The beneficial and adverse effects of some chemopreventive agents, such as
Vitamins A, C, E,
beta-carotene,
indole-3-carbinol,
capsaicin, garlic, and aloe are reviewed. Two large randomized trials with a
lung cancer endpoint, the
Alpha-Tocopherol,
Beta-Carotene (ATBC) Prevention Study and the
Beta-Carotene and
Retinol Efficacy Trial (CARET), suggested that
antioxidants might be harmful in smokers. However, the results of the Linxian study and of the ATBC or the CARET studies were significantly different in this respect, and therefore, the relationship between
antioxidant and
carcinogenesis remains open to debate.
Indole-3-carbinol has
cancer promoting activities in the colon, thyroid, pancreas, and liver, whereas
capsaicin alters the metabolism of chemical
carcinogens and may promote
carcinogenesis at high doses. Organosulfur compounds and
selenium from garlic have no or a little enhancing effect on
cancer promotion stage. Information upon chemopreventive mechanisms that inhibit
carcinogenesis is imperfect, although the causes and natures of certain human
cancers are known. Therefore, definitive preventive guidelines should be carefully offered for various types of
tumors, which properly consider ethnic variations, and the efficacies and the safety of chemopreventive agents.