No abstract

Vitamin D deficiency and the rapid increase in the prevalence of obesity are both considered important public health issues. The classical role of vitamin D is in Ca homoeostasis and bone metabolism. Growing evidence suggests that the vitamin D system has a range of physiological functions, with vitamin D deficiency contributing to the pathogenesis of several major diseases, including obesity and the metabolic syndrome. Clinical studies have shown that obese individuals tend to have a low vitamin D status, which may link to the dysregulation of white adipose tissue. Recent studies suggest that adipose tissue may be a direct target of vitamin D. The expression of both the vitamin D receptor and 25-hydroxyvitamin D 1a-hydroxylase (CYP27B1) genes has been shown in murine and human adipocytes. There is evidence that vitamin D affects body fat mass by inhibiting adipogenic transcription factors and lipid accumulation during adipocyte differentiation. Some recent studies demonstrate that vitamin D metabolites also influence adipokine production and the inflammatory response in adipose tissue. Therefore, vitamin D deficiency may compromise the normal metabolic functioning of adipose tissue. Given the importance of the tissue in energy balance, lipid metabolism and inflammation in obesity, understanding the mechanisms of vitamin D action in adipocytes may have a significant impact on the maintenance of metabolic health. In the present review, we focus on the signalling role of vitamin D in adipocytes, particularly the potential mechanisms through which vitamin D may influence adipose tissue development and function. The vitamin D endocrine system has been linked historically to the aetiology of rickets (1) . Growing evidence suggests that in addition to maintaining Ca homoeostasis and skeletal health (2,3) , vitamin D has pleiotropic actions that can affect multiple organs and metabolic processes, including in the cardiovascular, renal and immune systems (4 -7) . It has been argued that vitamin D deficiency as a global health issue may contribute to the pathogenesis of a number of disorders, including obesity, the metabolic syndrome and type 2 diabetes (8 -11) . Clinical and epidemiological studies show that obese individuals tend to have low vitamin D status (12 -17) .Although vitamin D bioavailability could be reduced in obesity due to increased sequestration by white adipose tissue (13,18) , the mechanisms underlying the inverse relationship between adiposity and vitamin D deficiency are largely unknown. Interestingly, recent studies suggest that (white) adipose tissue could be a direct target of vitamin D, and that the hormone may modulate adipose tissue formation and function (19 -23) . Given the multiplicity of functions of white adipose tissue, and the link between dysfunction of the tissue and the pathogenesis of obesity and its co-morbidities, clarifying the role of vitamin D in adipose tissue may lead to public health benefits.In the present article, we discuss recent advances in our understandin...