The role of vitamin D in human metabolism has expanded in recent years, with the discovery that metabolites of the hormone are required for proper immune function, cellular signalling and insulin receptor sensitivity. Therefore vitamin D is no longer considered just a requirement for skeletal health. Further, the current recommendations for dietary vitamin D intake have come under scrutiny because research has consistently shown that these amounts are too low for optimal vitamin D status. Because vitamin D can be produced in the skin, sunlight is an important provider of vitamin D. However, inhabitants of high latitudes do not have access to year round sun exposure, and are therefore at particular risk of developing a vitamin D deficiency during the winter months. This problem is exacerbated by those with darker skin, because they require more intense sunlight to synthesis endogenous vitamin D.
A large body of research has investigated the effects of vitamin D supplements on vitamin D status. However, little research has focussed on the amount of sunlight necessary to prevent vitamin D deficiency. Because of this, one study1 attempted to determine the effects of UVB radiation on the vitamin D status of South Asians in comparison to individuals with white skin. Artificial sunlight was provided at an intensity similar to that which would be experienced in the summer months in the United Kingdom and serum levels of 25 hydroxyvitamin D [25(OH)D] were measured on a weakly basis. At baseline, before any treatment had been administered all of the Asian subjects had insufficient vitamin D status with 25(OH)D plasma levels below 20ng/mL. In addition, 27% were vitamin D deficient with 25(OH)D plasma levels below 5ng/mL. These subjects were therefore at an increased risk of developing serious disease.
Administration of UVB radiation increased plasma levels of 25(OH)D in all subjects, however all South Asian subjects remained vitamin D insufficient despite this treatment, whereas 90% of the white subjects became vitamin D sufficient. The mean increase in 25(OH)D in the white subjects was 10.5ng/mL, compared to only 4.3ng/mL in the South Asian subjects. Interestingly, the authors noted the very low vitamin D intakes in the diets of all subjects enrolled on the study. This supports previous findings and likely relates to the limited food choices that contain high levels of vitamin D, including fish and fish products. These results would suggest that sunlight in the United Kingdom is not intense enough to raise levels of vitamin D in those with darker skin, even during the summer months. Supplementation of such groups with vitamin D3 would be a practical solution to improve vitamin D status.
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