The effect of different vitamin D treatments on serum vitamin D levels in early postmenopausal women.

The effects of vitamin D3 and 1,25 (OH)2D3 or l alpha-OHD3 on vitamin D metabolism were studied in a 12 month placebo controlled clinical study. 29 healthy women in their early menopause were randomized for treatment with either vitamin D3 (2000 IU/day, or 1,25(OH)2D3/l alpha-OHD3(0.25 micrograms/day), or placebo for 12 months. All participants were given a calcium supplement of 0.5 g/day throughout the study. Serum and urinary calcium increased in both vitamin D groups, whereas the posttreatment values in the placebo group were exactly the same as before treatment. Changes in the vitamin D metabolites only occurred in the vitamin D3 group, where 24,25(OH)2D and 25OHD3 were significantly increased, whereas the serum concentration of 1,25(OH)2D was unchanged. In the 1,25(OH)2D3/l alpha-OHD3 group no change in the serum vitamin D was found. The present data indicate that 1,25(OH)2D is subject to a tight feedback regulation; that 1,25(OH)2D is not the only vitamin D metabolite responsible for calcium absorption from the gut, and that 1,25(OH)2D given in physiological doses does not alter the metabolism of the other vitamin D metabolites.