Vitamin D is usually classified as a vitamin, nutritionally. However, it is not a true vitamin because it can be synthesised in the skin of humans from the action of ultraviolet light on cholesterol. Therefore vitamin D is not an essential dietary component, because as long as sunlight exposure is present, human have no need for dietary sources of the vitamin. Vitamin D is therefore conditionally essential, only being required under exceptional circumstances. In fact vitamin D is a steroid hormone, in much the same way as testosterone, oestrogen or progesterone. Because vitamin D is synthesised from cholesterol in animals and ergosterol in plants, it could be argued that it is a lipid, although more correctly it is a lipid derivative. It is certainly lipid soluble and enters the nucleus of cells where it binds with the vitamin D receptor to elicit genes expression and protein synthesis. As more is understood about vitamins D it is clear that it has physiological roles associated to regulation of insulin sensitivity.
Epidemiological evidence hints at the insulin sensitising role for vitamin D. In this regard, low plasma levels of the active biological form of vitamin D (25-hydroxyvitamin D) are associated with obesity, type 1 diabetes and type 2 diabetes. The way that vitamin D interacts with the insulin system is not fully understood, but low intakes of vitamin D may decrease the absorption of calcium, which in turn lowers plasma levels. This then triggers the release of parathyroid hormone with increases the expression of the 1-α-hydroxylase enzyme that causes an increase in the conversion of vitamin D to 1,25-dihydroxyvitamin D. This hormone then increases the absorption of calcium from the gut. However, if calcium intakes remain low, both parathyroid and 1,25-dihydroxyvitamin D also remain elevated, and this may stimulate an increase in cellular levels of calcium. This high cellular level of calcium in the presence of a chronic low intake of dietary calcium is termed the calcium paradox.
The calcium paradox is detrimental to fat loss because high intracellular calcium levels stimulate the de novo lipogenesis pathway and inhibit lipolysis, and this causes the creation of new fatty acids, and at the same time prevents their breakdown. The calcium paradox explains the association between high intakes of dairy products and high calcium intakes with lower levels of body fat that have been reported in epidemiological studies. But how do low intakes of vitamin D related to weight gain? Well as vitamin D is required for the correct absorption of calcium, it is possible that low intakes of vitamin D in turn cause a reduction in calcium absorption and this triggers the calcium paradox. Some evidence also suggests that vitamin D has antioxidant properties, and this may reduce oxidative stress, the latter being a cause of interference in the insulin signal cascade. The role of vitamin D as a lipid and as a steroid hormone is therefore still to be fully elucidated, although it clearly plays some role in the regulation of body weight.
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