Comparison of Effects of la,-Hydroxy-Vitamin D3 and 1,25-Dihydroxy-Vitamin D3 in Man

droxy-vitamin D3 [1CY (OH) D3] on intestinal absorption of 47Ca were compared in 41 experiments in normals and 72 experiments in patients with chronic renal failure. 11 patients were studied a second time after treatment for 2-5 mo. Doses varied from 0.14 to 5.4 ug/day to establish dose-response relationships. Urinary calcium was monitored in normal subjects, nine of whom re-ceived a constant calcium intake on a metabolic unit. There was an increase in intestinal absorption of 'Ca and urinary calcium in normals receiving 1,25 (OH)2D3, 0.14 fg/day or greater, and 0.28 ig/day or greater aug-mented intestinal absorption of "Ca in chronic renal failure. In contrast, 2.6 Ag/day of la(OH)Ds was re-quired to increase intestinal absorption of 'Ca in both groups. The increase in urinary calcium to maximal lev-els was delayed during treatment with la(OH)Ds, 5-10 days vs. 2-5 days with 1,25 (OH) 2Ds. Moreover, half times for urinary calcium to decrease to pretreatment levels after stopping treatment were greater after la-(OH)Da (1.5-2.7 days) than 1,25(OH)2D3 (1.1-2.0 days). With long-term administration there was a pro-gressive increase in intestinal absorption of "Ca in th