Epidemiology of retinoids and cancer.

A variety of in vitro and in vivo inquiries suggests that retinoids of various sorts may reduce the risk of cancer at various sites. Epidemiologic studies done by a number of different investigators similarly show a reduction in risk with increases in ingestion of vitamin A-containing diets. These reductions in risk are primarily of the squamous cell types of cancer: lung, larynx, mouth, cervix, and bladder cancers. Retinoids, particularly beta-carotene, may have a risk-reducing function for such sites. However, evidence has also been found that retinoids under certain conditions may increase the risk of prostate cancer as well as of Hodgkin's disease and leukemia. It is not an impossibility that, although beta-carotene could reduce risk under most circumstances, under certain other circumstances it could increase risk for different cancers. Obviously, more research is necessary, particularly with the carotenoids.