Vitamin D and its metabolites 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D are involved in cell differentiation and cell regulation. This role as a steroid hormone, may explain the association between low levels of vitamin D and a number of Western lifestyle diseases. In particular, research is showing that a high level of circulating 25-hydroxyvitamin D, the biological marker for vitamin D status, is associated with an increased risk of blood glucose disorders (including type 1 and type 2 diabetes), calcium disorders and cancer. For this reason studies have investigated the factors that influence vitamin D status. The most well understood factor to influence vitamin D status is the amount of sunlight exposure. This is fairly logical because vitamin D is synthesised from the action of ultraviolet light on the skin. Another factor that can influence vitamin D levels are vitamin D supplements, and these have also been shown to have a major impact on the vitamin D status of an individual.
Skin colour may be a significant influence on vitamin D synthesis rates, and therefore may determine vitamin D status. Studies have shown that darker skin requires higher levels of sun exposure in order to maintain vitamin D levels at the same level as someone with lighter skin. Genetic factors are therefore also influential of vitamin D status. Studies1 have analysed the genetic components relating to vitamin D status in African Americans, and results from this research suggests that a proportion of the variability in vitamin D levels is related to environment and a proportion to genetics. For example, season, site of blood draw, walking, a healthy diet, supplement use and dairy and cereal consumption are environmental variables that were associated with vitamin D status. In addition, gender and body mass index were genetic variables also associated with vitamin D status. These factors together accounted for 25 % of the variability of vitamin D status seen between individuals.
In this study, as had been shown previously many times, supplement users had significantly higher levels of vitamin D than non-supplement users. However, genetic factors were also shown to play a significant role, although the gene regions identified as influencing vitamin D status were not consistent with those identified in previous studies. These results therefore show that vitamin D status is difficult to predict because it is based on numerous influential factors that interact. However, general recommendations to expose the skin to sunlight and to consume dietary supplements or fortified foods is recommended whatever the underlying genetic make-up. Both sun exposure and supplements of vitamin D have been shown to increase blood levels of vitamin D into the sufficient range, and these should be the primary strategies used to maintain adequate vitamin D status. As with most human biochemical parameters, a wide variation in underlying genetic composition is therefore also apparent with vitamin D status.
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