Calcium absorptive effects of vitamin D and its major metabolites

The absorptive response to graded doses of vitamin D-3, 25(OH)D, and 1,25(OH)(2)D was measured in healthy adult men after treatment periods of eight, four, and two weeks, respectively. While no relationship was found between baseline absorption and serum vitamin D metabolite levels, all three vitamin D compounds significantly elevated Ca-45 absorption from a 300 mg calcium load given as part of a standard test meal. 1,25(OH)(2)D was active even at the lowest dose (0.5 mu g/day), and the slope was such that doubling of absorption would occur at an oral dose of approximately 3 mu g/day. 25(OH)D was also active in elevating absorption and did so without raising total 1,25(OH)(2)D levels. On the basis of the dose response curves for 1,25(OH)(2)D and 25(OH)D, the two compounds exhibited a molar ratio for physiological potency of approximately 100:1. The absorptive effect of vitamin D-3 was seen only at the highest dose level (1250 mu g, or 50,000 IU/day) and was apparently mediated by conversion to 25(OH)D. Analysis of the pooled 25(OH)D data from both the 25(OH)D- and vitamin D-3-treated groups suggests that approximately one eighth of circulating vitamin D-like absorptive activity under untreated conditions in winter may reside in 25(OH)D. This is a substantially larger share than has been predicted from studies of in vitro receptor binding.