1,25-Dihydroxycalciferol and calcium therapy in osteoporosis with calcium malabsorption. Dose response relationship of calcium absorption and indices of bone turnover.

We have studied some effects of small doses of 1,25-dihydroxycalciferol (calcitriol) and calcium, either combined or separately, in 45 patients with osteoporosis and malabsorption of calcium. In 24 patients on 0.25 micrograms/d of calcitriol the hourly fractional rate of radiocalcium absorption rose from 0.37 +/- 0.02 to 0.55 +/- 0.04 (p less than 0.001) and in 21 patients on 0.50 micrograms/d it rose from 0.36 +/- 0.02 to 0.69 +/- 0.05 (p less than 0.001). These responses were seen within 5 days and were not increased further at 6 weeks to 3 months. The combined therapy significantly reduced the fasting urinary hydroxyproline/creatine ratio (OHPr/Cr) and plasma alkaline phosphatase activity (ALP) in 6-12 weeks, the smaller dose of calcitriol (0.25 micrograms/d) being as effective as the larger one (0.50 micrograms/d) in this respect. Calcium alone (1 g/d) had no effect and calcitriol alone (0.25 mcg/d) had a lesser effect on both OHPr/Cr and ALP than the combined therapy. The falls in OHPr/Cr and ALP tended to be greatest in the cases with the highest initial levels but constituted only partial suppression of these variables towards the theoretical non-bone components. These results suggest that treatment with calcitriol and calcium suppresses bone resorption in osteoporosis associated with malabsorption of calcium and is more effective than calcium or calcitriol given alone.