The traditional role of vitamin D is considered to that of maintaining bone homeostasis. However, more recently, vitamin D has been shown to be involved in a far wider range of physiological roles including maintaining insulin homeostasis, cancer prevention and immunity. The historic recommendations for adult vitamin D intakes were based on the amount of the vitamin required to prevent rickets in children, but these intakes bare no resemblance to the physiological needs of healthy adult humans. This is especially true considering the newly discovered roles for the vitamin. Evidence continues to mount to suggests much higher intakes of vitamin D are required to maintain health than were previously considered necessary. It has been reported that obese subjects have lower levels of plasma 25-hydroxyvitamin D, the accepted biomarker for vitamin D status, than lean counterparts. Obese subjects may therefore be at increased risk of vitamin D deficient diseases, including those of the bone.
For example, the plasma 25-hydroxyvitamin D levels and bone histologies of 24 grossly obese subjects were investigated in comparison to lean healthy controls1 The 25-hydroxyvitamin D plasma levels of the obese subjects were significantly lower than lean controls [25.1 nmol/L (10.0 mg/mL) versus 32.5 nmol/L (13 ng/mL), respectively]. However, the mean values for the two groups were both very low and placed both groups at risk of vitamin D deficiency. In particular the mean value for the obese subjects was in the range of a severe vitamin D deficiency, while those of the healthy controls was insufficient to borderline deficient. Sufficient plasma vitamin D levels may be as high as 100 nmol/L (40 ng/mL). Of the obese patients, two had abnormal bone histologies and in one of these mild osteomalacia and secondary hyperparathyroidism, both symptoms of a vitamin D deficiency, were detected. The other obese patients all had evidence of increased bone turnover.
Therefore it appears that obese individuals may be at an increased risk of vitamin D deficiencies compared to normal healthy subjects. This poorer status may increase the risk of vitamin D associated conditions, particularly involving the bones. The reason for the poorer vitamin D status in obese subjects is not known but a number of hypotheses have been put forward. One suggestion is that the adipose tissue is able to store larger quantities of vitamin D in the obese due to its increased volume. Evidence for this hypothesis is supported by studies showing that when subjects are exposed to low vitamin D conditions some obese individuals appear to maintain plasma levels of 25-hydroxyvitamin D for longer suggesting that stored vitamin D is being released from adipose tissue. Alternatively, the poor vitamin D status of obese individuals may relate to a reduced exposure to sunlight on account of an unwillingness to expose themselves, or could simply be a reflection of a poor quality diet.
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