Diet-related reference values for plasma amino acids in newborns measured by reversed-phase HPLC.

We have measured by reversed-phase HPLC concentrations of amino acids in plasma in groups of 80 normal appropriate-weight term babies fed from birth either a casein formula (WhiteCap SMA, n = 26), a whey formula (Gold Cap SMA, n = 26), or breast milk (n = 28). They were studied from day 11 to week 15 postpartum. The trend was towards an increase in amino acid concentrations in plasma with age, more marked in formula-fed than in breast-fed infants. Reference values were derived for each group. Both formula-fed groups showed several differences from the breast-fed group. Detailed examination indicated that tyrosine, phenylalanine, and methionine concentrations were increased in the casein-fed group greater than 20% of the time, but only threonine was similarly increased in the whey-fed group. Other amino acids, different ones for each formula group, were increased less frequently. There were no consistent correlations with any aspect of infant growth. Appropriate reference values are important for interpreting amino acid concentrations in plasma from newborns and for evaluating the effects of any future dietary modifications to infant formulas. HPLC analysis provides a suitable highly sensitive method for undertaking such studies.