The importance of vitamin D to health has been highlighted in recent years in a growing body of research. The accepted clinical marker of vitamin D status is 25-hydroxyvitamin D[25(OH)D], a bioactive metabolite produced in the liver. Low plasma levels of 25(OH)D have been associated with various cancers, type 1 diabetes, type 2 diabetes, multiple sclerosis and metabolic syndrome. Vitamin D is produced in the skin via the action of ultraviolet B radiation, but can also be derived from dietary sources such as fish. Daily production or intake of vitamin D is not essential because adipose tissue can act as a reservoir, releasing the vitamin to plasma during the normal course of cellular turnover. For this reason serious vitamin D deficiencies can be delayed to the latter stages of winter in high latitudes if dietary vitamin D intake is not sufficient from sunlight.
An understanding of the tissue distribution of vitamin D allows better understanding of factors that can lead to deficiencies. To these ends, researchers1 have investigated vitamin D tissue distribution through the use of animal models, primarily centred around the pig. For example, growing pigs raised in confinement and fed a daily intake of 2000IU of vitamin D per day had mean plasma 25(OH)D levels of 45nmol/L, based on meta-analysis of a number of studies. Based on body size equivalencies, this would give the average 70kg women a total body store of 14,665IU of vitamin D, with 65% representing cholecalciferol (native vitamin D) and 35% as 25(OH)D. Nearly 75% of the total cholecalciferol in pigs was in fat, but 25(OH)D was more evenly distributed throughout the body (20% in muscle, 30% in serum, 30% in fat and 15% in other tissues).
The authors concluded that an intake of 2000IU would build up body stores enough to provide only 7 days worth of vitamin D to a pig. However the daily rate of vitamin D utilisation is different in humans. Data from human studies show that vitamin D appears to be released from and stored in adipocytes. This results in plasma levels of 25(OH)D taking longer to peak in obese individuals, compared to lean counterparts, as vitamin D is first stored in fat tissue. However, when sunlight is withdrawn, plasma levels of 25(OH)D remain higher for longer in obese individuals, due to the release of large stores of vitamin D from adipocytes. Data from pigs should be treated with caution, because animal physiology is quite different to that of man. However these pig studies confirm that vitamin D is sequestered in fat tissue, and this has important implications for winter months.
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