Local action of oral 1,25-dihydroxycholecalciferol on calcium absorption in osteoporosis.

To investigate whether low calcium absorption in osteoporosis improves by increasing 1,25-dihydroxyvitamin D both systemically in plasma and locally in gut, the effects of oral 25-hydroxycholecalciferol and oral 1,25-dihydroxycholecalciferol on plasma 1,25-dihydroxy-vitamin D (1,25-(OH)2D) and calcium absorption were studied in 20 postmenopausal patients with vertebral osteoporosis. In 10 patients taking oral 0.25 micrograms 1,25-dihydroxycholecalciferol twice daily for 7 d, calcium absorption increased more than in 10 patients taking oral 40 micrograms 25-hydroxycholecalciferol once daily for 7 d (p less than 0.02) despite both groups having a similar increase in plasma 1,25-(OH)2D. These results support the view that the major effects of oral 1,25-dihydroxycholecalciferol on absorption is due to a local action on the gut and that it is possible to increase calcium absorption in osteoporosis with oral 1,25-dihydroxycholecalciferol without increasing its undesirable action on bone resorption.