Also indexed as: Carotenes
What do they do?
Carotenoids are a highly colored (red, orange, and yellow) group of fat-soluble plant pigments. All organisms, whether
bacteria or plants, that rely on the sun for energy contain carotenoids. Their antioxidant effects enable these compounds to play a crucial
role in protecting organisms against damage during photosynthesis—the process of
converting sunlight into chemical energy.
In humans, carotenoids play two primary roles: All exert antioxidant activity, but some are
also converted into vitamin A. Of the 600 carotenoids that
have been identified, about 30 to 50 are believed to have vitamin A activity. Carotenoids the
body converts to vitamin A are referred to as "provitamin A" carotenoids. The most well known
of this group are beta-carotene and alpha-carotene.
Some of the better known carotenoids without provitamin A activity—but with very high
antioxidant activity—are lutein, lycopene, and zeaxanthin.1 2
Preliminary and experimental studies suggest that a higher dietary intake of carotenoids
offers protection against developing certain cancers
(e.g., lung, skin, uterine, cervix, gastrointestinal tract), macular degeneration, cataracts, and other health conditions linked to oxidative or
free radical damage.3 4
5 6 However, two double-blind studies have shown that supplementation with
isolated synthetic beta-carotene does not reduce the risk of lung cancer and may even increase that risk in
smokers.7 8 This finding suggests that foods that are high in
carotenoids may protect against cancer in humans for reasons unrelated to their carotenoid
content, that synthetic beta-carotene may have different effects from natural beta-carotene
(which is somewhat structurally distinct), or that carotenoids may need to be taken together
or with supportive antioxidants (e.g., vitamin C, vitamin E, selenium) in order
to reduce the risk of cancer. Researchers have yet to determine which of these possibilities
is true.
A high intake of carotenoids from dietary sources has been shown to be protective against
heart disease in several population-based
studies.9 10 However, a high level of these antioxidants might simply be
a marker for diets high in fruits and vegetables known to contain protective substances other
than carotenoids. Furthermore, a diet rich in carotenoids tends to be lower in saturated fat and cholesterol and higher in
fiber, which has also found to be protective against heart disease.
Because of their antioxidant activity, it has been
suggested that beta-carotene and other carotenoids
might protect against atherosclerosis by
preventing oxidative damage to serum cholesterol. However, research is conflicting in this
area. One thing is clear—carotenoids are significantly less effective in protecting
against damage to serum cholesterol than is vitamin E.
While feeding people beta-carotene has been shown to prevent oxidative damage to cholesterol
in some trials,11 other studies have reported that beta-carotene does not protect
cholesterol from oxidative damage.12 13
Just as in the case of cancer prevention, while a high intake of carotenoid-rich foods
appears to be protective against cardiovascular disease, the same is not true for
supplementation with synthetic beta-carotene. Double-blind intervention trials wherein people
are supplemented with beta-carotene alone or placebo have not found benefit for synthetic
beta-carotene supplementation. In fact, three of four trials have reported a higher
risk of cardiovascular disease in the
beta-carotene groups compared with those receiving placebo.14 15
16 17 While these outcomes prove that synthetic beta-carotene does not
protect against heart disease, the effects of natural beta-carotene and other carotenoids have
yet to be tested in intervention trials assessing effects on heart disease.
A potential problem with much of the research on carotenoids in cardiovascular disease has
been the focus on beta-carotene. A preliminary study found a strong association between
dietary sources of lycopene, not beta-carotene, and reduced
risk of heart attacks.18 Lycopene exerts
greater antioxidant activity compared to beta-carotene, and lycopene has also been reported to
protect cholesterol against oxidative damage.19
Where are they found?
