Although vitamin supplementation may be beneficial in patients with nutritional deficiencies (e.g., children, pregnant and lactating women, the elderly, patients with chronic illnesses), its usefulness in persons with healthy diets is less clear. Some epidemiologic evidence indicates that dietary factors may have a causative role in the development of cancer and cardiovascular disease, making vitamin intake a potential focus. The U.S. Preventive Services Task Force (USPSTF) reviewed prospective trials and observational studies on the routine use of vitamin supplementation to prevent cancer and cardiovascular disease.

According to the USPSTF, studies on the value of vitamin A alone in preventing cardiovascular disease have shown no positive effect. Observational studies that demonstrate the utility of vitamin A in the prevention of colon and breast cancer have design issues that make the conclusions uncertain. Observational studies on the effect of vitamin C on cardiovascular disease have had inconsistent results and have demonstrated no significant association between vitamin C supplementation and cancer risk. Vitamin E has not been shown to decrease the risk of cardiovascular disease or cancer. Beta-carotene does not decrease the risk of cancer or cardiovascular events. Studies evaluating antioxidant vitamin combinations have been conflicting and show no consistent value. Other vitamin combinations have been studied but also provide no consistent benefit.

Vitamin supplements have been shown to have potential harms. Moderate doses of vitamin A may reduce bone mineral density, and high doses can be teratogenic or hepatotoxic. Beta-carotene supplementation may be associated with increased mortality from lung cancer in smokers. Because adverse effects of vitamin supplementation rarely are reported in studies, the USPSTF notes that more studies are needed.

The USPSTF concludes that the evidence is inadequate to recommend supplementation of vitamins A, C, or E, multivitamins with folic acid, or antioxidant combinations to decrease the risk of cancer or cardiovascular disease. Beta-carotene supplements should not be used routinely in asymptomatic patients, because there is no evidence of risk reduction and some evidence that the supplements may cause harm in some patient groups.