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Teenagers are probably the least likely among us to eat enough fruits and vegetables or to get adequate amounts of vitamins and minerals from their diet. And children of lower-income households are at even higher risk of undernourishment, since they may not have access to regular or well-balanced meals.

That's why researchers at the University of California, Davis, studying multivitamin use among kids and teens expected to find that lower-income children would be more likely to take supplements to make up for their spotty diets. Instead, researchers found the opposite. When they analyzed the results of a national survey of more than 10,000 children and adolescents ages 2 through 17, they found that those who were most likely to take vitamin and mineral supplements were those who needed them least — in other words, youngsters in higher-income families, who ate consistent and nutritious meals. (See 9 kid foods to avoid.)

"We did find that vitamin and mineral supplements were used with more frequency in children who were underweight," says Dr. Ulfat Shaikh, lead author of the study. "But the rest was somewhat surprising."

About one-third of U.S. children take vitamins or supplements, according to the study, which was part of the government's National Health and Nutrition Examination Survey conducted between 1999 and 2004. Vitamin takers were more likely to be white; eat a low-cholesterol, high-fiber diet; come from a higher-income family; get plenty of exercise; and have better access to health care and health insurance. Which means that the bulk of these youngsters really didn't need supplements at all. (See the top 10 medical breakthroughs of 2008.)

While some children got unnecessary supplementation, the study found, those who might have reaped true benefit from vitamins didn't get them at all: kids from lower-income families who had less reliable, if any, access to health care, who relied on food stamps and were often forced to skip meals were less likely to take vitamins and minerals. "Children who face poverty, food insecurity and lack regular balanced meals have a high likelihood of benefiting from supplements," says Shaikh, but they typically don't have access to them because of cost.

Families who subsist on food stamps and live at or below the poverty level can't afford supplements, and there are many such families. According to Dr. Frank Greer, chair of the American Academy of Pediatrics Committee on Nutrition, 55% of infants born in the U.S. are eligible for the national Women, Infants and Children feeding program, which provides food for low-income mothers and their children up to age 5. But because the program is food-based, it does not include vitamin and mineral supplements.

That's a problem, particularly when it comes to vitamin D. Federal guidelines recommend that children get 400 I.U. of the vitamin a day, equivalent to four 8 oz. glasses of milk. But most American kids, including those eating regular balanced meals, don't get enough — and a multivitamin would be the easiest way to make up for the deficiency. As for the other minerals and vitamins typically found in supplements, however, Greer says, "We don't have deficiencies in the healthy U.S. population. Healthy kids who eat a well-rounded diet don't really need vitamin supplements."

What the study suggests is that doctors and health officials need to target nutrition messages more effectively to the proper parents. Families whose children get well-balanced meals should know that vitamin supplements won't make their kids any healthier. But parents who struggle to feed their children may need better access to vitamins and minerals to supplement daily meals.

Read "The Year in Medicine: From A to Z."

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• Alice Park
• A study finds that 34% of American kids take multivitamins, but the ones who need them most often don't