Measurement of true absorption, endogenous fecal excretion, urinary excretion, and retention of calcium in term infants by using a dual-tracer, stable-isotope method.

A dual-tracer, stable-isotope method was used to measure the percentage of true calcium absorption ( α ), true calcium absorption rate (V a ), endogenous fecal calcium excretion rate (V f ), urinary calcium excretion rate (V u ), and calcium retention rate (V r ). Twenty-seven infants with a mean gestation of 30.6±1.7 weeks and a mean birth weight of 1.4±0.21 kg were studied at 2 or 3 weeks of age, or both, during feedings of human milk (HM), fortified human milk (HMF), and commercially available formula (20 or 24 calories per ounce) for premature infants (EPF-20/780 and EPF-24/940) (part 1 of our study). Of 13 additional infants with a mean (±SD) birth weight of 1.26±0.25 kg and gestation of 29.6±2.5 weeks, 11 completed a crossover-design study at 2 and 3 weeks of age, receiving two identical formulas containing calcium, 940 mg/L, and phosphorus, 470 mg/L (EPF-24/940 formula) or calcium 1340 mg/L, and phosphorus, 680 mg/L (EPF-24/1340 formula) (part 2 of our study). The α value was higher in infants receiving HM (76.4±15.1%) or HMF (68.0±7.8%) than in those receiving EPF-20/750 formula (54.1%±5.6%) or in previously reported infants fed standard formula (47.1%±11.5%); those given EPF-24/940 formula had intermediate values (63.9%±13.9%, part 1; 56.1%±16.5%, part 2). No significant differences existed among groups for either V u or V f per kilogram. In the crossover study (part 2), no significant differences were seen between formulas for α and for V a , V f , or V f per kilogram. However, V u per kilogram was significantly decreased in infants receiving the higher mineral formula (EPF-24/940: 3.6±2.3; EPF-24/1340: 2.9±2.3 mg/kg per day; p = α , V u per kilogram, and V f per kilogram were not related to gestational age, age at study, calcium intake, or each other. However, V f per kilogram was inversely related to birth weight. Thus, α , V u , and V f appear to be independent and may be differentially affected by factors altering calcium dynamics. We conclude that increasing formula mineral content does not ensure increased retention; careful monitoring of individual infants remains indicated.