Impaired calcium absorption in uremic man: evidence for defective absorption in the proximal small intestine

Studies of intestinal absorption of calcium were carried out in 31 normal subjects, 30 patients with advanced renal failure, and 14 patients treated with hemodialysis. Absorption of radiocalcium was determined at 1, 2, 4, 6, and 25 hours after oral administration of the isotope with a 200 mg. calcium carrier, using a modification of an external arm-counting method. This method measures what is termed, during the first 4 hours after ingestion of 47 Ca and true absorption thereafter. In normal subjects, apparent absorption was most rapid 0 to 2 hours after ingestion of the isotope, while there was markedly impaired absorption in the patients with uremia. Between 2 and 6 hours, absorption did not differ between the patients with renal failure and the normal subjects; however, some absorption continued in the uremic patients between 6 and 25 hours, while little or no increase occurred in the normal subjects. There was no difference between 47 Ca absorption in patients with chronic stable renal disease and those regularly treated with hemodialysis. Administration of 1,25-dihydroxycholecalciferol (0.68 to 2.7 μg per day) for 6 to 15 days to 8 patients with renal disease caused a marked increase in calcium absorption, particularly during the 0 to 2 hour interval. These data indicate the existence of impaired intestinal absorption of calcium, with reversal of this abnormality following treatment with 1,25-dihydroxycholecalciferol. These observations are consistent with a defect in calcium transport in the proximal part of the small intestine where vitamin D may be most active in stimulating calcium transport. A defect in renal production of 1,25-dihydroxycholecalciferol may be responsible for this abnormality.