Low levels of vitamin D could be making our bodies more vulnerable to weight gain.

In a study published today, researchers in Portland and three other cities checked blood levels of vitamin D in more than 4,600 older women and tracked weight changes for nearly five years. Among the 571 women who gained weight, those with low vitamin D levels added about two pounds more than those with adequate vitamin D.

Last month, Norwegian researchers reported similar results in a study that followed 2,400 young and old women and men for 11 years. Those with low vitamin D were 73 percent more likely to become obese.

Both studies are observational, thus unable to establish cause and effect. "We need a lot more data before we recommend vitamin D to prevent weight gain," says Dr. Erin LeBlanc, an endocrinologist and researcher at the Kaiser Permanente Center for Health Research in Portland who led the study appearing today in the

.

The safety, low cost, and ready availability of vitamin D make it worthy of further study, LeBlanc says. Nearly four out of five women in her group's study had vitamin D levels less than 30 nanograms per milliliter, which is low but not deficient. At such a high rate of prevalence, an influence on weight gain would affect large numbers of people.

"Weight gain is such a huge problem, and so difficult to treat, looking for an intervention that can be so easily implemented is worth doing," LeBlanc says.

Lack of exposure to sun is a major cause of vitamin D deficiency. People with dark skin are more likely to be deficient because heavier pigmentation blocks the ultraviolet rays that drive the production of vitamin D in skin cells. Older people are more vulnerable to harm from deficiency.

Researchers say it's plausible that the vitamin plays a role in the regulation of body fat. Fat cells have receptors that bind vitamin D, and the vitamin can change the growth, metabolism and gene activity of human fat cells grown in a lab dish. Animals given extra vitamin D resist weight gain.

But the few human clinical trials to date on vitamin D supplements and body fat have produced mixed results. In the Women's Health Initiative, women who took daily vitamin D and calcium supplements maintained a lower body mass index and waist size after 7 years than women taking placebo pills. But in at least two clinical trials, vitamin D supplements had no effect on weight loss.

If vitamin D has a role in weight management, it could be limited to prevention of weight gain, according to Dr. Xiao-Mei Mai and collagues at the Norwegian University of Science and Technology, authors of

in the American Journal of Epidemiology.

Vitamin D requirements

Most Americans are getting enough vitamin D, according to

by the Institute of Medicine in 2010. But the risk of deficiency increases significantly after age 70. Here's what the experts agree on:

Daily vitamin D intake: At least 600 international units (IUs); and for those age 71 and older, at least 800 IUs per day.

Who needs testing: Many factors influence risk of deficiency, it's best to decide in consultation with a trusted primary care provider.

What blood level is deficient: Below 20 nanograms per millilter (ng/ml).

How much is too much: Excessive vitamin D can damage the kidneys and heart. Safe upper intake levels are 2,500 IUs per day for children age 1 through 3; 3,000 IUs daily for children 4 through 8 years old; and 4,000 IUs daily for all others.

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