Protein and amino acid metabolism in three- to twelve-month-old infants fed human milk or formulas with varying protein concentrations

Background: The metabolic response to different protein intakes from breast milk and/or formulas varying in protein concentrations, in combination with supplementary foods, has not been studied in infants who are in the second half of infancy. Methods: Healthy infants, exclusively breast-fed until 3 months old, were randomly assigned to one of three groups, F13, F15, or F18, and were given formulas with 13, 15, or 18 g/l of protein, respectively. Infants breast-fed (B) and mixed-fed (M) (breast milk and formula) at 6 months formed the fourth and fifth groups. All infants received the same supplementary foods and were studied from ages 3 to 12 months. Results: The concentrations of albumin, prealbumin, and transferrin were similar in all groups. At 6 months, serum and urine urea concentrations were lower in B and M, compared with urea levels in the formula-fed groups of infants. At 12 months, urine urea was lower in B + M than it was in F18. At 6 months, plasma concentrations of phenylalanine, tyrosine, and methionine were higher in all formula-fed groups; and those of valine, isoleucine, and threonine were higher in F18 and F15 than they were in B and M. Plasma concentrations of methionine, valine, and threonine were higher in F18 than in F13. At 12 months, plasma levels of tyrosine, methionine, valine, isoleucine, and leucine were higher in F18 than they were in B + M. Conclusion: Many indexes of protein metabolism were similar in groups F13, B, and M, particularly at 6 months. In contrast, the provision of a formula with 18 g/l of protein resulted in a different metabolic pattern, which could indicate unnecessarily high protein intakes.