Dynamics of calcium metabolism in infancy and childhood. I: Methodology and quantification in the infant

The dynamics of calcium metabolism in the human infant were determined using stable isotopes of calcium as tracers. Two isotopes, one given intravenously and the other orally were used. Tracer time-dependent dilutions were measured with high accuracy and precision using thermal ionization mass spectrometry. Similar studies were performed in infant rhesus monkeys using radio tracers. Analysis of the isotope dilutions using a compartmental model method showed quite similar results for the infants of both species. These similarities support the validity both of using stable isotopes in human studies, and the use of the rhesus monkey as a model for human calcium dynamics. It was found that the classical steady state compartmental model was inadequate for use in the rapidly growing infant, and the model was modified to account for the expansion of the body fluid calcium compartment during the time of the study. Comparison with adult human calcium metabolic parameters showed that urinary calcium flows are much higher in the adult than in the newborn, relative to other calcium metabolic parameters. Other parameters of infant calcium metabolism are all consistent with the growth and more rapid metabolism usually seen in the young.