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

The effects of short-term treatment with 25-hydroxy-vitamin D3 (25(OH)D3) on intestinal absorption of 47Ca were examined in 18 studies of normal subjects and 16 studies of patients with advanced renal failure. Doses of 25(OH)D3 were 20, 100, 500, or 1000 microgram/day given orally for 7--10 days. There was an increase in 47Ca absorption and urinary calcium in normal subjects receiving 20 microgram/day, while doses of 500 or 1000 microgram/day were required to augment 47Ca 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)D3 to be 1/125 as potent as 1,25-(OH)2D3 in the normal subjects and 1/400 as potent as 1,25(OH)2D3 in patients with chronic renal failure. Thus, pharmacologic doses of 25(OH)D3 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)2D3 and 25(OH)D3 may be explained by some conversion of 25(OH)D3 to 1,25(OH)2D3 in normal compared to uremic subjects, while 25(OH)D3 may act in large part via mass action in uremic patients.