Vitamin D and its major metabolites: Serum levels after graded oral dosing in healthy men

We determined the quantitative relationships between graded oral dosing with vitamin D-3, 25(OH)D-3, and 1,25(OH)(2)D-3 for short treatment periods and changes in circulating levels of these substances. The subjects were 116 healthy men (mean age, 28 +/- 4 years, with usual milk consumption of,<0.47 l/day and mean serum 25(OH)D of 67 +/- 25 nmol/l). They were distributed among nine open-label treatment groups: vitamin D-3 (25, 250 or 1250 mu g/day for 8 weeks), 25(OH)D-3 (10, 20 or 50 mu g/day for 4 weeks) and 1,25(OH)(2)D-3 (0.5, 1.0 or 1.0 mu g/day for 2 weeks). All treatment occurred between January 3 and April 3. We measured fasting serum calcium, parathyroid hormone, vitamin D-3, 25(OH)D and 1,25(OH)(2)D immediately before and after treatment. In the three groups treated with vitamin D-3, mean values for-circulating vitamin D3 increased by 13, 137 and 883 nmol/l and serum 25(OH)D increased by 29, 146 and 643 nmol/l for the three dosage groups, respectively. Treatment with 25(OH)D-3 increased circulating 25(OH)D by 40, 76 and 206 nmol/l, respectively. Neither compound changed serum 1,25(OH)(2)D levels. However, treatment with 1,25(OH)(2)D-3 increased circulating 1,25(OH)(2)D by 10, 46 and 60 pmol/l, respectively, Slopes calculated from these data allow the following estimates of mean treatment effects for typical dosage units in healthy 70-kg adults: an 8-week course of vitamin D-3 at 10 mu g/day (400 IU/day) would raise serum vitamin D by 9 nmol/l and serum 25(OH)D by 11 nmol/l; a 3-week course of 25(OH)(3) at 20 mu g/day would raise serum 25(OH)D by 94 nmol/l; and a 2-week course of 1,25(OH)(2)D-3 at 0.5 mu g/day would raise serum 1,25(OH)(2)D by 17 pmol/l.