Milk protein quantity and quality and protein requirements during development

Most currently available infant formulas have a protein concentration that provides intakes of nutritionally available protein that markedly exceed the requirements and also the protein intakes from human milk in breast-fed infants. Exclusively breast-fed infants have protein intakes of about 2 gm/kg/day during the first month of life and thereafter the intake decreases to about 1 gm/kg/day between the fourth and the sixth month. The amino acid composition of the nutritionally available proteins from human milk also differ significantly from those found in formulas based on bovine milk proteins. If the purpose is to produce human milk substitutes with a protein amino acid pattern similar to that of human milk and to produce a plasma amino acid profile and other metabolic indices of protein metabolism similar to that found in breast-fed infants, then the quantity of protein in formulas must be decreased, and furthermore the quality of the milk proteins must be modified and/or the whey-to-casein ratio changed. Preterm infants with a birth weight less than 1,500 gm grow at intrauterine rates (approximately 30 gm/day) on human milk protein intakes of 3 gm/kg/day when the caloric intake is 120 kcal/kg/day and mineral intakes adequate. At this protein intake, indices of protein metabolism and plasma amino acid profiles are similar to those found in breast-fed term infants. If currently available formula proteins are administered at similar intakes, the plasma amino acid profile of the infants will differ from that found in human milk-fed infants although growth rates and other metabolic responses are similar. This suggests that protein quality must be modified in formulas for VLBW infants. Urine area and urine amino acid excretion correlates directly with protein intake greater than 3.0 gm/kg/day in VLBW infants. By monitoring these metabolites in the urine, it may be possible in the future to adjust protein intakes to meet individual differences in requirements in VLBW infants.