uptake that
are consistent with the belief that carotenoids are transported by LDL cholesterol particles. In the human retina,
lutein and zeaxanthin predominate in the foveal region of
the macula, where essentially no beta-carotene or lycopene
is found, supporting the idea of tissue-specific functions for
the individual carotenoids (60).
Carotenoid accumulation in human lung tissue or bronchoalveolar lavage cells has been reported by Schmitz et al.
(61), who found mean concentrations of 0.11 H-g/g for both
beta-carotene and lycopene in the lung tissue of 13 persons
autopsied. Redlich et al. (62) found mean concentrations of
0.13 and 0.09 ug/g of beta-carotene and lycopene, respectively, in 21 US patients undergoing lung biopsy, the majority
of whom had a history of smoking. Total carotenoid concentrations in lung bronchoalveolar lavage macrophages of 12 of
these patients were considerably lower (mean of 6.85 ng/106
cells), with concentrations of the individual carotenoids
below the limits of detection at the time. We measured nine
carotenoids and isomers in human lung macrophages and
found slightly higher concentrations in 23 healthy nonsmokers (mean total carotenoids, 8.15 ± 3.33 ng/106 cells) (63).
Both lycopene and alpha-carotene lung macrophage concentrations were increased after 2-week daily supplementation
with vegetable juice (unpublished data).
How do lycopene and beta-carotene function in human
or mammalian systems?
The chief difference between beta-carotene and lycopene
is that beta-carotene is the main precursor for vitamin A in
the diet, whereas lycopene has no pro-vitamin A activity. In
many world populations, beta-carotene consumption is critical to preventing vitamin A deficiency. The beta-carotene
conversion factor to vitamin A is 12:1 (9).
The data on the role of lycopene or beta-carotene in protecting LDL cholesterol from ex vivo oxidation are equivocal and likely depend on the circumstances of each human
study (64—66). Even positive results were modest in effect.
Lycopene and beta-carotene appear to play no role in protecting the retina from age-related macular degeneration, as
do lutein and zeaxanthin. Beta-carotene seems to be
required for bovine fertility (67), and this carotenoid accumulates in the corpus luteum of the human ovary. Its function there is unknown.
Study of the effect of beta-carotene on immune function
is confounded by its precursor role for vitamin A. Vitamin
A has a well-documented role in adequate immune function
in humans. Beta-carotene supplementation studies have
shown positive effects on a variety of biomarkers of
immune function (68-70). Although a recent epidemiologic
study found that tomato intake reduced the risk of mortality and morbidity in Sudanese children (71), biomarkers of
immune function were not modified with tomato juice or
lycopene supplementation (72, 73).
What is known about the variable response of
subpopulations defined by genotype or phenotype?
One phenomenon that characterizes bioavailability of
carotenoids is the huge variability in plasma response to single or chronic doses of a variety of carotenoids from supplement or food sources in different persons. This variability has
led some investigators to identify "nonresponders" as persons who seem to have a negligible or very low increase in
carotenoid plasma concentrations in response to carotenoid
dosing (74, 75). This phenomenon appears to be less pronounced for lycopene compared with beta-carotene (76).
Since a few nonresponders can be found in small groups of
volunteers recruited for most bioavailability studies, the
question is raised as to whether nonresponders constitute a
subgroup at higher risk for various cancers for reasons independent of their lower carotenoid plasma concentrations.
What are the hypothesized functions of the compounds
in the carcinogenic process?
In a thorough evaluation of all literature related to
carotenoids and cancer, the International Agency for
Research on Cancer (7) decided that there was sufficient
evidence for cancer-preventive activity of beta-carotene in
experimental animals. This conclusion was based largely on
the more than 20 studies of the effect of beta-carotene on
skin carcinogenesis and another 20 positive studies on prevention of buccal pouch carcinogenesis in the hamster.
When other cancer sites and animal models were used in
these studies, positive findings were also supportive.
Because of very poor absorption of beta-carotene in these
rodents, these studies must be considered low exposure
despite the high doses of this carotenoid in the animals'
diets. The International Agency for Research on Cancer (7)
evaluated 10 animal studies in which lycopene was used and
found limited evidence for its cancer-preventive activity.
Seven of 10 showed statistically significant reductions in
tumor endpoints, and the others showed nonsignificant
reductions. The cancer sites included the colon, lung, liver,
and mammary gland.
A number of mechanisms have been proposed for
carotenoid interference in the carcinogenic process. Most
actively researched have been their antioxidant effects,
especially with respect to in vivo markers of lipid peroxidation in humans. Although beta-carotene supplementation
has produced modest reductions in some of these biomarkers but not others (7), their meaning for cancer prevention is
unclear. More relevant are biomarkers for oxidative DNA
Epidemiol Rev Vol. 23, No. 2, 2001
Lycopene and Beta-Carotene in Carcinogenesis
damage, since such damage can lead to point mutations in
the initiation, promotion, or progression of carcinogenesis.
Three measures have been used most frequently in human
studies: the single-cell gel electrophoresis or COMET assay,
which evaluates endogenous or agent-produced strand
breakage in DNA extracted most often in circulating leukocytes; measures of oxidation products of nucleic acids in
cellular DNA (predominantly lymphocytes or total leukocytes); and excretion of these oxidation products in urine.
Guanosine seems to be particularly vulnerable to oxidative
attack and, when oxidized, produces 8-hydroxydeoxyguanosine (80HdG). Urinary 80HdG is a function of
the extent of oxidative attack and its repair. It has been
found that 12-week beta-carotene supplementation had no
effect on 8OHdG excretion in smokers and nonsmokers (77)
or when combined with vitamins C and E (78). A number of
studies have determined that tomato-product supplementation reduces lymphocyte strand breaks induced by hydrogen
peroxide exposure (79, 80), endogenous strand breaks (81,
82), or leukocyte 8OHdG (36). Lycopene has been found to
protect CV1-P monkey cells from 8OHdG generation when
exposed to an oxidizing environment (83). These studies
point to the possibility that lycopene provides protection
from generalized DNA damage, but they may not be relevant unless the same protection is afforded to cancer target
tissues.
Nothing sets beta-carotene and lycopene apart more than
the pro-vitamin A function of beta-carotene. Retinoids have
been a subject for intensive research on cancer prevention
and treatment because of their role in cellular differentiation
and proliferation. Retinoids, especially retinoic acids, are
nuclear transcription factors that regulate gene expression.
