Malabsorption of calcium and phosphate in chronic renal failure: 32P and 45Ca studies in dialysis patients.

A modified double-isotope method was used to determine the absorption of calcium and phosphate in patients with chronic renal failure. Eight hemodialysis patients and six healthy control subjects received an intravenous dose of 45calcium and 32phosphate and an oral tracer dose of 45Ca and 32P was administered two weeks later. Timed plasma samples were obtained on each occasion to determine fractional absorption rates and cumulative absorption of either tracer from both tracer sets with deconvolution analysis. Stool collections were analyzed. Calcium absorption in normal subjects peaked at 12% over fifteen minutes at one hour and declined rapidly thereafter. Absorption was essentially complete at four hours and cumulative absorption at this time was 72 +/- 6%. The pattern of phosphate absorption was similar and cumulative absorption at 4 hours was 80 +/- 3%. Calcium absorption in dialysis patients was significantly impaired with a flattened profile, a maximal 15-min absorption rate of 2% and cumulative 4-h absorption of 20 +/- 2%. Phosphate absorption in dialysis patients was also impaired to a comparable degree with a maximal rate of 3.6% and a more delayed cumulative total of 35 +/- 5%. Stool data showed good agreement with deconvolution analysis in volunteers but always overestimated absorption in patients. Sequential double isotope analysis provided a simple and convenient method for the concurrent estimation of calcium and phosphate absorption in humans.