Isotopic Tracer Studies of Intestinal Calcium Absorption.
A method for measuring the retention of orally administered47Ca using a whole-body counter (WBC) is described. One microcurie47Ca in 90 ml of lowfal milk was given along with 5 μCi51Cr as a stool marker. The retention of47Ca was determined at 7 days if less than 2% of the administered51Cr was present in the body. Correlation between calcium retention by the whole-bodycounting and the excreta-recovery methods was highly significant (r=0.835,P<0.0001; N=17). In 5 subjects, each studied twice while on constant self-selected diets, calcium retention was measured by the WBC method with a precision (standard deviation) of±2.6% of the administered dose or±13.9% of the mean retained fraction. Whole-body measurement of47Ca retention has the advantages over other techniques that no collections of blood or stool are needed, no residency on a metabolic ward is required, and the radiation exposure (13 mrem) is so low that multiple studies may be performed in the same subject.