A\\-trans retinoic acid is the major ligand for the retinoic
acid receptors (a, |3), and 9-cis retinoic acid is the major ligand for the retinoid X receptors (a, P). These receptors are
usually associated and combine with areas in a variety of
genes called retinoic-acid response elements. In some cases,
gene expression is enhanced; in other cases, it is suppressed
possibly by cross-talk with a variety of other receptors that
include thyroid hormone receptors, vitamin D3 receptor, peroxisome proliferator receptor, and a number of orphan
receptors (84). Retinoids have been used successfully to retreat promyelocytic leukemia and head and neck cancers.
Retinol transport, uptake, and conversion to retinoic acid
within cells is under tight control, but the possibility that
beta-carotene and other carotenoids, because of their residual presence in target tissues, could bypass this control system and become retinoic acid or retinoid-like substances has
been the subject of some investigation.
Beta-carotene can be converted to retinol and subsequently to retinoic acid by two mechanisms. Central cleavage by a 15,15' dioxygenase has been found in the intestine
and liver, and now mRNA has been found in the small intestine, liver, kidney, and testes of the mouse. Surprisingly, an
additional excentric cleavage enzyme has been cloned and
identified that cleaves at the 9'10' double bond to yield betaapo-10' carotenal and beta-ionone and lycopene to
subsequently yield apolycopenals. This dioxygenase II is
more widespread, with detectable mRNA additionally found
Epidemiol Rev Vol. 23, No. 2, 2001
217
in the spleen, brain, lung, and heart (85). Excentric cleavage
may occur nonenzymatically as a result of a highly oxidative environment in a number of cells (86). It has also been
shown that beta-carotene can serve as the substrate for direct
conversion to retinoic acid (87, 88) in enterocytes. Since 9cis retinoic acid is a powerful ligand for the retinoid X
receptors, metabolism of the cw-isomers of beta-carotene to
cw-retinoids is also of interest.
In a recent study, Wang et al. (89, 90) explored the interaction of low and high doses of beta-carotene and cigarette
smoke with retinoid metabolism and function in ferrets. The
beta-carotene doses were carefully selected to represent the
lung tissue concentrations achieved in the AlphaTocopherol, Beta Carotene (ATBC) Cancer Prevention
Study (91) (high dose, equivalent to a human dose of 30
mg/day) or the amount of beta-carotene in five to nine servings of fruits and vegetables (low dose, equivalent to a
human dose of 6 mg/day). The high-dose beta-carotene plus
smoking and the high-dose beta-carotene-alone groups had
lower levels of retinoic acid lung tissue and showed 18-72
percent reductions in retinoic acid receptor p* gene expression (a tumor suppressor). Both c-Jun and c-Fos protooncogene expression (Ap-1) were increased three- to fourfold
over that of the control animals (92). Cell proliferation also
increased in these groups of ferrets. However, unlike the
high-dose groups, the low-dose beta-carotene plus smoking
group did not experience increased keratinized squamous
metaplasia of the lung and increased cyclin Dl (required for
cell proliferation). The smoking, high-dose group of ferrets
had threefold higher apocarotenal concentrations (the
excentric cleavage products from beta-carotene) in their
lung tissue.
These investigators hypothesized that at high doses, the
oxidative products of beta-carotene increase cytochrome P450 catabolism of retinol, leading to lower levels of retinoic
acid and loss of repression of metaplasia. Smoking increases
the concentration of these oxidative products. However, at
low concentrations, the apocarotenals may serve as precursors of retinoic acid and may explain the decreased metaplasia and cyclin-Dl expression these investigators found in
their low-dose, smoke-exposed ferrets. Since both vitamins
C and E have been shown to regenerate beta-carotene to its
unoxidized form (93, 94), these authors proposed that smokers with lower levels of these antioxidant vitamins would be
at higher risk from pharmacologic exposure to betacarotene. These data contribute to an emerging theme
related to carotenoids: high doses may be detrimental,
whereas low doses may be health promoting, and a mixture
of antioxidants must be present to obtain the full benefit.
Lycopene oxidation has just been shown to produce acycloretinal, and incubation with pig-liver homogenate generates acycloretinoic acid and a number of lycopenals (95).
Whether the acycloretinoids have retinoid-like activity and
whether lycopenals can decrease retinoic acid tissue concentrations have yet to be determined.
Tumor formation appears to be modulated by the function/activity of the insulin-like growth factor (IGF)-l receptor in target tissue cells (96). Beta-carotene has been able to
inhibit IGF-1-stimulated growth of a number of cancer cell
218
Arabetal.
lines at near physiologic concentrations (97). Lycopene has
increased membrane-associated IGF-binding proteins,
which negatively regulate IGF-1-receptor activation in cancer cells, and it has suppressed IGF-1-stimulated cell cycle
progression (98), an example of the direct action of
lycopene or perhaps its oxidation products on gene expression. It is highly unlikely that this effect is stimulated by a
retinoid-like fragment, although this hypothesis is under
active investigation.
An additional line of research indicating that carotenoids
may affect gene expression independent of their retinoid properties comes from a series of investigations of up-regulation of
connexin 43, one of a number of highly conserved proteins
that form the pores that connect one cell to another. The presence of beta-carotene, lycopene, canthaxanthin, and retinoids
in the cell medium upregulates connexin 43 and enhances
intracellular communication (99). Gap junctional communication is deficient in many human tumors, and its restoration
or up-regulation is associated with decreased proliferation.
Canthaxanthin was the most potent of the carotenoids and can
be converted to 4-keto-retinoic acid (shown to have strong
retinoic-acid-like activity), but an exhaustive investigation of
whether canthaxanthin is converted to 4-keto-retinoic acid in
vivo has been negative. However, the possibility still exists
that extremely small quantities of oxidation products of these
carotenoids are mediators for nuclear responses.
CANCER RISK ASSOCIATIONS WITH CONSUMPTION
OR PLASMA CONCENTRATIONS
Are there known or plausible adverse effects of these
compounds, and under what circumstances are they
expressed?
