Posted: Dec 17, 2010 6:39 PM
Dec. 17, 2010 -- New research adds to the evidence linking obesity with lower levels of vitamin D, and the finding could help explain why carrying extra pounds raises the risk for a wide range of diseases, researchers say.
The study suggests that people who are obese may be less able to convert vitamin D into its hormonally active form.
Researchers measured levels of vitamin D and its hormonally active form 1,25(OH)2D in almost 1,800 people being treated at a weight loss clinic in Norway.
Most of the people were overweight and 11% were considered morbidly obese, with a body mass index (BMI) of 40 or higher.
The more the study participants weighed, the lower their vitamin D levels tended to be, study researcher Zoya Lagunova, MD, of the Rikshospitalet-Radiumhospitalet Medical Center in Oslo, Norway, tells WebMD.
She says it makes sense that obesity is associated with lower vitamin D levels since vitamin D is a fat-soluble vitamin.
The study appears in the latest issue of the Journal of Nutrition.
"Much of the vitamin D produced in the skin or ingested is distributed in fat tissue," she says. "So obese people may take in as much vitamin D from the sun, food, or supplements as people who are not obese, but their [blood] levels will tend to be lower."
Along with calcium, vitamin D has long been recognized as important for bone health. But in recent years studies have suggested that the vitamin plays a part in a host of diseases, including heart disease, diabetes, and certain cancers.
Perhaps not coincidentally, obesity is a known risk factor for many of these diseases, Lagunova says.
The researchers concluded that 1,25(OH)2D levels might be a better measure of vitamin D in obese people than circulating levels of the vitamin.
But Clifford J. Rosen, MD, of the Maine Medical Center Research Institute, is unconvinced.
Rosen tells WebMD that 1,25(OH)2D is more difficult and costly to measure and is not a very good indicator of vitamin D status.
He adds that while people who are obese do tend to have lower vitamin D levels, the clinical relevance of this is unclear.
Lagunova and colleagues conclude that people who are obese may need more vitamin D in their diets or greater exposure to the sun than thinner people.
"Obese people may need more vitamin D to end up with the same levels as a person whose weight is normal," Lagunova says.
But it is far from clear if taking large doses of vitamin D in supplement form is beneficial or even safe, regardless of a person's body weight, says Rosen.
An osteoporosis researcher, Rosen served on an expert panel that reviewed the vitamin D research and concluded that most people get enough of the vitamin without taking supplements.
The panel, convened by the independent health policy advisory group Institute of Medicine (IOM), published its findings late last month.
The IOM committee found the evidence suggesting a role for vitamin D supplementation in the prevention of non-bone related diseases to be inconclusive. It also warned that intake of vitamin D over 4,000 IU (international units) per day increases the risk for harm to the body.
Rosen says there was no suggestion that obese people needed extra vitamin D to maintain adequate levels.
Rutgers University professor of nutritional sciences Sue A. Shapses, PhD, also served on the IOM panel. She calls the research suggesting a protective role for vitamin D against non-bone-related disease "compelling" but far from conclusive.
"We know that vitamin D deficiency is not good, but most people are not deficient," she says. "And there are just too many unanswered questions to recommend that people take high doses of vitamin D in supplement form."