Carotenoids as a dietary supplement

DEFINITION: Natural substance promoted as a dietary supplement for specific health benefits.

Overview

Carotenoids are red, orange, and yellow pigments found in fruits and vegetables. About 750 carotenoids have been identified, all with antioxidant properties. Some carotenoids can be converted in the body to vitamin A. These are called provitamin A carotenoids. The best-known carotenoids include beta-carotene, lutein, lycopene, astaxanthin, and zeaxanthin.

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. Still, studies in the twenty-first century did show some benefit, so scientific evidence on carotenoids continues to evolve.

Many proponents of alternative medicine considered this outcome paradoxical and attempted to explain it in various ways, such as that 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 study participants 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 all 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 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 hints that mixed carotenoid supplementation benefits people with human immunodeficiency virus infection, but the results were statistically weak. Thus, while eating fruits and vegetables is a good idea, it is not clear whether taking concentrated extracts of various substances found in fruits and vegetables provides any health benefits. Further, while observational studies of carotenoids were once the only evidence on which to draw conclusions, this is changing in the twenty-first century.

As the twenty-first century progressed, studies into the benefits of carotenoids emerged. Carotenoids were recommended for many conditions, including eye health, cancer prevention, cardiovascular health, skin health, immune function, and cognitive function. Myriad double-blind, placebo-controlled studies of carotenoids were undertaken. A 2023 study found benefits of supplementation with macular carotenoids in improving cognitive function in study participants. In a 2021 study, participants taking astaxanthin exhibited less exercise-induced muscle damage. A 2020 study showed that lutein helped patients improve their blood pressure and lipid levels, suggesting carotenoids may benefit individuals with cardiovascular disease.

Bibliography

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Kaulmann, Anouk, and Torsten Bohn. "Carotenoids, Inflammation, and Oxidative Stress--Implications of Cellular Signaling Pathways and Relation to Chronic Disease Prevention." Nutrition Research, vol. 34, no. 11, 2014, pp. 907-29, doi.org/10.1016/j.nutres.2014.07.010. Accessed 14 Sept. 2024.

Peterson, C. E., et al. "Combined Antioxidant Carotenoids and the Risk of Persistent Human Papillomavirus Infection." Nutrition and Cancer, vol. 62, 2010, pp. 728-733.

Srivastava, Ruby. "Physicochemical, Antioxidant Properties of Carotenoids and Its Optoelectronic and Interaction Studies with Chlorophyll Pigments." Scientific Reports, vol. 11, no. 1, 2021, pp. 1-14, doi.org/10.1038/s41598-021-97747-w. Accessed 14 Sept. 2024.