Vitamin D and Obesity: Current Evidence and Controversies.

Data from meta-analyses consistently support an inverse association of vitamin D levels with body weight. However, the impact of weight loss on improving vitamin D status is small, while studies on the supplementation with vitamin D after bariatric surgery have shown conflicting results regarding vitamin D status. Moreover, interventional studies do not support a beneficial effect of vitamin D supplementation on body weight. These findings warrant a cautious interpretation due to important methodological limitations and confounding factors, such as high heterogeneity of studies, variable methods of determination of vitamin D and definition of deficiency/insufficiency, use of various adiposity measures and definitions of obesity, and inadequate adjustment for confounding variables influencing vitamin D levels. The underlying pathogenetic mechanisms associating low vitamin D in obesity include volumetric dilution, sequestration into adipose tissue, limited sunlight exposure, and decreased vitamin D synthesis in the adipose tissue and liver. Experimental studies have demonstrated that low vitamin D may be implicated in adipose tissue differentiation and growth leading to obesity either by regulation of gene expression or through modulation of parathyroid hormone, calcium, and leptin. Obesity is associated with low vitamin D status but weight loss has little effect on improving this; vitamin D supplementation is also not associated with weight loss. Evidence regarding vitamin D status after bariatric surgery is contradicting. The link between vitamin D and obesity remains controversial due to important limitations and confounding of studies. More research is needed to clarify the complex interplay between vitamin D and adiposity.