NEW YORK (Reuters Health) - Low vitamin D stores may drain teenage girls of some of their muscle power, new study findings suggest.

Vitamin D may be best known for its role in aiding calcium absorption and maintaining healthy bones, but it is also important in normal muscle function.

In the new study, researchers found that among 99 girls, 12 to 14 years old, those with lower vitamin D levels in their blood performed more poorly in tests of jump height and leg-muscle power.

The findings suggest that a lack of vitamin D hinders the ability of muscles to contract, the researchers report in the Journal of Clinical Endocrinology & Metabolism.

The concern, they say, is that such effects may get in the way of young girls' bone development, since muscle force on the bones helps build bone mass and strength.

"We know vitamin D deficiency can weaken the muscular and skeletal systems, but until now, little was known about the relationship of vitamin D with muscle power and force," lead researcher Dr. Kate Ward, of the University of Manchester in the UK, said in a written statement.

None of the girls in the study had symptoms of an outright vitamin D deficiency, such as muscle pain. The majority had insufficient blood levels of the vitamin, however, and this appeared to be enough to affect their muscle function, the researchers point out.

"Vitamin D affects the various ways muscles work," Ward said, "and we've seen from this study that there may be no visible symptoms of vitamin D deficiency."

"Further studies are needed to address this problem and determine the necessary levels of vitamin D for a healthy muscle system," she added.

The American Academy of Pediatrics recently doubled the amount of vitamin D it recommends for infants, children and teenagers -- to 400 IU each day. A number of studies in recent years have suggested that many children may have inadequate vitamin D levels, possibly due to factors like spending little time outdoors or drinking soda at the expense of vitamin D-fortified milk.

Foods rich in vitamin D are limited, with oily fish being the primary natural source. Besides milk, some breakfast cereals and orange juice brands are fortified with the vitamin.

Experts recommend vitamin supplements for children who do not get enough vitamin D from food.

SOURCE: Journal of Clinical Endocrinology & Metabolism, February 2009.

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