Implications for 25-hydroxyvitamin D testing of public health policies about the benefits and risks of vitamin D fortification and supplementation.

Vitamin D status is of interest to physicians caring for patients in poor general health. The tool for assessing vitamin D status is the serum 25-hydroxyvitamin D (25(OH)D) concentration. Based on clinical trials and epidemiology the low end of the desirable concentration of this analyte generally ranges from 50 nmol/L to 75 nmol/L. Based on clinical trials, the high end of the safe concentration for 25(OH)D is at about 225 nmol/L, with an unspecified margin of safety beyond that. In the absence of sunlight, 225 nmol/L is achieved with prolonged consumption of about 10,000 IU/day (250 μg/day) of vitamin D3. Hence that intake should be regarded as safe, and in the absence of sunshine, comparable vitamin D-wise to abundant sun exposure. Government policy is very conservative, and consequently, the latest advice from the Institute of Medicine for Canada and the USA specifies that in the absence of sunshine, a recommended dietary allowance (RDA) of 600 IU/day (15 μg/day) of vitamin D will provide a serum 25(OH)D concentration of at least 50 nmol/L. Dietary-vitamin D-intake statistics for adult populations show that average intakes from food and supplements are 200-400 IU/day (5-10 μg/day), respectively. Therefore, adult populations are consuming vitamin D in amounts far below the RDA. Even if adults were to consume the RDA for vitamin D, it would still not be enough to ensure 25(OH)D > 50 nmol/L. The implication of all these things for the clinical laboratory is that there will continue to be a demand for 25(OH)D measurements for many years to come.