Excessive dietary intakes of beta-carotene and lycopene
can result in lycopenodermia and carotenodermia, both relatively harmless skin discolorations. Dietary consumption
of these carotenoids has not been associated with an
increased disease risk in humans. However, in two clinical
trials, use of supplements (beadlets) has led to serum concentrations not found with dietary intakes, and the interventions have proven detrimental. For the ATBC Study, the
baseline median concentration of beta-carotene was 0.32
H-mol/liter (17 |U.g/dl), which rose to 5.66 ^mol/liter (300
(jg/dl); for the Carotene and Retinol Efficacy Trial
(CARET), the median postintervention concentration was
3.96 |imol/liter (210 (ig/dl) (91, 100). Both studies showed
an excess of lung cancer in groups receiving supplementation. The finding that beta-carotene supplementation was
not only not helpful or innocuous but actually resulted in
18-26 percent increases in lung cancer risk gave cause for
alarm. A third beta-carotene study, the Physician's Health
Study, showed no excess of lung cancer in this population
(101). However, evidence from the two well-controlled
interventions in the United States and Finland was strong
enough to provide warnings. The DRI Committee on
Antioxidants concluded that beta-carotene supplements are
not advisable for the general population (9). Only short-term
studies have been performed with lycopene supplements or
tomato products, which have generated mean plasma con-
centrations of 0.5-0.75 (imol/liter (36, 37, 80). These concentrations are within the range of median concentrations
(0.56 nmol/liter) found in US populations not receiving supplements (102), a finding likely due to the lycopene depletion period used in bioavailability studies. It is not clear
whether, even in longer-term studies with highly bioavailable forms of lycopene, similarly high concentrations of
lycopene can be generated, as found in the beta-carotene
intervention trials.
Both beta-carotene and lycopene can act as prooxidants
as well as antioxidants. Circumstantial but prevalent is the
evidence of a detrimental effect of beta-carotene, especially
during oxidative stress (which may be the case for smokers,
drinkers of large quantities of alcohol, poor vitamin E and C
status, and a number of disease states associated with oxidative stress as well as prooxidative therapies such as
chemotherapy and radiography), particularly at higher
doses. Lycopene research has been insufficient to dismiss
the ability of high intakes to induce a prooxidative response
in vivo.
What is the evidence in humans for a role of betacarotene and lycopene in preventing cancer?
We searched for epidemiologic studies related to betacarotene, lycopene, and cancers at 12 organ sites. Several
review papers were used in the tabulation (103-105), and
new studies were identified through MEDLINE (National
Library of Medicine, Bethesda, Maryland) searches. For the
observational studies, we included only those with a cancer
endpoint, not precancerous endpoints such as colorectal adenomas. Because tomatoes are such a large contributor to
lycopene intake, studies that measured tomato and tomato
product consumption were included in the evidence relating
to lycopene. Over 200 observational epidemiologic studies
have reported on associations between beta-carotene exposure and cancer occurrence, and over 100 studies have
addressed lycopene and cancer. At least 23 placebocontrolled clinical trials with cancer or precancerous
endpoints have been conducted using beta-carotene supplementation, but we know of none that has been published in
which lycopene supplementation was used. Table 2 summarizes the results from the epidemiologic studies and clinical
trials for the different organ sites. Table 3 provides a more
detailed synopsis of the clinical trials.
Lung cancer observational epidemiologic studies. Lung
cancer is by far the most studied of the cancers in relation to
carotenoid exposure. Thirty-three of 37 case-control studies
examining beta-carotene intake or serum levels and lung
cancer risk found a protective effect; in 23, the effect was
statistically significant. One case-control study found a significant risk associated with consuming higher amounts of
beta-carotene (odds ratio (OR) = 2.0, n = 672 cases in
Shanghai, China) (106). Another study in China found a
significant risk associated with higher serum beta-carotene
concentrations when examined prospectively (107). In this
study, the highest tertile of serum beta-carotene was >19
(j.g/dl, and the highest risk was observed for alcohol
drinkers. Lycopene was undetectable in the serum of this
Epidemiol Rev Vol. 23, No. 2, 2001
Lycopene and Beta-Carotene in Carcinogenesis
TABLE 2.
219
Findings of epidemioiogic studies on the associations of beta-carotene and lycopene with cancer at various sites
Cancer site
and
type of study
IBeta-carotene
No.
of
studies
Negative association
Lycopene
Positive association
%
No.
%
No.
1
1
2
No.
Negative association
of
studies
%
17
9
0
Positive association
No.
%
41
0
0
7
0
0
0
Lung
Case-control
Cohort
Clinical trial
37
22
4
62
50
0
23
11
3
5
50
Stomach
Case-control
Cohort
Clinical trial
21
9
5
67
33
40
14
3
2
0
0
0
15
1
0
53
0
0
8
0
0
0
15
53
0
0
8
0
5
0
0
40
0
0
2
0
7
4
6
0
8
1
0
50
10
54
0
60
Colon/rectum
Case-control
Cohort
Clinical trial
14
9
7
50
0
0
7
0
0
0
6
1
0
50
0
0
3
0
0
0
Pancreas
Case-control
Cohort
Clinical trial
10
3
3
50
0
0
5
0
0
0
3
2
0
100
50
0
3
1
0
0
0
Cervix
Case-control
Cohort
Clinical trial
17
0
3
53
0
0
9
0
0
0
10
0
0
50
0
0
5
0
0
0
27
14
13
0
0
0
9
4
0
22
25
0
2
1
0
0
0
1
0
0
0
0
0
13
7
0
31
57
0
Esophagus
Case-control
Cohort
Clinical trial
Oral cavity, pharynx
Case-control
Cohort
Clinical trial
Breast
Case-control
Cohort
Clinical trial
0
2
13
1
0
0
0
0
0
0
0
0
0
0
0
1
48
21
0
Ovaries
Case-control
Cohort
Clinical trial
7
2
1
29
0
0
2
0
0
0
Prostate
Case-control
Cohort
Clinical trial
21
10
4
14
0
25
3
0
10
0
Bladder
Case-control
Cohort
Clinical trial
9
8
3
22
0
0
2
0
0
0
4
3
0
0
0
0
0
0
0
Skin
Case-control
Cohort
Clinical trial
7
5
5
29
2
1
0
0
0
1
1
0
0
0
0
0
0
0
20
0
3
1
Chinese population. The other 21 cohort studies of betacarotene and lung cancer risk found a protective effect, and
11 of these findings were statistically significant.
Epidemiol Rev Vol. 23, No. 2, 2001
1
No.
