Enormous progress in the investigation of vitamin D is currently being made from the
perspective of basic science to clinical medicine. The typical view of vitamin D function limited to
calcium metabolism and bone homeostasis has undergone extensive revision and it has been revealed
that vitamin D receptors exist in most tissues of the body. Nowadays, one of the most popular
aspects of vitamin D in research area is its role in glucose metabolism and insulin resistance.
The functional mechanism of vitamin D in metabolism includes genomic and rapid non-genomic
actions that are discussed in this review. Briefly, the modulatory action of vitamin D in the gene expression
of insulin signaling compartments and secretion of insulin hormone may point to its role
in the pathogenesis and development of type II diabetes. Vitamin D induced activation of the
PI3K/AKT pathway is through PTEN-mediated AKT downregulation. Also, allelic variations in
VDR and DBP might affect insulin secretion and diabetes occurrence. Vitamin D influences insulin
secretion from β-cell through calcium-dependent endopeptidases, which promotes the conversion
of pro-insulin to insulin; hence it can be declared that calcium and vitamin D are essential for
insulin exocytosis. Hypovitaminosis D in obese individuals is also associated with higher levels of
serum parathormone, through which this secondary hyperparathyroidism probably contributes to insulin
resistance associated with obesity. Moreover, vitamin D is an immune modulator that may affect
inflammation as a contributor to diabetes.
Keywords: Vitamin D, calcium, glucose metabolism, insulin resistance, PI3K/AKT pathway, GLUT4, insulin signalling.
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