Vitamin D is a steroid hormone, classified as a vitamin. As well as dietary sources of vitamin D, humans can synthesise vitamin D in the skin through the action of ultraviolet light acting on cholesterol. The biologically accepted marker for vitamin D is 25-hydroxyvitamin D. Estimates of the optimal plasma levels of 25-hydroxyvitamin D have recently been revised up such 40 to 80 ng/mL (100 to 200 nmol/L) are now recommended as the desired range for healthy adults. Vitamin D levels below 30 ng/mL (60 nmol/L) put the individual at serious risk of a vitamin D deficiency, that can develop into osteomalacia, the designated deficiency disease for vitamin D. However, plasma levels of 25-hydroxyvitamin D between 30 and 40 ng/mL are defined as insufficient. Within this range, it is thought that a number of metabolic dysfunctions develop, particularly regarding blood glucose homeostasis. For example, insufficient 25-hydroxyvitamin D plasma levels are now associated with the development of insulin resistance.
Studies have investigated the relationship between vitamin D status and non-bone related measures. For example in one study, the researchers measured the relationship between a number of parameters of glucose homeostasis and 25-hydroxyvitamin D plasma levels in 239 overweight and obese women1. The results showed that plasma 25-hydroxyvitamin D concentrations were inversely associated with fasting glucose, fasting and 2-hour insulin, HOMA-IR (a method used to quantify insulin resistance), visceral abdominal fat, percentage fat and triglycerides. The association of these factors with poor vitamin D status suggest that vitamin D is indeed associated with detrimental effects to glucose homeostasis. In addition the vitamin D status of the women was also inversely associated with parathyroid hormone status, suggesting that calcium homeostasis was also disrupted. As low plasma calcium may be related to poor insulin status, the two factors may be linked (please see the calcium paradox: here).
One interesting thing about this study was the fact that the authors observed a threshold for 25-hydroxyvitamin D, below which glucose homeostasis was disrupted. This may be one of the first reports to publish such a threshold for this particular physiological dysfunction. Those women with 25-hydroxyvitamin D plasma levels below 26 ng/mL (65 nmol/L) had disruptions to their normal glucose homeostasis, while those with levels above this did not. As has been found in other studies, black subjects had lower plasma levels of 25-hydroxyvitamin D compared to white subjects independent of other variables. This has been suggested to relate to the lower synthesis rates of vitamin D is darker skin relative to the available sunlight. The evidence that vitamin D supplementation is effective at causing weight loss in those who experience an increase in plasma levels of 25-hydroxyvitamin D (here), may be due to the beneficial effects of vitamin D on blood glucose homeostasis.
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