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After taking the supplement, blood levels of the vitamin peaked in both groups, but they dropped more rapidly in the obese group. Their vitamin Dmeasurements were significantly lower in the obese even after 24 hours.

The study results suggest that obese individuals produce and absorb just as much of the vitamin as do lean people, but store it differently in the body. The researchers speculate that the increased fat stores the vitamin D, removing it from the bloodstream where it would be available for use. For this reason, overweight people may need to take a supplement.

"These findings did surprise us in a way because one would think that a person who is overweight would be less likely to have nutritional problems," says Wortsman, an endocrinologist and associate professor of medicine at Southern Illinois University in Chicago. "In uncomplicated obesity, this may not be a problem," he tells WebMD. "But in obese people placed on strict [diets], it could be a big problem." People who have undergone weight-reduction surgery, such as intestinal bypass procedures, already have trouble digesting fat; this problem may "exacerbate the vitamin deficiency ... and increase the risk of bone disease."

"I think this is cutting-edge work," says Hal Seim, MD, medical director of the Medical Weight Management Center in St. Paul, Minn. "Not only probably do obese people spend less time exposing their bodies to the sun, when they do, they seem to get less benefit," says Seim, who was not involved in the study. "The bottom line is that we probably should start supplementing our obese patients with vitamin D."

But Richard Dickey, MD, an endocrinologist in Hickory, N.C., sees these findings in a different light. "Obese patients do not have as much osteoporosis as other people," he tells WebMD. "Therefore, there must be other mechanisms involved [in maintaining their bones]."

Dickey, who is immediate past president of the American Academy of Clinical Endocrinologists, also points out research is pointing to more and more cases of vitamin D deficiency in the U.S. and Northern Europe. "We now have an epidemic of vitamin D deficiency in our elderly and possibly our younger people." Vitamin D deficiency in the young can also coincide with the peak time in bone formation as well.

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