Micronutrients and cancer chemoprevention.

Accumulating evidence suggests that a number of micronutrients may decrease the incidence of cancer of epithelial cell origin. These include vitamins A, C and E, beta-carotene and selenium. Few lines of research, apart from means to reduce cigarette smoking or heavy alcohol consumption, are as promising in terms of substantially decreasing human cancer incidence as studies of such micronutrients as dietary inhibitors of cancer. In this paper, we review the mechanisms by which these micronutrients may act to inhibit carcinogenesis, as well as the current animal and epidemiologic evidence supporting their role as chemopreventive agents. With respect to areas of future study, valuable information can be obtained from basic research as well as observational analytic epidemiologic studies utilizing dietary questionnaires which are more specifically focused and have been more rigorously evaluated than those previously employed. Another promising avenue of epidemiologic research is prospective studies of cancer incidence in relation to biochemical parameters in stored blood samples collected at baseline. The most reliable way to test directly whether a micronutrient prevents the development of human cancer is a large, randomized placebo-controlled trial among individuals with no previous history of the disease. The timely conduct of such trials is particularly important, since regular intake of nutritional supplements is already widespread in the United States, despite the lack of reliable evidence as to their benefits.