Amino acid responses to dietary intake in the first 72 hours of life.

Plasma free amino acid concentrations were measured in cord blood and at 24, 48, and 72h of age in term infants who were either breastfed or fed a whey- or casein-predominant standard term-infant formula. There was a significant initial decrease in the concentrations of almost all amino acids compared with cord blood. Statistically significant effects of the type of feeding were observed in the concentrations of methionine, cystine, tyrosine, alanine, and p-serine; similarly, significant effects of postnatal age were noted in the concentrations of all amino acids with the exception of cystine. These data suggest that the metabolic milieu changes rapidly after delivery, influenced both by postnatal age and by type of feeding. Cord blood urea and histidine concentrations were significantly higher in female infants than in male infants; the route of delivery (cesarean section v vaginal delivery) affected only cord blood concentrations of alanine (vaginal greater than C-section). These data suggest that amino acid responses to the quality of dietary protein observed in infancy (for example, high tyrosine concentrations in response to feeding a casein-predominant formula) develop in the immediate postnatal period.