Vitamin D is naturally produced by the human body when exposed to direct sunlight. Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis in the skin, and it is important for individuals with limited sun exposure to include good sources of vitamin D in their diet.
Extra vitamin D is also recommended for older adults and people with dark skin. Individuals having a high risk of deficiency should consume 25 μg (1000 IU) of vitamin D daily to maintain adequate blood concentrations of 25-hydroxyvitamin D. In the United States and Canada, for example, fortified milk typically provides 100 IU per glass, or a quarter of the estimated adequate intake for adults over age 50.
In the United States (U.S.), the 100% Daily Value used for product labels is 400 IU/day and typical diets provide about 100 IU/day. Although milk is usually fortified, the average daily consumption by most Americans is insufficient in obtaining levels of vitamin D recommended by various medical authorities.
While adequate intake has been defined as 200 IU/day for ages infant to 50, 400/day for 51-70, and 600/day over 70, the American Academy of Pediatrics argues that these recommendations are insufficient and recommends a minimum of 400 IU, even for infants. The NIH has set the safe upper limit at 2000 IU, but acknowledges newer data supporting a UL as high as 10,000 IU/day. The Institute Of Medicine is revisiting vitamin D and calcium recommendations with a report expected to be released in spring 2010.
Measuring nutritional status
A blood calcidiol (25-hydroxy-vitamin D) level is the accepted way to determine vitamin D nutritional status. The optimal level of serum 25-hydroxyvitamin D is 35–55 ng/ml (or 90-140 nmol/l); with some debate among medical scientists for the slightly higher value.
For instance, a later classification is:
- 0-14.9 ng/ml = Severely deficient
- 15.0-31.9 ng/ml = Mildly deficient
- 32.0-100.0 ng/ml = Optimal
- >100.0 ng/ml = Toxicity possible
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