Nature of Vitamin D's Role in Weight Loss Yet Unknown


WASHINGTON — People who are insufficient in vitamin D lose less weight on a low-calorie diet than do those with higher vitamin D levels, a small study has found. But whether or not the low vitamin D level is the cause of less weight loss remains unclear.

“Vitamin D deficiency is closely linked with obesity,” Dr. Shalamar Sibley of the University of Minnesota, Minneapolis, the study's lead author, said at the annual meeting of the Endocrine Society. “But the cause-and-effect nature of this relationship is unclear. This association may be due to sequestration of vitamin D in adipose tissue, and this is a likely contributor. But the other way to look at the question is, is it possible that the lack of vitamin D is promoting obesity or inhibiting weight loss?”

Several small studies have addressed this issue, according to Dr. Sibley.

One study of 60 overweight or obese women comparing a cereal diet with a vegetable diet found that women with higher baseline levels of 25-hydroxyvitamin D lost more weight than did those with lower levels (Br. J. Nutr. 2008;100:269-72).

Another study of 24 overweight women designed to look at the effects of dietary versus supplemental calcium on total energy expenditure with a hypocaloric diet found that women with higher 25-hydroxyvitamin D levels showed increased thermic activity following meal consumption, compared with those with lower vitamin D levels (Obesity 2008;16:1566-72).

To further explore the issue, Dr. Sibley and colleagues recruited 20 women and 18 men with a body mass index of at least 27 kg/m

Participants were screened at baseline to assess usual dietary intake, physical activity, and anthropomorphic and body composition measurements, including height, weight, fat and tissue mass (with dual-energy x-ray absorptiometry), abdominal fat mass distribution (with single-slice CT), and plasma 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D. On average, participants were in the “insufficient” range for vitamin D, but were not outright deficient, Dr. Sibley said.

At the end of the study, women had lost an average of about 4 kg and men about 7 kg. The researchers found levels of both forms of vitamin D at baseline predicted the magnitude of weight loss, after adjustment for sex.

Dr. Sibley added that 1,25-dihydroxyvitamin D “did in fact predict abdominal fat loss in particular, and both forms [of vitamin D] predicted subsequent fat mass loss in abdominal subcutaneous compartments.”

Furthermore, “no significant correlations between baseline BMI, age, or season with baseline vitamin D concentrations or weight loss” were shown, according to Dr. Sibley.

In addition, 1,25-dihydroxyvitamin D levels did not change from baseline to post-weight loss; neither did 25-hydroxyvitamin D levels, she said. Both 1,25-dihydroxy and 25-hydroxyvitamin D levels were closely correlated with each other, both at baseline and after weight loss.

The study's limitations were that it was observational, and that the results could be confounded by the “healthy cohort” effect, she said.

The study was sponsored by the National Institutes of Health, the University of Minnesota, and the Pennock Family Endowment.

Dr. Sibley said he had no relevant conflicts of interest to disclose.

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