Sunlight—Can It Prevent as well as Cause Cancer?1

Kxccssive exposure tu sunlight is known to damage the skin. However, the emphasis of most studies has heen on the consequences of sunlight exposure to fair-skinned individuals, and the situation of people with heavy skin pigmentation residing in, or migrating to, geographic locations with limited sunlight incidence has been largely neglected. Recent epide miolÃ3gica! studies suggested the hypothesis that sunlight deprivation, and the associated reduction in the circulating levels of vitamin I), (vit l),i derivatives may lead to the increased incidence of the carcinomas of the hreast, colon, and prostate. Two endocrine pathways may mediate these effects. The pineal function can potentially he involved, hut the formation of vit I), derivatives is gaining credibility as a mechanism for the retar dation of cancer progression. Kvidence is accumulating that such com pounds, c..!;.. 1,25-dihydroxyvitamin II, (1.25I),) induce differentiation of several neoplastic cell types, arrest or retard their proliferation, and act as chemoprcventivc agents in animal carcinogenesis. We also propose that the antineoplastic effects of vil I), derivatives are exerted at several steps in tumor progression and that immunomodulating effects of I.-51), may contribute to these effects of sunlight. The recent findings that common cancers, e.g., carcinoma of the prostate and the breast, behave more aggressively in black Americans than in white Americans may be ex plained on this basis. Although more data are needed on the effects of sunlight on the circulating levels of 1.25D,, a corollary of this hypothesis is that there should be no broad condemnation of moderate sun light exposure, as it may be available in insufficient amounts to some Americans.