Evidence is accumulating that the vitamin D level of a large percentage of the Western population is insufficient to meet normal metabolic function. Vitamin D is a hormone synthesised in the skin from the action of ultra violet light on cholesterol, and factors such as high latitude (>35°), sunscreen use, working inside and lack of recreation time in the sun, can all reduce plasma levels of vitamin D. The accepted clinical marker for vitamin D status is 25-hydroxyvitamin D [25(OH)D] and current recommendations are for levels to be above 40 nmol/L (16 ng/mL) for optimal health. Although levels below 25 nmol/L (10 ng/mL) are classified as a severe deficiency, evidence suggests that many people fall into the insufficient plasma range between 25 nmol/L and 40 nmol/L. Diabetes and cardiovascular disease are two diseases that are associated with insufficient plasma 25(OH)D levels.
Because metabolic syndrome is associated with an increased risk of diabetes and cardiovascular disease, researchers are interested in the association between metabolic syndrome and vitamin D. Researchers1 investigated this association in 808 non-diabetic patients by measuring plasma 25(OH)D, fasting and postprandial glucose and insulin levels, insulin sensitivity, as well as plasma adiponectin, triacylglycerol and HDL concentrations. After making adjustments for age, gender and body mass index, 25(OH)D was inversely associated with fasting plasma glucose and insulin concentrations and insulin resistance. If adjustments were made for gender and age, plasma 25(OH)D was associated with insulin sensitivity, plasma adiponectin and HDL cholesterol, and inversely associated with plasma triacylglycerols. Compared to the subjects in the lowest tertile for plasma 25(OH)D, those in the highest tertile had a 1.6, 12.7 and 9.8% lower concentration of fasting glucose, concentration of fasting insulin and insulin resistance score, respectively.
These results suggest that plasma 25(OH)D is associated with markers for metabolic syndrome in non-diabetic subjects. This supports previous studies that have found associations between diabetes and plasma 25(OH)D concentrations. It is not known how 25(OH)D might affect insulin sensitivity, but it is possible that low 25(OH)D increases parathyroid levels, which in turn raise intracellular calcium concentrations causing interference with normal cellular metabolism. Alternatively, 25(OH)D may play a role in expression of the insulin receptor and thus have a direct relationship with the sensitivity of the cell to insulin. Low levels of vitamin D are also associated with systemic inflammation, which may result in insulin resistance. There is also strong evidence that 25(OH)D play a central role in the secretion of insulin from the β-cells of the pancreas. These results with others add to the growing literature linking vitamin D to diabetes.
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