0
0
0
4
4
0
0
0
Seventeen case-control studies have examined tomato or
lycopene intake or plasma concentrations and lung cancer.
All showed odds ratios of less than 1.0, and seven produced
E 3. Findings of randomized controlled trials on the role of beta-carotene in human cancer chemoprevention*
r(s), year, and reference no.
Population
No. of subjects
Intervention
Endpoint
Result(s)
p
chetal., 1984(137)
Philippines—betel quid users
25
18
Beta-carotene, 180 mg/week Oral micronuclei
Placebo
i,t 66%
i, 1%
<0
chetal., 1985(138)
Inuits—smokeless tobacco users
23
31
Beta-carotene, 180 mg/week Oral micronuclei
Placebo
4, 60%
4, 0.5%
<0
chetal., 1988(139)
India—betel nut chewers
31
51
Beta-carotene, 180 mg/week Oral micronuclei
Beta-carotene and retinol,
100,000 lU/week
Placebo
4, 71%/1, 75%$
4, 71%/i, 71%
<0
<0
30
chetal., 1988(139)
India—betel nut chewers
27
51
33
dze et al., 1993(140)
Uzbekistan—oral leukoplakia,
esophagitis
384
14.8%
Beta-carotene, 180 mg/week Oral leukoplakia
(complete remission) 27.5%
Beta-carotene and retinol,
100,000 lU/week
Placebo
3%
Beta-carotene, 40 mg/day,
retinol, 100,000 IU/
week, and vitamin E,
80 mg/week vs. placebo
ewal et al., 1999 (142)
Larty et al., 1995 (143)
Leukoplakia prevalence
ORt = 0.62 (95% Clt:
33% of subjects
52% of subjects
10% of subjects
46
42
43
Beta-carotene, 360 mg/week Oral leukoplakia
(complete
Vitamin A, 300,000 lU/week
regression)
Placebo
United States
11
12
Beta-carotene, 60 mg/day
Placebo (following 6 months
of 60 mg/day of betacarotene for all subjects)
Oral leukoplakia
progression
18%
755 total
Beta-carotene, 50 mg/day
and retinol, 25,000 IU
every other day vs.
placebo
Sputum atypia
OR = 1.24 (95% Cl:
0.78, 1.96)
114 total
Beta-carotene, 20 mg/day
vs. placebo
Sputum micronuclei
i , 27% (95% Cl: 9, 41)
Second head-and-neck
cancer
RRt = 0.69 (95% Cl:
0.39, 1.25)
Poppel et al., 1992 (144) The Netherlands—male smokers
yne et al., 2001 (124)
United States
BCt study, 1994 (91)
Finland—male smokers
264 patients Beta-carotene, 50 mg/day
vs.placebo
29,133 total
<0
OR = 0.66 (95% Cl:
0.37,1.16)
India
United States—male asbestos
workers
N
<0
0.39, 0.98)
Progression/stable vs.
regression
291
nkaranarayanan et al.,
997(141)
T,t 8%/4, 7%
Lung cancer
Beta-carotene, 20 mg/day
with or without vitamin E,
Bladder cancer
50 mg/day vs. placebo
Colorectal cancer
Prostate cancer
Stomach cancer
Pancreatic cancer
<0
<0
N
17%
Beta-carotene: RR = 1.18
(95% Cl: 1.03, 1.36)
RR = 1.0
RR = 1.0
RR = 1.2
RR = 1.3
Incidence: RR = 0.75
Mortality: RR = 0.81
<0
<0
29,133 total
Beta-carotene, 20 mg/dl with Pancreatic cancer
or without vitamin E, 50
mg/day vs. placebo
RR (incidence) = 0.75
RR (mortality) = 0.81
talahti et al., 1999 (127)
Finland—male smokers (ATBC)
enn, 1996 (100)
United States—smokers and
18,314 total
asbestos workers (CARETt)
Beta-carotene, 30 mg/day
and retinol, 25,000 IU/
day vs. placebo
Lung cancer
RR = 1.28(95%CI:
104, 1.57)
nekens, 1996 (101)
United States—male physicians 22,071 total
Beta-carotene, 50 mg/day
every other day vs.
placebo
Prostate cancer
Lung cancer
Bladder cancer
Colorectal cancer
Pancreas cancer
Melanoma
No association
No association with any
cancers
(PHSt)
N
N
<0.
United States—male
physicians^
22,071 total
Beta-carotene, 50 mg/day
every other day vs.
placebo
Prostate cancer
32% reduction in risk
among those in the
lowest quartile of
baseline plasma betacarotene levels
United States—females#
39,873 total
Beta-carotene, 50 mg/day
every other day vs.
placebo
Breast cancer
Colorectal cancer
Lung cancer
Ovarian cancer
Bladder cancer
Pancreatic cancer
Cervical cancer
Stomach cancer
Melanoma
No association with any
cancers
enberg et al., 1990 (145) United States—prior skin cancer
1,805 total
Beta-carotene, 50 mg/day
vs. placebo
Second skin cancer
R R = 1.05 (95% Cl:
0.91, 1.22)
en et al., 1999 (146)
1,383 total
Beta-carotene, 30 mg/day
vs. placebo
Basal cell skin cancer
R R = 1.04 (95% Cl:
0.73, 1.27)
R R = 1.35 (95% Cl:
0.84, 2.19)
oketal., 1999(136)
etal., 1999(125)
ling et al., 2000 (147)
Queensland, Australia
Squamous cell skin
cancer
United States—PHS
Vet etal., 1991 (128)
Netherlands
mney et al., 1997 (129)
Bronx, New York (United States
22,071 total
Beta-carotene, 50 mg on
alternate days vs.
placebo
278 total
Beta-carotene, 10 mg/day
vs. placebo
69 total
Beta-carotene vs. placebo
Basal cell skin cancer
Squamous cell skin
cancer
R R = 0.99 (95% Cl:
0.92, 1.06)
R R = 0.97 (95% Cl:
0.84, 1.13)
CINf regression
O R = 0.68 (95% Cl:
.60)
0.28, 1.
CIN regression
<0.
O R = 1.53
Table co
3.
