Antioxidant micronutrients in cancer prevention.

Antioxidant micronutrients are one of the body's primary defenses against free radicals and reactive oxygen molecules. Carotenoids, vitamin C, and vitamin E trap these molecules, and selenium is an essential component of an antioxidant enzyme. There is considerable support from animal studies for a protective effect of antioxidant micronutrients on cancer. However, the role of these micronutrients in cancer prevention in humans is less clear. Diet studies suggest protective effects of fruits and vegetables on risk of cancer at several sites. Inverse associations between dietary carotenoids and serum beta-carotene and lung cancer have been observed repeatedly. Vitamin C has also been consistently inversely associated with risk of oral and esophageal cancer in diet studies and with stomach cancer in both diet and plasma studies. It remains unknown, however, whether carotenoids and vitamin C or some other component of fruits and vegetables, the primary sources of these micronutrients, prevent cancer in humans. Selenium has been inversely correlated with cancers at numerous sites in ecologic studies, but observational studies do not provide strong support for a protective effect of selenium on cancer at any site. There also is not strong support for a protective effect of vitamin E on cancer in humans. Results of studies on the association of antioxidant micronutrients with cancer at many sites are inconsistent. This could be due to lack of a true protective effect or could be related to methodologic problems in assessing dietary intake in epidemiologic studies.