Intake and Excretion of Calcium and Phosphorus by Infants; Calcium Retention and Model

Abstract

Intake, excretion, and retention of calcium and phosphorus were determined for 30 infants by a metabolic balance study in the home. In addition, the average accumulation of calcium during the first year was computed, and a simple model of calcium metabolism was developed from these values and published tracer data. The corrected average daily intake was 0.50 gm calcium and 0.46 gm phosphorus. These values are appreciably lower than recommended intakes based on cows' milk, mainly because the premodified-milk formulas and infant foods in the infants' diets contained less calcium and phosphorus than cows' milk. Both items are commonly used in the United States and may result in calcium and phosphorus intakes below recommended levels for many infants; human milk provides even lesser amounts of these two minerals. The respective corrected average daily fecal excretions of calcium and phosphorus were 0.29 and 0.16 gm, and urinary excretions were 0.031 and 0.18 gm. The amounts excreted in feces were related linearly to the intake. Phosphorus in urine was also correlated linearly with intake, but calcium was not. Corrected average daily retention of calcium and phosphorus was 0.16 and 0.10 gm, respectively. Calcium and phosphorus retention increased slightly with intake; for example, changing the daily calcium intake from 0.40 to 0.80 gm is predicted to increase daily retention from 0.14 to 0.21 gm. Percent retention, however, decreased with increasing intake. Calcium and phosphorus retention did not change between the third and tenth month of life. The measured values were 7% higher for calcium and 8% higher for phosphorus than estimated by Fomon for boys in their first year. 24 Accumulated calcium increased almost linearly with age. Based on the commonly cited calcium content at birth of 28 gm and observed accumulation of 43 gm between the first and tenth months, the calcium content at age 1 year was estimated to be 83 gm. Leitch and Aitken13 estimated 82 to 85 gm at this age. Relative to body weight, based on a value of 8.2 gm per kilogram at birth, the computed calcium content reached a minimum of 6.5 gm per kilogram in the third month and then increased to 8.0 gm per kilogram at the end of the tenth month. A calcium model was developed for the average age and weight of the infants in the study–164 days and 7.4 kg, respectively. The two compartments consisted of 47 gm calcium in non-exchangeable bone and 3.3 gm in the exchangeable pool (extracellular fluid, soft tissue, and bone surfaces). The half lives for replacing a single dose of calcium tracer were computed to be 2.5 and 600 days. Transfer fractions from gastrointestinal tract to blood (f1) and from blood to bone (f2') were estimated to be 0.5 and 0.8, respectively.