Continued
r(s), year, and reference no.
tetal., 1993(121)
al., 1993(123)
Population
Linxian County, China—general
population
Linxian County, China—
esophageal dysplasia
No. of subjects
29,584 total
3,318 total
Intervention
Endpoint
Beta-carotene, 15 mg/day,
vitamin E, 30 mg/day,
and selenium, 50 u.g/
day vs. placebo
Stomach cancer death
Beta-carotene, 15 mg/day
and multivitamin/multimineral vs. placebo
Stomach cancer death
Esophageal cancer
death
Esophageal cancer
death
Result(s)
RR = 0.79 (95% Cl:
0.64, 0.99)
RR = 0.96 (95% Cl:
<0
N
0.79, 1.18)
RR = 1.18(95%CI:
0.76, 1.85)
RR = 0.84 (95% Cl:
0.54,1.29)
reaetal., 2000 (122)
Colombia
773 total
Beta-carotene, 30 mg/day
vs. placebo
stad et al., 1998 (148)
Norway—adenoma
116 total
Polyp growth and
Beta-carotene, 15 mg/day
recurrence
with vitamin C, 150 mg/
day, vitamin E, 75 mg/day,
selenium, 101 ug/day,
and calcium, 1.6 g/day
vs. placebo
No difference between
placebo and intervention arm (no
separate betacarotene arm)
enberg et al., 1994
149)
United States—resected
adenoma
751 total
Recurrent adenoma
Beta-carotene, 25 mg/day
with or without vitamin C,
1 g/day and vitamin E,
400 mg/day vs. placebo
Beta-carotene: RR = 1.01
(95% Cl: 0.85, 1.20)
Lennan et al., 199
150)
Australia—resected adenoma
390 total
Beta-carotene, 20 mg/day
with or without wheat
bran, 25 g/day, with or
without low-fat diet vs.
placebo
Recurrent adenoma
Beta-carotene: OR = 1.4
(95% Cl: 0.8, 2.3) for 2
years; OR = 1.3 (95%
Cl: 0.8, 2.2) for 4 years
endall et al., 1991 (151)
United States—resected
adenoma
132*
125
Beta-carotene, 15 mg/day
Placebo
Recurrent polyps
29%
24%
Intestinal metaplasia
regression
p
RR = 3.4(95%CI:1.1,9.8)
<0
able was adapted from Mayne and Lippman (152).
negative association; T, positive association; NS, not significant; OR, odds ratio; Cl, confidence interval; RR, relative risk; ATBC, Alpha-Tocopherol, Beta Carotene;
ne and Retinol Efficacy Trial; PHS, Physicians' Health Study; CIN, cervical intraepithelial neoplasia.
hange in leukoplakia and normal mucosa, respectively.
his is considered a positive outcome, because all subjects received supplementation with 60 mg/day of beta-carotene for 6 months prior to randomization into place
ment groups for another 6 months. Only two subjects in both the placebo and supplemented groups progressed following the second 6 months of the trial.
ased on nested case-control analyses following intervention.
his trial was stopped prematurely after an average of 2.1 years of intervention.
umber completing 36 months.
Lycopene and Beta-Carotene in Carcinogenesis
statistically significant results. Only two of seven cohort
studies found tomato consumption or lycopene to be protective for lung cancer, and none of the findings was statistically significant.
Of the 12 case-control studies that measured both betacarotene and lycopene exposures in relation to lung cancer,
most (n = 7) found a significant protective effect of betacarotene intake or plasma concentrations, while only four
reported significant protection from lycopene or tomatoes.
Seven cohort studies measured both beta-carotene and
lycopene, and only two found significant effects for either
carotenoid. One study demonstrated that higher serum concentrations of beta-carotene were protective (relative risk
(RR) = 0.44, p < 0.05) and that higher serum lycopene concentrations were not (RR = 1.01, not significant) (108). A
recent prospective analysis used data from the Health
Professionals Follow-up Study and the Nurses' Health
Study and found a reduced risk of lung cancer with higher
intakes of lycopene and beta-carotene, although the findings
for lycopene only were statistically significantly (109).
Thus, for lung cancer, the observational evidence is mixed
but appears to be more consistent for beta-carotene. In light
of the two observational studies showing an increased risk
of lung cancer with increasing beta-carotene exposure, it
may be interesting to examine the relation more closely in
China.
Lung cancer clinical trials of beta-carotene. On the basis
of abundant evidence accumulated in the 1980s of a protective effect of a high fruit and vegetable and hence a high
beta-carotene diet on lung cancer risk, clinical trials investigating beta-carotene supplementation in high-risk smokers
and asbestos workers were undertaken. The ATBC study, a
randomized clinical trial in which White male smokers in
Finland received 20 mg/day of beta-carotene (in the form of
10 percent water-soluble beadlet capsules) for an average of
6 years, found an 18 percent increased risk of lung cancer in
those receiving the supplements compared with those
receiving placebos (91). This increase in contrast to the
expected decrease in lung cancer within such a short period
of observation suggests that beta-carotene can play an
important role in the latter stages of carcinogenesis—tumor
growth and progression.
The CARET study was terminated early (after less than 4
years) because of the ATBC study results and preliminary
analyses of the CARET data, which showed a 26 percent
increased risk of lung cancer in those smokers and asbestos
workers taking the 30-mg beta-carotene and 25,000-IU
retinol supplement (110). Further investigation of the ATBC
study data revealed that those subjects who smoked the most
and who drank alcohol were at highest risk (111). In contrast, the Physician's Health Study found no effect, positive
or negative, of beta-carotene supplementation on lung cancer risk in a group of 22,000 male physicians, half of whom
used 50-mg beta-carotene supplements every 2 days for 12
years (101). Only 11 percent of these subjects were current
smokers. Differences in the study populations and in the
dose and subsequent plasma concentrations obtained
(median values of 120 u_g/dl in the Physician's Health Study,
210 u.g/dl in the CARET study, and 300 |ig/dl in the ATBC
Epidemiol Rev Vol. 23, No. 2, 2001
223
Study as compared with the 15-30 (ig/dl value that the
observational studies found to be protective) may explain
some of the differences in the results observed in these trials
and suggest a threshold plasma concentration beyond which
beta-carotene is not protective and may in fact be harmful
(112, 113). Longer follow-up of all three of these study populations should answer the question of whether there is a
preventive effect of high exposures earlier in the development of cancers, which would be consistent with an antioxidant role preventing initiation of lung cancer in vulnerable
groups.
