Link between obesity and vitamin D clarified

The link between obesity and vitamin D deficiency appears to be a one-way street. A large study of the genetics underpinning both conditions finds that obesity may drive down vitamin D levels, but a predisposition to the vitamin deficiency doesn’t lead to obesity. The findings also suggest that boosting vitamin D levels won’t reverse obesity.

An association between the two has been observed for years, but determining cause and effect has been difficult. “I find this very plausible and a correct interpretation of the data,” says Robert Heaney, an endocrinologist at Creighton University in Omaha, Neb. “I think it’s worth reporting.”

In the new study, researchers tapped into a huge international database, accessing the genetic profiles of more than 42,000 people. The scientists noted whether a person harbored any of 12 genetic variants associated with being overweight. Not surprisingly, people with these variants were more likely to be obese than those without them. People with these obesity-associated gene variants were also apt to have low vitamin D levels, Elina Hyppönen, an epidemiologist and nutritionist at University College London, and colleagues report online February 5 in PLOS Medicine.

When the researchers tested for four genetic variants linked to low vitamin D levels, they found that people with the variants were not necessarily prone to obesity. The researchers checked both findings against a separate database of people and got similar results.

The data suggest that losing weight could potentially reverse vitamin D deficiency in obese people, says Hyppönen. Or, she says, obese people could improve their overall health by having their vitamin D checked and correcting any deficiency. Vitamin D is important for, among other things, fighting infection, absorbing calcium and maintaining a healthy immune system.

The underlying mechanism that would explain why being obese suppresses vitamin D is still a matter of debate. Because vitamin D is fat soluble, some scientists had assumed that it got sequestered in fatty tissues, Heaney says. If so, less vitamin D would reach the bloodstream. But while the vitamin is indeed stored in fat, there’s no strong evidence that fatty tissue sequesters and retains vitamin D, Heaney says.

His work suggests that vitamin D gets diluted throughout the body, and that dilution shows up as a deficiency in overweight and obese people, who have more mass than normal-weight people. In people who garner most of their vitamin D from sun exposure, Heaney says, the deficiency could be especially bad. As a person adds weight and volume, the amount of their skin surface area doesn’t expand proportionately, and so they fail to maintain adequate levels, Heaney and colleagues report in the December 2012 Obesity.