BACKGROUND: The Food and Nutrition Board of the National Academy of Sciences states that 95 microg vitamin D/d is the lowest observed adverse effect level (LOAEL). OBJECTIVE: Our objective was to assess the efficacy and safety of prolonged vitamin D3 intakes of 25 and 100 microg (1000 and 4000 IU)/d. Efficacy was based on the lowest serum 25-hydroxyvitamin D [25(OH)D] concentration achieved by subjects taking vitamin D3; potential toxicity was monitored by measuring serum calcium concentrations and by calculating urinary calcium-creatinine ratios. DESIGN: Healthy men and women (n = 61) aged 41 +/- 9 y (mean +/- SD) were randomly assigned to receive either 25 or 100 microg vitamin D3/d for 2-5 mo, starting between January and February. Serum 25(OH)D was measured by radioimmunoassay. RESULTS: Baseline serum 25(OH)D was 40.7 +/- 15.4 nmol/L (mean +/- SD). From 3 mo on, serum 25(OH)D plateaued at 68.7 +/- 16.9 nmol/L in the 25-microg/d group and at 96.4 +/- 14.6 nmol/L in the 100-microg/d group. Summertime serum 25(OH)D concentrations in 25 comparable subjects not taking vitamin D3 were 46.7 +/- 17.8 nmol/L. The minimum and maximum plateau serum 25(OH)D concentrations in subjects taking 25 and 100 microg vitamin D3/d were 40 and 100 nmol/L and 69 and 125 nmol/L, respectively. Serum calcium and urinary calcium excretion did not change significantly at either dosage during the study. CONCLUSIONS: The 100-microg/d dosage of vitamin D3 effectively increased 25(OH)D to high-normal concentrations in practically all adults and serum 25(OH)D remained within the physiologic range; therefore, we consider 100 microg vitamin D3/d to be a safe intake.
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