Intestinal calcium absorption: Concurrent use of oral and intravenous tracers and calculation by the inverse convolution method

Serum calcium specific activity curves following concurrent oral and intravenous doses of 2 radioisotopes of calcium were obtained in 20 normal women averaging 41 years of age, and in 10 osteoporotics averaging 65 years. Calcium absorption rate was calculated over a 4 hour period using an inverse convolution function of the 2 specific activity curves. The maximum rate of absorption occurred at an average of 15.6 minutes after ingestion and decreased monoexponentially thereafter. This finding is consistent with animal studies indicating that calcium absorption occurs maximally in the first part of the small intestine. Fractional absorption of the test load was calculated from integration of the absorption rate curve and was found to be identical both to the average equilibrium quotient, ⁴⁷Ca/⁴⁵Ca, observed in the blood and urine samples collected from these patients over several days, and to absorption estimated from external recovery of the oral tracer. Absorption was higher in the normal women than in 6 untreated osteoporotics, but 5 osteoporotics given ethane-1-hydroxy-1,1-diphosphonate had a higher rate than either group. Neither of these differences was statistically significant.