ISOTOPIC DETERMINATION OF INTESTINAL CALCIUM ABSORPTION IN NORMAL SUBJECTS

True gastrointestinal absorption of radiocalcium was determined in a group of normal subjects employing simultaneously three different methods: the cumulative fecal excretion, and the double isotope ratio in plasma and urine between orally administered 47Ca and intravenously injected 45Ca. The measurement of 45Ca fecal excretion was used to correct endogenous contribution to 47Ca fecal content. The results obtained showed that the three methods are equally reliable in the assessment of radiocalcium absorption. On the other hand, peak plasma values of oral 47Ca did not reflect with accuracy the true gastrointestinal absorption of the isotope. The results observed by other workers are reviewed and summarized and the establishment of a provisional normal reference standard for radiocalcium absorption is attempted. © 1969.