Vitamin D in adult health and disease: a review and guideline statement from Osteoporosis Canada

After synthesis in the skin or dietary ingestion, vitamin D is removed from the bloodstream into various tissues, including the liver, adipose tissue and muscle. Its biologic half-life is about 60 days,4 and it is eventually converted to 25-hydroxy - vitamin D in the hepatocytes.4,5 Vitamin D3 (cholecalciferol) is the molecule synthesized in the skin in response to ultraviolet B radiation, whereas vitamin D2 (ergocalciferol) is derived from irradiation of certain fungi. Both vitamin D2 and vitamin D3 create 1,25-dihydroxyvitamin D, the active form, although there is some evidence that vitamin D2 may not be used in the body as efficiently as vitamin D3.6 In Canada, most vitamin D supplements consist of vitamin D3, but high-dose preparations, available by prescription, are vitamin D2. In this paper we use the term "vitamin D" to refer to both forms, unless a distinction is warranted. There has been a marked increase in the clinical use of 25- hydroxyvitamin D assays.11 However, serum 25-hydroxyvitamin D should be measured only if deficiency is suspected or would affect the person's response to therapy (e.g., in cases of impaired intestinal absorption, such as celiac disease, or osteoporosis requiring pharmacologic therapy). In treating deficiency, serum 25-hydroxyvitamin D will indicate the effectiveness of vitamin D therapy. The half-life of 25- hydroxyvitamin D in the body is 15-20 days.4 With standarddose supplementation, serum 25-hydroxyvitamin D plateaus after three to four months.12 Therefore, serum 25-hydroxyvitamin D should be checked no sooner than three months after standard-dose treatment is initiated (level 2 evidence, grade B recommendation). After high-dose oral or parenteral vitamin D replacement is administered (e.g., 500 000 IU), the peak 25-hydroxyvitamin D level may be achieved in one month.13 Patients taking daily doses above Health Canada's "tolerable upper intake level" (currently set at 50 µg [2000 IU]) should undergo monitoring of serum 25-hydroxyvitamin D (level 4 evidence, grade D recommendation). For healthy Canadians, the dose recommendations for routine supplementation in this paper should result in adequate blood levels. Monitoring of routine supplement use and routine testing of otherwise healthy individuals as a screening procedure are not indicated (grade D recommendation). When supplements are used to treat vitamin D insufficiency, the amount should be great enough to increase 25- hydroxyvitamin D to desirable levels. Daily