Enterohepatic physiology of 1,25-dihydroxyvitamin D3 metabolites in normal man.

Within 6 hr of the intravenous administration of radiolabeled 1,25(OH)2D3 to five normal vitamin D-replete human subjects, 15.6% of the injected dose appeared in bile as polar metabolites of 1,25(OH)2D3. Of the injected dose, 27% and 7.5% appeared in the feces and urine, respectively, at 24 hr. In another two subjects, biliary radioactivity was sampled at two jejunal sites separated by a distance of 40 cm; a 24.8% decrease in radioactivity over this segment of bowel was noted. These data demonstrate that products of 1,25(OH)2D3 are excreted in normal human bile. Furthermore, these products are reabsorbed as such or as free 1,25(OH)2D3 in the intestine and re-excreted as polar products in bile. Our data suggest that there is an enterohepatic circulation of the products of 1,25(OH)2D3 in normal man.