[Body weight and bone/calcium metabolism. Obesity and vitamin D.]

Obesity is associated with lower serum 25(OH)D level via several mechanisms including sequestration of fat soluble vitamin D in increased fat mass. Since obesity is the major cause of insulin resistance and type 2 diabetes, lower serum 25(OH)D level is also associated with these conditions. Non-surgical weight reduction, especially that results in decreased visceral fat mass, is associated with an improvement in insulin resistance and a small but significant increase in serum 25(OH)D level. Whether the latter is independently associated with the former is not known. Plural meta-analyses reported that vitamin D supplementation per se without life-style intervention is not associated with a significant weight reduction. However, recent meta-analyses of randomized controlled trials in which large doses vitamin D over 2,000 IU/day supplemented to type 2 diabetes patients revealed a small but significant improvement in indices of insulin resistance and glycemic control. The beneficial effects of vitamin D supplementation on glucose metabolism appeared to be more prominent in non-obese subjects in whom higher serum 25(OH)D level were attained, suggesting potential benefits of vitamin D on glucose metabolism is not mediated by weight or fat mass control.