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.