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Q: I think I eat pretty healthful, balanced meals. Do I still need a multivitamin and mineral supplement?

A: Here’s the bottom line. Most people who are healthy and eat healthfully don’t need a daily multivitamin and mineral supplement. They don’t need individual vitamin or mineral supplements, either. According to the Dietary Guidelines for Americans, most children older than 9 and adults who eat sufficient amounts of nutrient-dense foods (that’s mainly fruits, vegetables, whole grains, legumes/beans, low-fat dairy foods) and consume at least 1,600 calories can meet their nutrition needs from foods.

Yes, this sounds like heresy or, at best, counterintuitive to consumers faced with the marketing prowess of the dietary supplement industry. The mantra “If a small amount is good for you, then more must be better” hasn’t, according to research, been wise advice.

Over the past couple of decades, one study after another has set out to research the health effects of large doses of vitamins or minerals, particularly antioxidants. And they’ve found no significant benefits. Worse yet, some have uncovered health concerns, such as increased risk of heart disease or cancer.

An explanation? “Oxidation, the process by which the body uses oxygen to convert food to energy, creates free radicals, often billed as evildoers in our bodies,” says Paul Offit, chief of the division of infectious diseases at Children’s Hospital of Philadelphia and author of “Do You Believe in Magic? The Sense and Nonsense of Alternative Medicine.” Those free radicals can damage cells and the lining of arteries, one reason they’ve been linked with aging, certain cancers and heart disease. But, Offit says, we’ve discovered we need free radicals to kill bacteria and cancer cells. When people take large doses of antioxidants (such as selenium, beta-carotene, and vitamins A, C and E), “they can tip the balance towards an unnatural state in which the immune system is less able to kill harmful invaders.”

Yet Americans continue to swallow boatloads of supplements.

A study this year in the Journal of American Medical Association Internal Medicine analyzed the use of supplements among a large sample of American adults. “We found half of U.S. adults use dietary supplements, most commonly a multivitamin and mineral supplement,” says the lead author, Regan Bailey, a dietitian and nutritional epidemiologist in the Office of Dietary Supplements at the National Institutes of Health. Bailey adds, “Our study reveals that adults use supplements primarily for their assumed health benefits, yet there remains a lack of sufficient research on this.” Interestingly and somewhat predictably, Bailey’s research showed that the adults studied tended to report eating more healthfully, practicing moderation with alcohol, exercising and abstaining from smoking.

Another important concern raised by Bailey’s study is that only a quarter of the supplements taken by adults were recommended by their health-care provider. This can be dangerous because of potential interactions of supplements, prescribed medications and other therapies. Your health-care provider should be asking about your supplement use, but if he or she doesn’t, bring it up.

Bailey and Offit agree that most people who are healthy and eat healthfully don’t need a multivitamin or mineral supplement. Offit’s suggestion: Start by eating sufficient fruits and vegetables. They’re low in calories and packed with nutrients.

The Academy of Nutrition and Dietetics, in its position on nutrient supplementation, concurs with this food-first approach. However, the academy and experts agree on this important caveat: Some people, based on their nutrition intake, medical issues and other circumstances, may need to fill nutrition gaps with supplements. Consider women of childbearing years, women who are pregnant, people who are vegan or strict vegetarians and people who don’t consume sufficient calories, to name a few.

When it comes to choosing the foods you eat, make your calories count.

For a trusted source on supplements, visit the Web site of the Office of Dietary Supplements at the National Institutes of Health: ods.od.nih.gov.

Warshaw, a registered dietitian nutritionist and certified diabetes educator, is the author of numerous books published by the American Diabetes Association and the blog EatHealthyLiveWell found on her Web site, www.hopewarshaw.com.

Have a nutrition question? Send an e-mail to localliving@washpost.com. Put “Nutrition Q&A” in the subject line and tell us where you live.