INTAKE, EXCRETION, AND RETENTION OF STABLE STRONTIUM

Abstract

The average daily strontium intake by 25 infants observed for consecutive periods of approximately 7 months each was 0.60 mg, at a strontium/calcium ratio of 1.24 mg per gram. The amount of strontium and the strontium/calcium ratio in the diet varied widely, which suggests that extensive sampling is needed to obtain representative values for a large population. Milk and formula supplied most of the dietary strontium, but other components of the diet–especially water–contained appreciable amounts of strontium. Average strontium concentrations and strontium/calcium ratios are given for individual infant foods. Strontium balances were obtained for 10 infants in 56 28-day periods. Average corrected intake, fecal excretion, urinary excretion, and retention were 0.66, 0.50, 0.068, and 0.086 mg per day, respectively. Strontium/calcium ratios were 1.46, 2.08, 3.16, and 0.50 mg per gram, respectively. The ratios show strontium to be less readily absorbed and retained than calcium and more effectively excreted in urine. Strontium intake and fecal excretion increased gradually with age, but urinary excretion and retention remained constant. Strontium fecal excretion varied linearly with strontium intake, and strontium urinary excretion was proportional to the square root of the product of calcium intake and urinary excretion. There was a small but significant correlation between strontium and calcium retention. The strontium/calcium ratio in retained material, however, showed no correlation with the strontium/calcium ratio in the diet, so that the ORbody/diet was not constant. In a supporting study, strontium/calcium ratios were measured in bones of Cincinnati infants. Average values for a total of 44 samples were 0.29 ± 0.018 mg per gram at average age 1 day, 0.30 ± 0.013 mg per gram at 34 days, and 0.38 ± 0.029 mg per gram at 190 days. A simple compartment strontium model was constructed by combining information from the calcium and strontium balances in this study, results of several published tracer calcium studies, and average strontium/calcium ratios measured in the bones of Cincinnati infants. The compartments consisted of an exchangeable pool and of non-exchangeable bone, as proposed in several models for adults. For the 10 infants whose strontium balances were obtained, the model coefficients yielded strontium half lives of approximately 2 and 300 days, an ORbody/diet of 0.34 and an ORbone/diet of 0.27 for an average age of 148 days and an average weight of 7.5 kg. Coefficients estimated for a model pertaining to all 30 infants differed only slightly in detail from those for the 10-infant subgroup. Coefficients estimated at 30-day age increments between ages 30 and 300 days indicated that the two characteristic strontium half lives increased gradually with age, and provided an extrapolated strontium body burden at age 1 year of 36 mg or 0.43 mg per gram calcium. The average strontium/calcium ratio in the diet of the infants under study was higher than values reported for surveys of infants' diets in other areas. The ratio in bones collected at Cincinnati was also higher than that reported for other locations. The ORbody/diet in this study was consistent with values obtained in Argentine, British, and other United States' surveys. The discrimination factors and observed ratios that were measured directly or inferred from the model were generally similar to values accepted for adults. Discrimination against strontium relative to calcium occurred during both absorption from the gastrointestinal tract and renal clearance. In summary, this study describes the strontium content of an average infant diet that was freely selected by 30 parents and their pediatricians, and strontium balances for 10 of these infants. Because of ad libitum dietary intake, strontium and calcium consumption varied from period to period, and the effect of strontium and calcium intakes and intake ratios on strontium retention could be evaluated. Variations in the retention of strontium among infants were related to variations in calcium retention. The ORbody/diet, although convenient for predicting average strontium/calcium ratios in retention from average intake ratios, was not useful in predicting variations in retention among the infants. Coefficients for a simple compartment model were derived to compare the infant to the adult and to compute radiostrontium turnover. Balance studies with constant diets at several calcium and strontium levels to test the constancy of the ORbody/diet were suggested, as well as double-tracer experiments with short-lived radiocalcium and radiostrontium at low doses to evaluate the models.