Many epidemiological studies have shown an association between diets rich in
carotenoids and a reduced incidence of many forms of
cancer, and it has been suggested that the
antioxidant properties of these compounds are a causative factor. Attention has focused on the potential role of one specific
carotenoid,
beta-carotene, in preventing
cancer, and numerous publications have described in vitro experiments and animal studies which suggest that not only can this
carotenoid protect against the development of
cancer, but also several other
chronic diseases. Since the immune system plays a major role in
cancer prevention, it has been suggested that
beta-carotene may enhance immune cell function. Several human trials, using dietary
beta-carotene supplementation with a wide range of intakes, have been undertaken to address this hypothesis. The general conclusion of these studies is that this compound can enhance cell-mediated immune responses, particularly in the elderly. The present article will review some of these human studies and, hopefully,
complement the reviews of other authors associated with the present symposium, some of whom will also describe work in this area. Potential mechanisms for the effects of
carotenoids on immune function will also be reviewed. 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.