Effect of 1,25-dihydroxyvitamin D₃ on calcium absorption in the colon of healthy humans

Calcium absorption in patients with short bowel syndrome is significantly higher when the colon is left intact. To study calcium transport in the large bowel, we investigated whether exogenous 1,25-dihydroxyvitamin D₃ [1,25(OH₂)D₃] can induce or enhance colonic calcium absorption in healthy subjects ingesting a normal diet. Steady-state colon perfusion studies were performed before and after 1 wk of 1,25(OH)₂D₃ administration (2 μg/day, 10 subjects). Serum 1,25-dihydroxyvitamin D concentration rose from 23.0 ± 2.2 to 39.5 ± 4.3 pg/ml (mean ± SE, P < 0.01). In the basal state the mean net movement of calcium was not significantly different from zero when a 5 mM calcium gluconate solution was perfused 100 ± 84 μmol·h^-1·entire colon secreted^-1). Vitamin D administration resulted in a significant change toward calcium absorption (106 ± 47 μmol·h^-1·entire colon absorbed^-1, P < 0.02). 1,25(OH)₂D₃ had no effect on colonic magnesium, phosphate, water, and electrolyte movement. This study demonstrates that in healthy humans exogenous 1,25(OH)₂D₃ can change colonic calcium movement toward absorption. We suspect that similar changes in colonic calcium transport are caused by endogeous 1,25(OH)₂D₃ when calcium deficiency has occurred in short bowel syndrome.