Carotenoids: functions and fallacies

Many epidemiological studies have demonstrated an association between a high intake of fruits and vegetables and a low risk of diseases, particularly epithelial cancers (Hennekens, 1986). Initially these foods were considered only as a source of retinol, that is vitamin A, since vitamin A-deficient diets have been linked with cancer for almost 60 years (for review, see Moon, 1989). Furthermore, retinoids (compounds with a chemical structure based on that of retinol) are well-established chemoprotective agehts for experimental carcinogenesis. In 1981, however, Pet0 et al. (1981) questioned the assumption that vitamin A was the protective ingredient in plant material. Their main argument was that most people in the Western World had satisfactory vitamin A status and feeding more vitamin A whether in the form of plant material or preformed retinol itself had very little, if any, effect on circulating vitamin A levels since the amount in blood is endogenously regulated. The authors pointed out, however, that provitamin A, that is p-carotene, was not similarly controlled. Circulating levels of F;-carotene were influenced by the dietary intake of plant foods and, therefore, @carotene might be the cancer-preventive ingredient. The reader will be forgiven for thinking that the only carotenoid of any importance is p-carotene. Most nutritionists were weaned onto the subject of nutrition on the understanding that the only carotenoid of any importance is p-carotene. Indeed, p-carotene is the major source of vitamin A for the majority of the world’s population and although there may be fifty or so provitamin A carotenoids (Olson, 1989), none of them is as effective or as important as p-carotene as a source of vitamin A. The purpose of the present review is to discuss some of the other functions of carotenoids and I hope to dispel some of the fallacies. In any discussion of function one must consider the nature of the molecule and its localization within tissues and cells in relation to possible physiological action. The carotenoids are compounds which, although similar in structure, may well have quite different activities at the cellular level. Therefore, it is important to know which carotenoids are found in man and what specific properties these may have in relation to those functions which might be important.