A number of hypotheses for the failure of clinical trials to
show a protective effect have been suggested and reviewed
elsewhere. The protective effect of fruit and vegetable consumption on cancer risk observed in epidemiologic studies
may have been the result of another nutrient or phytochemical or a combination of phytochemicals in fruits and vegetables rather than beta-carotene. If, in fact, other
carotenoids besides beta-carotene are protective, then high
dosing of beta-carotene may have competed with the other
carotenoids for absorption, thus increasing risk. From in
vitro and animal studies, we know that beta-carotene can act
as a prooxidant at high doses and under high oxidative stress
conditions as may occur in smokers (89,90,114). In support
of this hypothesis, further analyses of the ATBC study data
showed that the increased risk of lung cancer in subjects
receiving supplementation was limited to those who smoked
one or more packs of cigarettes per day (111).
Little is known about the oxidative metabolites of betacarotene. The elegant study of Liu et al. (115) that used
smoke-exposed ferrets points to two possible routes of interference for pharmacologic doses of beta-carotene. One is
through the direct effect of carotene oxidation products,
which may up-regulate API; another is by depressing retinoic
acid concentration, which down-regulates retinoic acid receptor/retinoid X receptor activity, which again up-regulates
API. As a result of the up-regulation of cyclin Dl, increased
lung cell proliferation occurs. In these studies, high doses of
beta-carotene designed to approximate those in the ATBC
trial, combined with smoke exposure, yielded the most proliferative sequence of events and lung metaplasia, whereas lowdose beta-carotene plus smoke exposure had no detrimental
effects and may even have afforded weak protection. It is possible that a metabolite could inhibit retinoic-acid-induced
transcription of tumor suppressor genes by blocking the binding of retinoic acid to its receptor (116). A sequence of events
leading to the change in gene expression and cell proliferation
by the combination of beta-carotene and tobacco smoke has
been proposed (117). It is possible that lifelong smoking
habits produce too strong of a cumulative effect on carcinogenesis to make a short-term intervention effective late in life
(113). In fact, beta-carotene may be protective only in the initiation phase of carcinogenesis, although in vitro studies suggest it is also effective in inhibiting cell proliferation (97,
118). Evidence of antioxidant depletion being related to lower
tumor numbers and sizes via inhibition of tumor cell apoptosis has also recently been reported in mice (119). While all of
these hypotheses are plausible, the unexpected results of the
lung cancer trials remain unexplained.
224
Arab et a!.
Stomach cancer. For stomach cancer, the evidence from
observational studies supportive of a protective effect is
similar for both beta-carotene and lycopene. All 21 casecontrol studies measuring beta-carotene found protective
effects ranging from a 17 to 95 percent risk reduction for the
highest levels of beta-carotene exposure. Over half (n = 1 4 )
of these results were statistically significant. For lycopene,
12 of the 15 case-control studies found odds ratios of less
than 1.0 for dietary intake of tomatoes or lycopene, and
eight of these odds ratios were statistically significant. No
case-control studies found significant adverse effects of
beta-carotene or lycopene for stomach cancer.
The cohort studies for stomach cancer are limited in
power because of small sample sizes. Stomach cancer is a
relatively rare disease, particularly in economically developed countries. Even so, three of the nine cohort studies
examining beta-carotene found a significant reduction in
risk of about 70 percent for those subjects in the highest
plasma or dietary beta-carotene intake group. However, the
most recent and largest cohort study (and the only one to
measure lycopene intake) did not find a protective effect for
either carotenoid; for those in the highest quintile of betacarotene intake, the relative risk of stomach cancer was 1.6
in the multivariate analyses (not significant,/? = 0.13), and
the relative risk for lycopene intake was 1.1 (not significant,
p = 0.16) (120).
The results from clinical trials regarding beta-carotene
and stomach cancer have been mixed. In the trial in Linxian,
China, where stomach cancer is still common and malnutrition prevalent, a reduction was found in stomach cancer
deaths for subjects receiving a 15 mg/day beta-carotene, 30
mg/day vitamin E, and 50 fig/day selenium supplement versus placebo (121). However, the large ATBC Study found
somewhat more cases of stomach cancer in those subjects
receiving a 20 mg/day beta-carotene supplement versus
placebo (RR = 1.3, not significant) (91). A more recent trial
in Colombian subjects with intestinal metaplasia found that
those treated with 30 mg/day of beta-carotene for 72 months
were three times more likely to experience regression than
placebo-treated subjects (122). The effect of beta-carotene
was similar to that of anti-Helicobacter pylori treatment and
ascorbic acid supplementation. Future clinical trials in this
research area will be helpful in confirming this finding and
determining whether the effect is evident only in undernourished populations or in other populations as well.
Esophageal cancer. Esophageal cancer etiology has
been studied less frequently in relation to carotenoids than
stomach or lung cancer has. Fourteen of the 15 case-control
studies of beta-carotene found protective associations, and
eight reported statistically significant findings. Only two of
five case-control studies measuring lycopene found a significant protective effect of tomato consumption on esophageal
cancer risk. No known cohort studies have reported on this
disease in relation to exposure to these carotenoids. Two
clinical trials in Linxian, China, did not find a significant
protective effect of 15 mg/day beta-carotene in combination
with vitamin E, selenium, or a multivitarnin/multirnineral
supplement on esophageal cancer death (121, 123). In addition, exposure estimates may be biased by disease status in
case-control studies. Nevertheless, there is no clear association between beta-carotene or lycopene and esophageal cancer risk.
Oral cavity, pharyngeal cancers. For oral cavity and
pharyngeal cancers, 12 of 13 case-control studies reported
reduced odds ratios for the highest levels of beta-carotene
intake, and seven found statistically significant odds ratios.
For lycopene, seven of eight case-control studies observed a
reduction in risk associated with the highest level of
lycopene; in four (50 percent), findings were statistically
significant. Only two prospective studies have measured
beta-carotene or lycopene in relation to oral cancers, and
neither found statistically significant associations. The small
numbers of cases in these studies may have limited their
power to detect an effect.
