HUMAN-MILK PROTEIN DOES NOT LIMIT GROWTH OF BREAST-FED INFANTS

Background: The efficiency with which breast- and formula-fed infants utilize dietary nutrients is likely to provide insight into their rela tive requirements for the growth process. Methods: We measured longitu dinal changes in growth, body composition, and dietary intakes in brea st-and formula-fed infants and estimated the gross efficiency with whi ch dietary nitrogen and energy were used for lean body mass and body f at deposition. Lean body mass and body fat were determined in 10 breas t-fed and 10 formula-fed infants at 6-week intervals during the first 24 weeks of life by the O-18 dilution technique. Dietary nitrogen and energy intakes were determined from the amount of milk and food consum ed and the nutrient content of the feedings. The gross efficiency of n utrient utilization was calculated for each infant from the cumulative dietary intake and the change in body composition with time. Results: Length and weight gains and lean body mass and body fat accretion dur ing the first 24 weeks of life were similar between breast-and formula -fed infants despite significantly higher nitrogen and energy intakes of the formulated group. The gross efficiency of dietary nitrogen util ization for lean body mass deposition was almost two-fold lower in for mula-than in breast-fed infants, whereas the efficiency of dietary ene rgy utilization for lean body mass and body fat deposition was similar between groups. Despite apparent differences in the efficiency of nit rogen utilization, there was no association between lean body mass dep osition and dietary protein intake, implying that human milk protein d oes not limit growth quantitatively in breast-fed infants. Conclusions : These differences in nutrient utilization illustrate the biologic ad aptability of human infants who are quipped with mechanisms that promo te normal growth despite the variability of their nutrient intake.