Carotenoids are a group of plant compounds some of which show vitamin A activity in humans. Structurally, carotenoids are carbon chains that can be xanthophylls or carotenes, the former differing from the latter in that they contain an oxygen molecule in their structure. In plants carotenoids play a role in harvesting light during periods of low solar illumination by absorbing light in the 400 to 500 nm range. Carotenoids also protect plants from free radical damage by scavenging singlet oxygen. In humans carotenoids are bioavailable, as evidence shows they are absorbed from the gastrointestinal tract and accumulate in plasma and tissues. Studies have shown that carotenoids are effective in vivo antioxidants and chemically protect humans cells from singlet oxygen in a similar manner to plant cells. The antioxidant ability of carotenoids may explain their antitumour effects, whereby intakes of β-carotene are inversely associated with cancer risk.
It is thought that carotenoids in the skin are able to interact with free radicals generated through the action of ultraviolet light and dissipate the energy through interaction with their structure, ultimately producing heat. This effect was alluded to in a study published in the American Journal of Nutrition in 19881. Subjects had their carotenoid levels measured and were then exposed to ultraviolet A radiation in the 320 to 400 nm range and ultraviolet B radiation in the 280 to 320 range. The results showed that ultraviolet exposure cause a significant reduction in plasma carotenoids in the subjects, suggesting that carotenoid concentrations had been depleted. The authors were unsure as to why this might occur, but suggested that free radical generation had depleted the plasma of carotenoids, which may have been structurally altered by a photochemical process. Interestingly the depletion of carotenoids had no effect on vitamin A status.
These results support other data to show that exposure to environments that cause the generation of free radicals depletes plasma and tissue concentrations of antioxidants. In addition to ultraviolet light, this is also know to be the case for smoking. As long as antioxidants are repleted, then it is likely that free radical tissue damage may be limited and long-term health maintained. However, diets low in antioxidant plant compounds such as carotenoids, flavonoids and vitamin E may lead to depletion of antioxidant defences, damage to cells and tissue and ultimately DNA damage. For example, the inverse association between smoking and plasma antioxidants may explain the increased risk of cancer in smokers. Because antioxidants interact and have overlapping but unique roles within the tissues of the body, it is important that a variety of such compounds are included in the diet in order to protect from disease.
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