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Obese People May Be Caught in Vitamin D Dilemma


WebMD Health News
Reviewed by Merle Diamond, MD

Sept. 1, 2000 -- Do obese individuals run a higher-than-average risk of being deficient in vitamin D? A new study suggests that they might, because their bodies process the vitamin differently whether it is received from sunlight or diet.

Vitamin D is important in building and maintaining bone strength and also acts as a hormone to regulate the growth and development of other tissues. In children, deficiency causes rickets, where impaired bone growth results in deformity. Adults with deficiency can develop soft -- and sometimes broken -- bones. Increasing attention is also being paid to vitamin D's potential role in preventing cancer.

Many foods contain vitamin D, but it can also be manufactured in the skin in response to the ultraviolet light of the sun. In fact, most of the vitamin D that we need comes from exposure to the sun -- not from food. Because it is known that overweight people have lower levels of the vitamin, researchers wanted to know why, and whether supplements might be needed in obese people.

In this study, lead author Jacobo Wortsman, MD, and his co-authors compared how vitamin D is used in 19 healthy, obese volunteers to that of 19 healthy, normal-weight people. They measured vitamin D levels during the winter, so that the effect of natural sunlight was minimal. To assess the skin's ability to manufacture vitamin D, they had the subjects sit in a tanning bed and measured blood levels of vitamin D 24 hours before and 24 hours after the session.

The initial blood levels of vitamin D did not differ significantly between the two groups before they entered the tanning bed. Both groups also responded with a significant increase in vitamin D within 24 hours of exposure. However, those levels were 57% lower in the obese people compared with the normal-weight participants.

In order to see if weight affects the body's ability to absorb and use the vitamin, they had the subjects take a vitamin D supplement following a 12-hour fast and after abstaining from dairy products --which are usually high in vitamin D -- for the previous week. This part of the study was done a month after the tanning bed test.

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.

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