Comparative analysis of free amino acid concentrations in colostrum and transitional breast milk from mothers of term and preterm infantsa

This study aimed to comprehensively characterize the free amino acid (FAA) composition of human breast milk and to compare profiles between term (TBM; milk of term infants) and preterm (PBM; milk of preterm infants) samples across two lactation stages, colostrum and transitional milk. A total of 200 breast milk samples were collected on the 3rd and 10th postpartum days from 50 healthy mothers of term infants and 50 healthy mothers of preterm infants in Erzurum Province. Forty-five FAAs were quantified using a validated liquid chromatography–tandem mass spectrometry (LC–MS/MS) method. Significant differences in FAA composition were observed between TBM and PBM. Glutamic acid, the predominant FAA in all samples, was approximately 82% higher in TBM compared to PBM (p < 0.001). TBM also exhibited markedly higher levels of bioactive amines, including anserine, serotonin, and dopamine, whereas PBM was enriched in several amino acids such as aspartic acid (> 200%) and branched-chain amino acids, particularly leucine and isoleucine (p < 0.05–0.001). Among essential amino acids, histidine was more abundant in TBM, while leucine, isoleucine, lysine, and phenylalanine were significantly higher in PBM (p < 0.05–0.001). Lactation stage–dependent variations were evident in both groups. Colostrum exhibited a significantly richer FAA profile than transitional milk in both TBM and PBM (p < 0.05–0.001). In TBM, colostrum contained higher concentrations of several amino acids, including arginine, aspartic acid, asparagine, glycine, proline, serine, histidine, and valine, whereas transitional milk showed higher levels of glutamic acid, beta-alanine, beta-aminoisobutyric acid, and dopamine (p < 0.05–0.001). In PBM, colostrum was particularly enriched in essential amino acids such as lysine, methionine, phenylalanine, threonine, tryptophan, and branched-chain amino acids, while several non-essential amino acids decreased during the transition to transitional milk (p < 0.001). Overall, these findings demonstrate that both gestational age and lactation stage significantly influence the free amino acid composition of human breast milk, highlighting distinct bioactive and nutritional profiles in term and preterm milk with potential implications for early infant nutrition. © The Author(s) 2026.