Calcium Absorptive Effects of Vitamin D and Its Major Metabolites*

The absorptive response to graded doses of vitamin D3, 25(OH)D, and 1,25(OH)2D was measured in healthy adult men after treatment periods of eight, four, and two weeks, respectively. While no rela-tionship was found between baseline absorption and serum vitamin D metabolite levels, all three vitamin D compounds significantly elevated 45Ca absorption from a 300 mg calcium load given as part of a standard test meal. 1,25(OH)2D was active even at the lowest dose (0.5 mg/day), and the slope was such that doubling of absorption would occur at an oral dose of approximately 3 mg/day. 25(OH)D was also active in elevating absorption and did so without raising total 1,25(OH)2D levels. On the basis of the dose response curves for 1,25(OH)2D and 25(OH)D, the two compounds exhibited a molar ratio for physiological potency of approximately 100:1. The absorptive ef-fect of vitamin D3 was seen only at the highest dose level (1250 mg