Observational studies have consistently reported an increased risk of vitamin D deficiency in those who are obese. However, the underlying explanations and direction of causality are unclear. Does obesity cause vitamin D deficiency, does vitamin D deficiency cause obesity or is it a little of both?
Activated vitamin D (1,25-dihydroxyvitamin D) may influence the mobilization of free fatty acids from the adipose tissue as well as metabolism of fat cells. Rat experiments have shown that large doses of vitamin D lead to increases in energy expenditure, a mechanism of weight loss. However, three randomized controlled trials testing the effect of vitamin D supplementation on weight loss in obese or overweight individuals showed no effect. On the other hand, there have been randomized controlled trials that have shown a very modest weight reduction after vitamin D supplementation, too.