Carotenoids are found in all plant foods. In general, the greater the intensity of color,
the higher the level of carotenoids. In green leafy vegetables, beta-carotene is the predominant carotenoid. In the orange
colored fruits and vegetables—such as carrots, apricots, mangoes, yams, winter squash—beta-carotene concentrations
are high, but other pro-vitamin A carotenoids typically predominate. Yellow vegetables have
higher concentrations of yellow carotenoids (xanthophylls), hence a lowered pro-vitamin A
activity; but some of these compounds, such as lutein, may
have significant health benefits, potentially due to their
antioxidant effects. The red and purple vegetables and fruits—such as tomatoes, red
cabbage, berries, and plums—contain a large portion of non-vitamin
A–active carotenoids. Legumes, grains, and
seeds are also significant sources of carotenoids. Carotenoids are also found in various
animal foods, such as salmon, egg yolks, shellfish,
milk, and poultry. A variety of carotenoids
is also found in carrot juice and “green
drinks” made from vegetables, dehydrated barley greens, or wheat grass.
Synthetic beta-carotene is available as a supplement. Mixed carotenoids (including the
natural form of beta-carotene) are also available in supplements derived from palm oil, algae,
and carrot oil.
Carotenoids have been used
in connection with the following conditions (refer to the individual
health concern for complete information):
Who is likely to be deficient?
Carotenoid deficiency is not considered a classic nutritional deficiency like scurvy or
beri-beri (severe vitamin C and vitamin B1 deficiencies, respectively). However, given the
possible health benefits of carotenoids, most doctors recommend adequate intake. People who do
not frequently consume carotenoid-rich foods or take carotenoid supplements are likely to be
taking in less than adequate amounts, though optimal levels remain unknown. Also, deficiency
may be found in people with chronic diarrhea or other
disorders associated with impaired absorption.
How much is usually taken?
Whether people who already consume a diet high in
fruits and vegetables would benefit further
from supplementation with a mixture of carotenoids remains unknown. While smokers clearly
should not supplement with isolated synthetic
beta-carotene, the effect in smokers of taking either natural beta-carotene or mixed
carotenoids is not clear.
Nonetheless, based on health-promoting effects associated with these levels in preliminary
research, some doctors recommend that most people supplement with up to 25,000 IU (15 mg) per
day of natural beta-carotene and approximately 6 mg each of alpha-carotene, lutein, and lycopene.
Are there any side effects or interactions?
Carotenoids are generally regarded as safe, based primarily on studies with beta-carotene. Increased consumption of carotenoids may cause
to the skin to turn orange or yellow—a condition known as “carotenodermia.”
This occurrence is completely benign and is unrelated to jaundice—the yellowing of the
skin that can result from liver disease or other causes.
Until more is known, people especially smokers should not supplement with synthetic
beta-carotene. Two double-blind studies have shown that supplementation with isolated
synthetic beta-carotene may increase the risk of lung
cancer in people who smoke.20 21 Moreover, three of four studies
have found small increases in the risk of
heart disease in people assigned to take synthetic beta-carotene compared with those
assigned to take placebo.22 23 24 25
Are there any drug
interactions?
Certain medicines may interact with carotenoids. Refer to drug interactions for a list of those medicines.
References
1. Krinsky NI. The antioxidant and biological properties of the
carotenoids. Ann NY Acad Sci 1998;854:443-7 [review].
2. Russell RM. Physiological and clinical significance of carotenoids.
Int J Vitam Nutr Res 1998;68:349-53 [review].
3. Cooper DA, Eldridge AL, Peters JC. Dietary carotenoids and lung
cancer: a review of recent research. Nutr Rev 1999;57:133-45 [review].
4. Gerster H. Anticarcinogenic effect of common carotenoids. Int J
Vitam Nutr Res 1993;63:93-121.
5. Rock CL. Carotenoids: biology and treatment. Pharmacol Ther
1997;75:185-97 [review].
6. Landrum JT, Bone RA, Kilburn MD. The macular pigment: a possible role
in protection from age-related macular degeneration. Adv Pharmacol
1997;38:537-56.
