Growth, serum biochemistries, and amino acids of term infants fed formulas with amino acid and protein concentrations similar to human milk.

We tested the hypothesis that amino acid intake from infant formulas modified to be similar to human milk would result in indices of protein metabolism more like those in human milk-fed infants. Formula-fed infants received for 12 weeks one of three isocaloric formulations of a whey-adapted formula that differed in protein concentration: 11, 13, or 15 g/L. Infants consumed similar volumes of formula or human milk. Serum urea nitrogen concentrations reflected the protein content of the diets. Plasma indices of protein nutritional status were normal and did not differ among groups. Growth rates of all infants were normal and similar. Serum indicators of protein nutritional status varied with age, which made comparisons of formula-fed infants with human milk-fed infants difficult. Plasma concentrations of leucine and isoleucine at 4 weeks of age were higher in infants fed the formula containing 15 g protein/L when compared with those of infants fed the other two formulas or human milk. At 8 and 12 weeks of age, all formula-fed infants had plasma amino acid profiles that did not differ significantly from each other except for isoleucine, which was lower in the 11-g/L group. We found that providing formulas with an amino acid pattern similar to that of human milk did not produce a plasma amino acid pattern identical to that of the breast-fed infant. This observation suggests that other factors, such as the hormonal response to feeding, differing nutritional bioavailability of amino acids from human and bovine milk proteins, and the changing quantity and type of amino acids with advancing lactation, influence plasma amino acid concentrations.