he adult recommended intake for vitamin D is based on obsolete studies performed on children using cod liver oil supplements to prevent rickets. As a result the recommended intake of vitamin D does not provide enough of the vitamin for adults. In equatorial countries this is not a concern as the body can synthesise vitamin D from the action of ultra violet light on cholesterol in the skin. However, in more northerly latitudes endogenous vitamin D production is not as efficient and this can lead to long term vitamin D insufficiency if dietary intake does not compensate. Although high numbers of white individuals living in high latitude countries are vitamin D insufficient, the numbers of individuals of African decent are even higher. This is because those with dark skin require increased levels of sunlight to synthesise adequate vitamin D as the high melanin content of the skin prohibits this process somewhat.
Supplementation is increasingly being seen as the only solution to maintain correct vitamin D status. This is due to the range of foods containing vitamin D is severely limited. Because people of African descent require additional vitamin D, researchers have investigated the amount of supplemental vitamin D required to maintain optimal plasma concentrations. For example, in one study researchers1 measured the serum concentrations of 25-hydroxyvitamin D [25(OH)D)], the accepted biological marker for vitamin D status, in 47 health individuals while administering 4000 iu per day of vitamin D. At baseline more than 90 % of African American subjects had serum 25(OH)D concentrations below 32 ng/mL and around 75 % had serum concentrations below 20 ng/mL. Supplementation with 4000 iu per day for 1 year was required to remove the disparity in vitamin D status between the black and white men in the study.
These results support the growing body of evidence that optimal vitamin D status can only be maintained at higher latitudes with vitamin D supplements. Current recommendations suggest that 2000 iu or above may be required to maintain optimal concentrations, which is roughly 10 times the current recommended vitamin D intake. For those with darker skin this requirement may increase to 20 times the recommended intake. The higher requirement for vitamin D may explain the higher risk for cardiovascular disease and diabetes seen in black individuals, as vitamin D insufficiency is increasingly being linked to these conditions. As has been shown previously, those with the lowest baseline 25(OH)D concentrations had the largest increase in status with supplements. Production of endogenous synthesis of vitamin D decreases as circulating levels reach 40 ng/mL suggesting that this might be the optimal circulating level.
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