Effects of carotenoids on human immune function

Many epidemiological studies have shown an association between diets rich i n carotenoids and a reduced incidence of many forms of cancer, and it has b een suggested that the antioxidant properties of these compounds are a caus ative factor. Attention has focused on the potential role of one specific c arotenoid, beta-carotene, in preventing cancer, and numerous publications h ave described in vitro experiments and animal studies which suggest that no t only can this carotenoid protect against the development of cancer, but a lso several other chronic diseases. Since the immune system plays a major r ole in cancer prevention, it has been suggested that beta-carotene may enha nce immune cell function. Several human trials, using dietary beta-carotene supplementation with a wide range of intakes, have been undertaken to addr ess this hypothesis. The general conclusion of these studies is that this c ompound can enhance cell-mediated immune responses, particularly in the eld erly. The present article will review some of these human studies and, hope fully, complement the reviews of other authors associated with the present symposium, some of whom will also describe work in this area. Potential mec hanisms for the effects of carotenoids on immune function will also be revi ewed. Finally, possible reasons for the failure of three major prospective studies to demonstrate a beneficial effect of beta-carotene supplementation on lung cancer risk will be discussed.