7. Albanes D, Heinone OP, Taylor PR, et al. Alpha-tocopherol and
beta-carotene supplements and lung cancer incidence in the Alpha-Tocopherol, Beta-Carotene
Cancer Prevention Study: effects of base-line characteristics and study compliance. J Natl
Cancer Inst 1996;88:1560-70.
8. Omenn GS, Goodman GE, Thornquist MD, et al. Effects of a combination
of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J
Med 1996;334:1150-5.
9. Kritchevsky SB. Beta-carotene, carotenoids and the prevention of
coronary heart disease. J Nutr 1999;129:5-8 [review].
10. Palace VP, Khaper N, Qin Q, Singal PK. Antioxidant potentials of
vitamin A and carotenoids and their relevance to heart disease. Free Radic Biol Med
1999;26:746-61.
11. Levy Y, Ben-Amotz A, Aviram M. Effect of dietary supplementation of
different beta-carotene isomers on lipoprotein oxidative modification. J Nutr Environ
Med 1995;5:13-22.
12. Reaven PD, Ferguson E, Naveab M, Powell FL. Susceptibility of human
LDL to oxidative modification. Effects of variations in beta-carotene concentration and oxygen
tension. Arterioscler Thromb 1994;14:1162-9.
13. Reaven PD, Khouw A, Beltz WF, et al. Effect of dietary antioxidant
combinations in humans. Protection of LDL by vitamin E but not by beta-carotene.
Arterioscler Thromb 1993;13:590-600.
14. Greenburg ER, Baron JA, Karagas MR, et al. Mortality associated with
low plasma concentration of beta carotene and the effect of oral supplementation.
JAMA 1996;275:699-703.
15. Omenn GS, Goodman GE, Thornquist MD, et al. Effects of a combination
of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J
Med 1996;334:1150-5.
16. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group.
The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in
male smokers. N Engl J Med 1994;330:1029-35.
17. Hennekens CH, Buring JE, Manson JE, et al. Lack of effect of
long-term supplementation with beta carotene on the incidence of malignant neoplasms and
cardiovascular disease. N Engl J Med 1996;334:1145-9.
18. Kohlmeier L, Kark JD, Gomez-Gracia E, et al. Lycopene and myocardial
infarction risk in the EURAMIC Study. Am J Epidemiol 1997;146:618-26.
19. Argarwal S, Rao AV. Tomato lycopene and low density lipoprotein
oxidation: a human dietary intervention study. Lipids 1998;33:981-4.
20. Albanes D, Heinone OP, Taylor PR, et al. Alpha-tocopherol and
beta-carotene supplements and lung cancer incidence in the Alpha-Tocopherol, Beta-Carotene
Cancer Prevention Study: effects of base-line characteristics and study compliance. J Natl
Cancer Inst 1996;88:1560–70.
21. Omenn GS, Goodman GE, Thornquist MD, et al. Effects of a combination
of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J
Med 1996;334:1150–5.
22. Greenburg ER, Baron JA, Karagas MR, et al. Mortality associated with
low plasma concentration of beta carotene and the effect of oral supplementation.
JAMA 1996;275:699–703.
23. Omenn GS, Goodman GE, Thornquist MD, et al. Effects of a combination
of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J
Med 1996;334:1150–5.
24. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group.
The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in
male smokers. N Engl J Med 1994;330:1029–35.
25. Hennekens CH, Buring JE, Manson JE, et al. Lack of effect of
long-term supplementation with beta carotene on the incidence of malignant neoplasms and
cardiovascular disease. N Engl J Med 1996;334:1145–9.
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The information presented in Healthnotes is for informational
purposes only. It is based on scientific studies (human, animal, or in vitro),
clinical experience, or traditional usage as cited in each article. The results reported may
not necessarily occur in all individuals. For many of the conditions discussed, treatment with
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before making any changes in prescribed medications. Information expires March 2005.
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