Carotenoids are red, orange, and yellow pigments found in fruits and vegetables. About 600 carotenoids have been identified. All of them have
properties; some of them can be converted in the body to
, and these are called “provitamin A” carotenoids.
Some of the most well-known carotenoids are
and zeaxanthin. The results of some but not all
suggest that a diet high in these carotenoids can reduce the risk of developing various illnesses, including cardiovascular disease, age-related vision loss, and various types of cancer.
These findings led to large-scale studies of synthetic beta-carotene for preventing
(especially lung cancer),
. To everyone’s surprise, the results showed at best no benefit, and, at worst, a possible increase in risk.
Many proponents of alternative medicine considered this outcome paradoxical, and attempted to explain it away in various ways:
- Beta-carotene alone may not be as useful as mixed carotenoids (and other healthful substances) found in fruits and vegetables.
- Synthetic beta-carotene may be less effective than natural beta-carotene.
- The participants in these studies were the wrong group of people (generally, smokers).
However, while any of these explanations may be correct, it is also quite possible that carotenoids simply do not provide any of the healthful effects attributed to them. Observational studies are notoriously unreliable for proving a treatment effective. Such studies only find associations between events, rather than cause and effect. It is quite possible, for example, that people who tend to eat more fruits and vegetables may be healthier in various other ways than those who do not, and that these other factors account for the apparent improvements. Consider the history of medical beliefs about hormone replacement therapy (HRT) for menopausal women. Observational studies had found evidence that women who used hormone replacement therapy had less heart disease, and on this basis millions of women were prescribed HRT. However, when proper double-blind studies were done, the results indicated that HRT actually caused heart disease. (For information on why double-blind studies are the best type of study, see
Why Does This Database Rely on Double-blind Studies?
Similarly, nothing more reliable than observational studies underlies the widespread belief that lycopene can prevent
and lutein can do the same for
. One double-blind study does hint that mixed carotenoids is beneficial for people with
, but the results were statistically weak.
Thus, while it seems to be a good idea to eat fruits and vegetables, it is not at all clear that taking concentrated extracts of various substances found in fruits and vegetables provides any benefits.
For more information, see the individual articles on
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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.
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Age-Related Eye Disease Study Research Group. A randomized, placebo-controlled clinical trial of high-dose supplementation with vitamins C and E and beta carotene for age-related cataract and vision loss: AREDS report no. 9.
Austin J, Singhal N, Voigt R et al. A community randomized controlled clinical trial of mixed carotenoids and micronutrient supplementation of patients with acquired immunodeficiency syndrome.
Eur J Clin Nutr.
2006 May 24 [Epub ahead of print].