Amino acid composition of preterm and term breast milk during early lactation.

The amino acid composition of the protein nitrogen (PN) and non-protein nitrogen (NPN) fractions of human milk obtained from mothers delivering maturely and at term was studied weekly until 44 weeks postconceptual age. Preterm milk contained significantly more total nitrogen (TN) and PN than the term milk; the NPN component was similar between the two groups. The amount of PN decreased significantly in both term and preterm milk during the first weeks of lactation (from approximately 300 mg/100 ml at day 7 to approximately 220 mg/100 ml at 8 weeks), while the level of NPN remained constant (approximately 33 mg/100 ml). The concentrations of almost all amino acids in the PN fraction also decreased during early lactation, while those in the NPN portion remained relatively constant. Glutamate (1630 microM) and taurine (398 microM) were the most plentiful amino acids found in the NPN fraction. No differences in the amino acid composition of the PN were detectable between the preterm and term group. Amino acids measured in this study accounted for 67% of the TN present. Whether the remaining 33% of the nitrogen can be attributed to nitrogen-containing substances other than amino acids and/or methodologic error is not known.