Milk protein quantity and quality in the term infant. II. Effects on acidic and neutral amino acids

The response of the plasma and urine concentrations of acidic and neutral amino acids to two bovine protein formulas (1.5 gm/100 ml) containing, respectively, 60% whey proteins and 40% caseins or 18% whey proteins and 82% caseins was measured in term infants. These two groups of infants were compared with a group of infants that were breast-fed; all infants were fed ad libitum. Concentrations of citrulline, threonine, phenylalanine, and tyrosine in the plasma and urine were higher in the formula-fed infants than in the breast-fed infants and were a reflection of the amount of protein provided. Concentrations of tyrosine and phenylalanine were higher in the plasma and urine of infants fed the casein-predominant formulas than they were in those fed whey-predominant formulas. The opposite was true for threonine, which was present in higher concentrations in plasma and urine of infants fed whey-predominant formulas than it was in infants fed casein-predominant formulas. Concentrations of taurine were lower in the plasma and urine of formula-fed infants than they were in breast-fed infants. These differences give further evidence that formulas now in common use may provide a protein intake in excess of protein requirements and that there is a dietary requirement for taurine in man which is not satisfied by such formulas. Although these differences have not been established as nutritionally deleterious, neither can they be assumed to be entirely acceptable.