Effect of 25-hydroxy-vitamin D3 on intestinal absorption of calcium in normal man and patients with renal failure.

Abstract The effects of short-term treatment with 25-hydroxy-vitamin D 3 (25(OH)D 3 ) on intestinal absorption of 47 Ca were examined in 18 studies of normal subjects and 16 studies of patients with advanced renal failure. Doses of 25(OH)D 3 were 20, 100, 500, or 1000 μg/day given orally for 7–10 days. There was an increase in 47 Ca absorption and urinary calcium in normal subjects receiving 20 μg/day, while doses of 500 or 1000 μg/day were required to augment 47 Ca absorption in renal failure patients. During treatment, plasma levels of 25(OH) D increased to similar levels in both normal and uremic subjects. A comparison of the dose-response curves found 25(OH)D 3 to be 1 125 as potent as 1,25(OH) 2 D 3 in the normal subjects and 1 400 as potent as 1,25(OH) 2 D 3 in patients with chronic renal failure. Thus, pharmacologic doses of 25(OH)D 3 are active in both normal and uremic patients, although relatively greater quantities are necessary in uremia. This difference in relative potency of 1,25(OH) 2 D 3 and 25(OH)D 3 may be explained by some conversion of 25(OH)D 3 to 1,25(OH) 2 D 3 in normal compared to uremic subjects, while 25(OH)D 3 may act in large part via mass action in uremic patients.