NEW YORK (Reuters Health) - Vitamin supplements are meant to fill-in where diet may be lacking, but a new study finds that U.S. kids may not be getting some of the most needed nutrients from their vitamin pills and the kids taking vitamins may not be the ones who need them the most.

Looking at the diets and supplement use of more than 7,000 kids, researchers found that between the ages of nine and 18 many had low levels of certain vitamins and minerals, and few took supplements, while younger kids had adequate levels of most nutrients and were possibly getting too much of some vitamins and minerals.

Most children under eight, for instance, got the nutrition they needed from the food they ate, regardless of whether or not they took supplements, the study found.

Even with the use of supplements, however, more than a third of children failed to get sufficient calcium and vitamin D. And users sometimes overloaded on essential nutrients such as iron and vitamin A.

Zinc and folate were consumed to excess across all age groups.

"We don't know if these excessive amounts cause any harm," said Regan Bailey, a nutritional epidemiologist at the National Institutes of Health in Bethesda, Maryland, who led the study.

But, Bailey and her coauthors note in The Journal of Pediatrics, their findings suggest that makers of children's vitamins should consider reformulating their products to better match kids' needs and modern diets.

Bailey's team used dietary surveys to assess mineral and vitamin intake among children between the ages of two and 18 who took part in a major government health survey between 2003 and 2006.

About 40 percent of children between ages two and eight take a supplement, the study found. Supplement usage was lower among older kids - 29 percent of kids between nine and 13 years old took them and 26 percent of teenagers did.

White children were more likely than their black or Hispanic counterparts to take a supplement, commonly a multivitamin/mineral.

Overall, supplement users and nonusers got roughly similar levels of 15 different vitamins and minerals from food sources alone. But users were also more likely to get recommended doses of calcium, magnesium, phosphorus, vitamin A and vitamin C from food than nonusers.

When supplement use was considered, researchers found users had much better nutrient levels than nonusers. Across all age groups, taking supplements improved intake of calcium and vitamins A, C, D and E.

Calcium and vitamin D consumption often fell below recommended levels, however, even among kids who took supplements.

More than a third of kids using supplements did not get enough calcium. The recommended amount is between 200 and 1,000 milligrams a day for children up to age eight, and 1,300 mg per day for kids age nine and up and teens.

Most children between ages two and eight who didn't take supplements failed to get adequate levels of calcium and vitamins D and E.

"This is unfortunate," Bailey said. "Calcium and vitamin D are critical during this age for bone growth and development."

Teens fell short on several other essential nutrients as well, including magnesium, phosphorus and vitamins A, C and E.

Vitamin E is involved in immune function, vitamin A in vision and bone growth and magnesium promotes nerve function and cellular energy production.

At the same time, supplement users were more likely to boost consumption of certain nutrients - such as zinc, folate, iron and vitamin A - above recommended upper limits.

The long-term effects of high doses of these micronutrients in children are unknown, the report noted.

The bottom line, Bailey explained, was that for some kids who would have been deficient in some vitamins and minerals, supplements were a help.

"For older children, taking supplements added nutrients for which intake would have been inadequate from food alone," she told Reuters Health.

Bailey added that the problem of overloading affected children under age eight the most.

The findings highlight the gap between supplement formulations for children and their needs as identified in national research data, Bailey explained.

"Manufacturers should tailor levels of nutrients in their products to help consumers meet, but not exceed, nutrient recommendations," she said, adding: "This is particularly salient for calcium and vitamin D."

Bailey, who works for the federal Office of Dietary Supplements, declined to elaborate on what specific changes could be made by supplement makers or parents.

"The ODS is here to help consumers; we do not make recommendations on supplement use," she said.

SOURCE: The Journal of Pediatrics, online June 20, 2012.