There is insufficient evidence to support the use of supplements to prevent the following conditions:
Cancer. Two large trials published in 2009 came up empty. In one, sponsored by the National Cancer Institute, researchers reported that vitamin E and the mineral selenium failed to prevent prostate cancer. In fact, researchers noted possible increased risks of prostate cancer from vitamin E, and of type 2 diabetes from selenium. The second study, the Physicians' Health Study II, found that neither vitamin C nor E reduced the risk of colon, lung, prostate, or other cancers in men. And women haven't fared much better: Folic acid and certain other B vitamins provided no protection against breast or any other cancer in one study, and calcium and vitamin D had no effect on breast-cancer risk in another.
Some research suggests that vitamin and mineral supplements may pose particular risks to people who are being treated for cancer. While many cancer patients take antioxidant supplements such as vitamins C, E, and beta-carotene with the hope of reducing the toxicity of chemotherapy and radiation, a 2008 review found that the practice may protect cancer cells as well as normal ones. As a result, many oncologists now advise patients not to use antioxidant pills during those treatments.
Heart disease. Folic acid and other B vitamins failed to prevent heart attacks, strokes, and death from cardiovascular disease in women at risk for heart disease in a 2008 trial by the Harvard Medical School. And neither vitamin C nor E prevented those events in men in the Physicians' Health Study II. Vitamin E, however, was linked to an increased risk of hemorrhagic stroke.
Antioxidant supplements were previously thought to prevent fatty buildups in arteries, but research now suggests that they may worsen cholesterol levels and blunt the effects of cholesterol-lowering drugs. A 2009 review found that diets rich in those vitamins protected people from heart disease but supplements of them did not, underscoring the power of a healthy diet.
Type 2 diabetes. In a 2009 trial, vitamin and mineral pills didn't reduce the risk of metabolic syndrome--a cluster of symptoms including abdominal obesity and high levels of LDL (bad) cholesterol and blood sugar--that can lead to type 2 diabetes. Additional 2009 studies found that vitamin pills didn't prevent type 2 diabetes and might undermine the ability of exercise to improve blood sugar levels. And while many people with diabetes take supplements of the mineral chromium to control blood sugar and lose weight, that benefit is unproven.
Cognitive decline. B vitamins didn't slow Alzheimer's disease, and vitamin E failed to prevent dementia in people with cognitive impairment, according to trials from the Alzheimer's Disease Cooperative Study, an NIH-funded consortium. Another study suggested that high doses of vitamin E slowed the progression of moderate dementia, but most specialists are wary because of the potential risks, says Paul Aisen, M.D., director of the consortium.
On the other hand, a deficiency of vitamin B12 or thyroid hormone can cause cognitive impairment, so if you're declining more than normal for your age, you should be tested for those conditions. "But speak to your physician," Aisen says. "You can't necessarily replenish low B12 stores on your own."
Immune function. The evidence on whether vitamin and mineral supplements can enhance immunity is contradictory, especially for people who eat adequately. And while supplements can boost immune response in older people with nutritional deficiencies, it's still not known if that results in fewer infections. "In the vast majority of people, taking megadoses is not recommended, and may be harmful," says Kevin High, M.D., chief of infectious diseases at Wake Forest University School of Medicine in Winston-Salem, N.C. An excess of vitamin E, for instance, can backfire.