Vitamin A and selenium intake in relation to human cancer risk.

The possibility that dietary intake of certain vitamins and minerals may influence the occurrence of human cancer is receiving considerable scientific attention. One prominent hypothesis, that increased dietary intake of vitamin A reduces the occurrence of cancer, has received support from a large number of epidemiologic studies in which an inverse association was observed. The largest body of evidence relates to lung cancer. However, when examined in further detail, this apparent protective effect appears primarily attributable to higher intakes of green and yellow vegetables, which contain the carotenoid precursors of vitamin A. In contrast, there is little evidence to support an association between preformed vitamin A intake and cancer risk. In several studies based on prospectively collected sera, retinol levels were inversely related to subsequent cancer risk. However, these have not been supported by further investigations and appear to be the result of methodologic artifact. Available evidence thus suggests that factors associated with green and yellow vegetables provides modest protection against certain forms of cancer; beta-carotene is a likely candidate and is the focus of considerable research activity. Stimulated by the results of many animal studies and ecologic comparisons, we and other investigators have examined the association of serum selenium levels with subsequent risk of cancer. In the three published prospective studies an inverse association was observed, with a 2-to 5-fold increase in overall cancer risk among those with lowest selenium levels. As would be predicted by animal studies, the combination of low selenium and low vitamin E appears to be particularly deleterious.(ABSTRACT TRUNCATED AT 250 WORDS)