What are carotenoids as a dietary supplement?

Overview

Carotenoids are red, orange, and yellow pigments found in fruits and
vegetables. About six hundred carotenoids have been identified, and all of them
have antioxidant properties. Some carotenoids can be converted in the body to
vitamin
A, and these are called provitamin A carotenoids. The
best-known carotenoids include beta-carotene, lutein,
lycopene, astaxanthin, and zeaxanthin.

The results of some observational studies 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 cancer (especially lung cancer), heart disease, cataracts, strokes, and macular degeneration. The results showed, at best, no benefit and, at worst, a possible increase in disease risk.

Many proponents of alternative medicine considered this outcome paradoxical and attempted to explain the outcome in various ways, such as beta-carotene alone may not be as useful as mixed carotenoids (and other healthful substances) found in fruits and vegetables, and synthetic beta-carotene may be less effective than natural beta-carotene. Also, the participants in these studies were inappropriate for the trials (generally, they were 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 HRT 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.

Similarly, nothing more reliable than observational studies underlies the
widespread belief that lycopene can prevent prostate
cancer and that lutein can do the same for cataracts.
One double-blind study does hint that mixed carotenoid supplementation is
beneficial for people with human immunodeficiency virus infection,
but the results were statistically weak. Thus, while it is a good idea to eat
fruits and vegetables, it is not clear that taking concentrated extracts of
various substances found in fruits and vegetables provides any health
benefits.

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.” Archives of Ophthalmology 119 (2001): 1439-1452.

Austin, J., et al. “A Community Randomized Controlled Clinical Trial of Mixed Carotenoids and Micronutrient Supplementation of Patients with Acquired Immunodeficiency Syndrome.” European Journal of Clinical Nutrition 60 (2006): 1266-1276.

Epstein, K. R. “The Role of Carotenoids on the Risk of Lung Cancer.” Seminars in Oncology 30 (2003): 86-93.

Hak, A. E., et al. “Plasma Carotenoids and Tocopherols and Risk of Myocardial Infarction in a Low-Risk Population of US Male Physicians.” Circulation 108 (2003): 802-807.

Peterson, C. E., et al. “Combined Antioxidant Carotenoids and the Risk of Persistent Human Papillomavirus Infection.” Nutrition and Cancer 62 (2010): 728-733.