This review discusses the processes that were followed to determine both the normal circulating 25(OH)D levels as well as the dietary recommendations and the inherent flaws surrounding both. This review provides a synopsis of the research leading to the normal levels as well as the research which supports a new, higher recommendation for circulating and dietary vitamin D.
In 1991 normal levels of 25(OH)D were determined by looking at a diverse group of subjects who appeared to be free from disease, based on a Gaussian distribution. This approach to determining a normal level is no longer an acceptable practice. Early studies into vitamin D status identified that individuals with increased sun exposure had higher than “normal” levels. The author recommends that nutritional vitamin D deficiency be defined as <80mnol/L .
The author notes that since naturally occurring dietary vitamin D is rare in our food supply, it is likely that humans evolved to produce vitamin D primarily through cutaneous generation and therefore to determine the normal levels for vitamin D, one needs to look at those who have increased sun exposure. Multiple studies have have demonstrated that a total body minimal erythemic dose of sunlight can provide 10000-20000IU of vitamin D-3 within 24 hours of exposure.
The basis for the 1989 Recommended Daily Allowance (precursor to the RDI) of 400IU for infants and children was that this was approximately the amount found in a teaspoon of cod liver oil which was considered effective in the prevention of rickets. In 1997 the DRI was set at 200IU for persons aged up to 50years. Several subsequent studies have shown this level is insufficient to maintain an acceptable circulating 25(OH)D level. Based on multiple studies, the author concludes that the adult requirement for Vitamin D likely exceeds 2000IU/d.
This article very clearly identifies the flaws in the current DRI’s for vitamin D. Higher values for a normal circulating level of 25(OH)D have already been accepted in the medical and scientific community but the recommendations for dietary vitamin D lag are insufficient to maintain these levels. The vast majority of people do not achieve adequate sunlight for cutaneous generation. Many physicians routinely recommend supplementing vitamin D however these recommendations may not be adequate to achieve a physiological effect.
A report by the Institute of Medicine on the DRIs for Vitamin D and Calcium is expected in November of 2010.
Impact on Practice
Vitamin D deficiency has been identified across the stages of CKD. Although this review does not address chronic kidney disease specifically, a higher DRI for Vitamin D will also apply to those with CKD and assessment of the nutritional adequacy of the renal diet and supplementation will have to take this into account.
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