The most promising evidence of an effect of betacarotene on oral cancers comes from the 10 placebocontrolled clinical trials that have been conducted. In six of
these trials, supplementation with beta-carotene alone
resulted in either significant improvements or prevention of
oral leukoplakia; in another study, beta-carotene plus retinol
resulted in complete remission of oral leukoplakia. In these
studies, supplementation doses ranged from 20 mg/day to
360 mg/week. The most recent trial examined the effect of
supplementation with 50 mg/day of beta-carotene for an
average of 51 months on second head and neck cancers in
264 subjects (124). There was a suggestion of a reduced risk
of second head and neck cancers in the subjects receiving
supplementation compared with placebo-treated subjects,
but the association was not statistically significant (RR =
0.69, 95 percent confidence interval: 0.39, 1.25). Thus,
while the observational data are mixed, results of clinical trials suggest that supplementation with beta-carotene can
improve the chances of remission from early lesions of oral
cancer.
Colorectal cancer. Half of the case-control studies of
colorectal cancer and both beta-carotene and lycopene have
found a significant risk reduction for those subjects exposed
to the highest levels of these carotenoids. However, the
cohort and clinical trial data do not strongly support these
results. None of the nine cohort studies measuring betacarotene or the one measuring lycopene found significant
effects of either carotenoid. In fact, five of the nine cohort
studies of beta-carotene reported relative risks of more than
1.0. Similarly, seven clinical trials found that supplementation with 20-25 mg/day of beta-carotene resulted in no
effect or a slight suggestion of an increased risk of recurrent
adenoma, and no effect on colorectal cancer rates was
observed in the ATBC Study (91), the Physician's Health
Study (101), or the Women's Health Study (125). Thus,
beta-carotene does not appear to be protective against colorectal cancer, and there are too few data on lycopene to
reach a definite conclusion.
Pancreatic cancer. While pancreatic cancer has not
been studied as frequently as some of the other cancers in
relation to carotenoids, the effect of lycopene or tomato
exposure on risk reduction is consistent. All three casecontrol studies of lycopene and pancreatic cancer found a
significant reduced risk of 77-84 percent for those subjects
Epidemiol Rev Vol. 23, No. 2, 2001
Lycopene and Beta-Carotene in Carcinogenesis
with the highest lycopene intakes or serum concentrations.
One of the two cohort studies of lycopene reported a statistically significant risk reduction of 81 percent, and the other
reported an inverse association between tomato consumption and pancreatic cancer risk but did not report the relative
risk estimate.
For beta-carotene, all 10 case-control studies showed
inverse associations with pancreatic cancer (five were statistically significant), except for a subgroup analysis in one
study that observed a positive association between betacarotene intake and pancreatic cancer for women in
Louisiana (126). However, the cohort studies of betacarotene did not find significant associations, nor did the
three clinical trials, the ATBC Study (127), the Physician's
Health Study (101), and the Women's Health Study (125),
which included pancreatic cancer as an endpoint. Thus, it
appears that lycopene exposure may be more important than
beta-carotene in reducing the risk of pancreatic cancer.
Cervical cancer. While there does seem to be evidence
of a protective effect of beta-carotene and lycopene on cervical cancer, the data are mixed. Of the nine case-control
studies that measured both carotenoids, four found a significant protective association of beta-carotene but not
lycopene, and three found a significant protective association of lycopene but not beta-carotene. These data point to
possibly large differences in whole population exposures to
these carotenoids. Low overall exposure may limit the ability to evaluate a risk reduction effect. There are no known
cohort studies to help clarify the associations. However,
three clinical trials found no significant effect of betacarotene supplementation on the incidence of cervical cancer or on the regression of cervical intraepithelial neoplasia
(125, 128, 129).
Breast cancer. Thirteen of 27 case-control studies found
significant inverse associations between beta-carotene and
breast cancer. Only two of nine case-control studies of
lycopene found significant protective associations. One was
a small study that used breast adipose tissue concentrations
of the two carotenoids as the exposure and found an approximate 70 percent reduction in risk for both carotenoids
(130). The cohort studies do not strongly support an association between either carotenoid and breast cancer. Only
three of 14 cohort studies measuring beta-carotene found a
significant reduction in risk, and only one of four measuring
lycopene observed a significant risk reduction. One clinical
trial of beta-carotene supplementation followed breast cancer as an endpoint but found no association between supplementation and breast cancer incidence (128).
In a recent meta-analysis, Gandini et al. examined the
association between breast cancer and diet (131). A number
of studies were excluded from the analyses of beta-carotene
results because no cutpoints for categories of beta-carotene
intake were provided in the studies. This is a problem
throughout the literature. By not providing cutpoints, it is
impossible to quantitatively compare the results across studies or determine the threshold levels whereby intake may in
fact become harmful, as evidenced by the clinical trials.
Ovarian cancer. Ovarian cancer is the least studied of
the cancers reviewed here in relation to carotenoid exposure.
Epidemiol Rev Vol. 23, No. 2, 2001
225
Five of the seven case-control studies of beta-carotene
found odds ratios of less than 1.0, although only two were
statistically significant. A small, nested case-control study
measuring serum carotenoids found a nonsignificant
increased risk of ovarian cancer with higher serum lycopene
concentrations (OR = 1.36) and no association of betacarotene (OR = 0.93) (132). No significant effects of betacarotene were observed in two cohort studies, and no known
cohort studies have measured lycopene exposure in relation
to ovarian cancer. Thus, more research is needed regarding
this cancer before conclusions can be drawn.
Prostate cancer. The protective association of lycopene
and tomato intake with prostate cancer has received significant attention in the past few years. Interestingly, very few
case-control studies support this association; only four of 13
have found a significant reduced risk of prostate cancer for
men consuming the highest amounts of tomatoes or
lycopene. The evidence for an association is strongest in the
cohort studies, however. Except for tomato juice examined
in a large prospective cohort study in the United States,
Giovannucci et al. found risk reductions of 25^40 percent
for men consuming the greatest amounts of tomatoes and
tomato products (133). However, another more recent study
from the Netherlands found no protective effect of tomato
intake on prostate cancer risk (134). We conducted a metaanalysis of the epidemiologic studies of prostate cancer and
lycopene and found a modest effect (unpublished observation). The fact that lycopene is the most abundant carotenoid
in prostate tissue supports the hypothesis that it might play
a role in prostate cancer prevention. Nevertheless, only randomized trials can fully answer this question.
Evidence that beta-carotene may protect against prostate
cancer is very weak. Only three of 21 case-control studies
found a significant protective association, and none of the
10 cohort studies found a significant reduction in risk.
