Recommended Dietary Intake for Vitamin D

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Due to the very small and insignificant amounts of vitamin D secreted in human milk, it has historically been concluded that there is no evidence that lactation increases maternal requirements for vitamin D. Therefore, the current recommended adequate intake remains similar to nonlactating adults and is set at 200 IU/day [21]. Since the establishment of this recommended dietary intake of vitamin D in 1997, concerns about the wide spread prevalence of vitamin D deficiency have surfaced in the medical and scientific literature. Furthermore, the basis of these recommendations was made prior to the use of circulating 25-hydroxyvitamin D as an indicator of vitamin D status. To date, there is no scientific literature available pertaining to the minimum vitamin D intake needed to maintain normal concentrations of maternal circulating 25-hydroxyvitamin D. The appropriate dose of vitamin D during lactation appears to be greater than the current dietary reference intake of 200 IU/day. Supplemental intake of 400 IU vitamin D per day has only a moderate effect on maternal blood concentrations of 25-hydroxyvitamin D [52]. Many experts agree that a desirable 25-hydroxyvitamin D concentration is >75 nmol/l (30 ng/ml) [52], and attainment of these levels requires an additional intake of approximately 1,700 IU/day [53].

Currently, the US Institute of Medicine considers an intake of 2,000 IU/day for lactating women to be the tolerable upper intake level. The upper tolerable level, as defined by the US Institute of Medicine, is the highest level of continuing daily nutrient intake that is likely to pose no risk of adverse health effects in almost all individuals. Hathcock and colleagues [54] recently focused on the risk of hypercalcemia and demonstrated that the margin of safety for vitamin D consumption for adults is likely greater than ten times any current recommended level. These authors conclude that the tolerable upper limit for vitamin D consumption by adults should be set at 10,000 IU/day [54].

Furthermore, vitamin D is a fat-soluble vitamin and is stored in body fat. As a result, several studies have linked obesity with poorer vitamin D status, as demonstrated by lower circulating 25-hydroxyvitamin D concentrations [55-58]. A study conducted by Wortsman and colleagues [58] confirmed that obese patients had lower basal 25-hydroxyvitamin D and higher serum parathyroid hormone concentrations than nonobese patients. Following exposure to an identical amount of UVB radiation, the blood concentration of vitamin D was 57% less in obese than in nonobese subjects. It was proposed that the lower serum 25-hydroxyvitamin D levels seen among obese subjects were the result of increased sequestering of vitamin D in fat tissue. Likewise, body mass index (BMI) was inversely correlated with peak blood vitamin D concentrations after oral dosing. In conclusion, obese subjects may have a greater requirement for vitamin D than their nonobese counterparts do.

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