Carotenoid

Carotenoids are naturally occurring pigments that are found in plants and other photosynthetic organisms such as algae and some bacteria. Approximately 600 carotenoids have been identified, 50 of which are consumed by humans. The vital role that carotenoids play as accessory pigments in photosynthesis has been an active area of research for many years. In addition, some carotenoids are metabolic precursors to retinol in humans, and thus have important health roles in the prevention of vitamin A deficiency, one of the most important nutrient deficiencies on a worldwide level. Carotenoids are widely distributed in plants, including fruits and vegetables, where they account for most of the red/orange/yellow colors in these foods. Many of the carotenoids ingested from foods are absorbed and accumulate in human blood and tissues. Whether non-provitamin A carotenoids have health effects in humans is not clear.

Carotenoids are generally classified into two categories, the carotenes (hydrocarbon carotenoids) and the xanthophylls (oxygenated carotenoids). The most well-known carotenes include alpha- and beta-caro-tene, found in carrots, and lycopene, found in tomato products. The most well-known xanthophyllic carotenoids include lutein and zeaxanthin, which are found in deep-green leafy vegetables such as spinach, and cryptoxanthin, found in oranges. Carotenoids serve an important antioxidant function in plants and photosynthetic organisms; whether they serve a similar antioxidant function in humans is less clear.

Some chronic conditions such as cancer, age-re-lated macular degeneration, and aging are thought to result from oxidative stress. Many epidemiological, mechanistic, and clinical studies are actively investigating whether people who consume more carotenoids (from food or supplemental sources) have a reduced risk of developing chronic disease. As an example, the human macula (part of the retina) specifically concentrates two of the carotenoids found in human blood, lutein, and xeaxanthin. Recent evidence suggests that higher concentrations of these two pigments (obtained from diet) in the macula may protect against photo-induced damage, reducing the risk of macular degeneration. Human supplementation trials are currently underway to evaluate this hypothesis.

Another carotenoid that has been of particular interest for its health effects is lycopene. Many, but not all, epidemiological studies find that persons with greater lycopene intake/status are at lower risk of developing chronic diseases, including prostate cancer. This relationship is promising but not yet convincing.

The only carotenoid that has been evaluated in long-term supplementation trials in humans for disease prevention is beta-carotene. It was hypothesized that this carotenoid might help to reduce the incidence of lung and other tobacco-related cancers, as cigarette smoking is known to be a potent oxidative stressor. Results showed that contrary to the hypothesis, supplementation with high-dose beta-carotene increased lung cancer incidence in high-risk populations (e.g., smokers). It is now recognized that carotenoids, particularly at high concentrations, can form breakdown products with unexpected physiological effects. Based on these findings, it is generally advised that humans obtain carotenoids from foods rather than from supplements, with correspondingly lower exposures. Exceptions to this are for prevention of vitamin A deficiency in at-risk populations, and in the setting of clinical trials (e.g., for age-related macular degeneration), with careful monitoring for health risks and benefits.

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