One, a small study from Finland, actually found a significant increased risk of prostate cancer for men with the
highest plasma concentrations of beta-carotene (135).
Three of the four clinical trials found no association of
beta-carotene supplementation with prostate cancer incidence. However, in a reanalysis of Physician's Health
Study data, Cook et al. reported a 32 percent reduction in
prostate cancer incidence for those men receiving betacarotene supplementation who were in the lowest quartile
of plasma beta-carotene when the study began (136). This
insight is interesting and lends support to the idea that
beta-carotene may be protective at the doses available
from dietary intake alone rather than from megadoses in
supplement form. It also may reflect the idea that circulating beta-carotene is a marker for other healthful lifestyle
behaviors or protective dietary components.
Bladder cancer. There is little evidence to support a role
of beta-carotene in preventing bladder cancer. Two of nine
case-control studies found significant inverse associations
between increasing dietary intake of beta-carotene and bladder cancer. Similarly, four case-control studies of lycopene
found a risk reduction of 10-30 percent, although none of
the results was statistically significant. None of the eight
cohort studies of beta-carotene or the three cohort studies of
226
Arab et al.
lycopene revealed a significant reduction in the risk of bladder cancer. The ATBC Study (91), Physician's Health Study
(101), and Women's Health Study (125) clinical trials all
included bladder cancer as an endpoint, but all found no
effect of beta-carotene supplementation on bladder cancer
incidence.
Skin cancer. Few observational studies have examined
the relation of carotenoid exposure and skin cancer in
humans. On the basis of the small number of studies that
have been conducted to date, neither beta-carotene nor
lycopene appears to be protective for melanomas or for
basal cell or squamous cell skin cancers. The cohort studies
are plagued by small numbers of cases, limiting their power
to detect an effect. In contrast, several clinical trials have
been conducted, but none has found an effect of betacarotene supplementation on either second skin cancer or
skin cancer incidence.
Summary. In summary, our tabulation of studies examining cancer at 12 different organ sites found 283 risk estimates for beta-carotene exposure and 121 risk estimates for
lycopene or tomato exposure. As noted in table 1, a similar
proportion of studies found significant protective effects for
both beta-carotene (40 percent) and lycopene (35 percent).
Three studies of beta-carotene reported an increased risk of
cancer (lung and prostate) for those subjects in the highest
quantile of exposure, but no studies found that lycopene was
significantly associated with an increased risk at any organ
site.
Although the inverse association between lycopene and
prostate cancer has received considerable attention, the
consistency of the evidence in observational studies of
prostate cancer is similar to that for other organ sites such
as pancreatic, cervical, colorectal, oral, and stomach.
However, fewer cohort studies of these sites have been
conducted.
The marked discrepancy between the clinical trials findings, which have been mostly null (with the exception of oral
and pharyngeal cancers) and the primarily protective effect
observed in the epidemiologic studies may be related to differences in dose and delivery of beta-carotene, measurement
error, and confounding. Large doses of beta-carotene from
highly bioavailable beadlets, as given in the clinical trials,
elevate plasma levels significantly more than those levels
observed in epidemiologic studies. Furthermore, epidemiologic studies are often limited by poor or semiquantitative
measurement of dietary carotenoid intakes. In addition, they
have, to date, rarely controlled adequately for other lifestyle
factors correlated with carotenoid consumption that may be
confounding the associations.
What elements must be included in the next generation
of ecologic, case-control, and cohort studies to advance
our knowledge about carotenoids and cancer?
The lack of consistent findings in observational epidemiologic studies regarding the role of carotenoids in carcinogenesis can reflect a number of factors, including a weak or
nonexistent underlying association, confounding by related
behaviors, mismeasurement of carotenoids, or heterogeneity
of effect such that associations in a subpopulation of respon-
ders are being diluted by nonresponders in the study population. If these issues are not resolved, more epidemiologic
studies of the same genre cannot be expected to add significantly to our knowledge.
Improvements in the measurement of bioavailable
carotenoids may provide the most significant contribution to
the next generation of studies. To date, few, if any, studies
include dietary assessment approaches that support ascertainment of the bioavailability of the carotenoids under
study. Assessment of whether the food source is consumed
raw or cooked, the preparation method used, and the presence of fat at the time of ingestion would improve estimates
of the true and available dose of the nutrient.
In light of current and increasing levels of food coloring
and food fortification, these approaches need to be known
and included in the estimates. Because supplements are
becoming more popular in the United States and because
beta-carotene and lycopene are now available in multivitamins and individually, studies should accurately assess and
calculate the contributions from these supplemental sources
(e.g., vitamins, natural supplements) to the total exposure
measures.
Studies will also become more valuable when relevant
covariates and potential confounders are assessed and are
incorporated into the analyses. For example, if lycopene
intake is derived from pizza and pasta that are rich in fat and
may negatively contribute to the dietary profile, dietary fat
types should be controlled for in the analyses. Similarly, if
people who consume pizza and pasta with tomato sauces
more frequently differ from others in other aspects of their
diets and lifestyles, these items may be confounders camouflaging a protective association. To date, sophisticated nutritional epidemiologic analyses have not been applied to
control for dietary colinearity and behavior clustering, and
this deficit can readily be overcome.
Intelligent use of exposure biomarkers in lieu of dietary
assessment with subjective methods to using carotenoids,
membranes, blood lipids, and adipose tissue in studies while
considering the factors influencing availability and catabolism can also enhance our understanding. Use of exposure
biomarkers of this nature, which may in fact be markers of
eating patterns, is not free of the concerns about other dietary
covariates. However, these biomarkers can reduce measurement error and bias. In addition, the information on cis- and
fra/w-isomers available in biomarker measures may enhance
our understanding of both their role and their mechanisms of
effect. If we can begin to identify responders, based on the
presence of a genetic polymorphism or active phenotype,
studies can be designed to include only this population, from
which a stronger effect can be expected if carotenoids are
anticarcinogenic.
Finally, in this as in all areas of nutritional epidemiology,
the success of future epidemiologic study contributions
depends on their design being biologically driven, on choosing and acquiring the most accurate and unbiased exposure
measures. As we move toward greater understanding of
interactions and gradients of individual risk, future studies
must be designed for diversity, not, as they have in the past,
for central tendencies.
Epidemiol Rev Vol. 23, No. 2, 2001
Lycopene and Beta-Carotene in Carcinogenesis
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