Taking vitamins won't reduce the fat, sugar or salt in your diet that could contribute to disease. And supplements won't provide the disease-fighting phytochemicals found in fruits, vegetables and other foods. Though supplement manufacturers and consumer magazines may claim that supplements fight disease, the evidence for many of these claims isn't sufficient. For example, a 1996 clinical trial found that the mineral selenium could reduce colon, prostate and lung cancer risk by half. And it's possible selenium supplements only would help prevent cancer in individuals who don't get the mineral from food.
More research needs to be done on many supplements.
A few vitamin and mineral supplements, through studies, have shown promise in making a significant disease-prevention impact for certain groups of people. Research has shown that folic acid can prevent neural tube birth defects in newborns. The U.S. Public Health Service has recommended that all women capable of becoming pregnant consume 400 micrograms of folic acid per day. Many foods such as breakfast cereals and pasta are fortified with folic acid, but you'd have to eat a large amount of some of these foods to consume the recommended value. Therefore, taking a folic acid supplement may be a good idea.