Does beta-carotene prevent cancer? A critical appraisal

The possible role of beta-carotene as a protective nutrient against cancer is reviewed. Human prospective and retrospective studies strongly indicate that beta-carotene protects against lung cancer and probably against stomach cancer. It may also be protective against cancer of the ovary, cervix, breast and other cancers, but not the colon or rectum. The protective factor appears to be beta-carotene itself, rather than total vitamin A. Experiments using a variety of animal models also show that beta-carotene is anticarcinogenic and appears to act at several stages of the process. Possible mechanisms of action are discussed, namely that it must first be converted to vitamin A, that it alters carcinogen metabolism, that it is an anti-oxidant and that it enhances the immune defenses.