Billions of people around the world take dietary supplements. Even though supplements have become astoundingly popular, researchers have a tough time proving their benefits. The effects of supplements depend on the level of nutrients already being absorbed from the diet, as well as factors that influence nutrient absorption and metabolism.

And just because these supplements are sold over the counter doesn't mean they are safe or that you can't get associated with problems, such as osteoporosis, which is caused by too much vitamin A, says Michael Roizen, MD, Cleveland Clinic's chief wellness officer, and coauthor of the You: The Owner's Manual series. And because drugs and nutrients share the same route of absorption and metabolism in our bodies, supplements may interact with medications.

Research does support supplements for some people:

• Pregnant women and women of childbearing years require extra folic acid to help prevent birth defects.

• Older adults, especially postmenopausal women, may want to take calcium and vitamin D. While a recent US Preventive Services Task Force report indicated that there wasn't sufficient evidence that the typical doses of calcium supplements prevent osteoporotic fractures, some studies show that vitamin D plays an important role in the absorption of calcium and in boosting bone health.

• People over 50 may benefit from vitamin B12 supplements

• Young women with anemia may benefit from iron supplements

• People on very restricted diets or with many allergies may want to take a multivitamin

• Those at risk for age-related muscular degeneration - an eye problem - may benefit from antioxidant and zinc supplements

If you fall into any of these categories, ask your doctor about appropriate doses. In general, though, it is best to get your vitamins and minerals from food rather than pills. Supplements appear to be less effective than eating foods rich in the same nutrients; there is a good deal of evidence that antioxidant supplements, in particular, are usually not helpful, and may in fact be harmful. Plus, foods contain other important nutrients, such as fiber, essential fatty acids, and antioxidant phytochemicals.

The Bottom Line

• It is best to get your nutrients from food, rather than supplements

• Most healthy adults don't need supplements, as long as they eat a balanced diet

• If you think you might need a supplement, talk to your doctor first to make sure it doesn't interact with any medications you are taking