Free amino acids in full-term and pre-term human milk and infant formula

Although the nutritional value of human milk has been thoroughly studied, few reports describing its free amino acid (FAA) content have been published. Although infant formulas are designed to approximate the nutrient composition of human milk, the content and concentration of free amino acids are unknown. We compared the FAA concentrations of milk from mothers of preterm and full-term infants with those in several infant formulas.

Human milk was obtained during three different stages of lactation (colostral, transitional and mature milk). Sixty-seven samples were collected from 44 healthy mothers of term infants and 23 mothers of premature infants 29 to 36 weeks gestation (mean 33 weeks). Two brands of powdered term formula (TF-A and TF-B) and two brands designed for preterm infants (PTF-A and PTF-B )were also studied. Ion exchange chromatography was used for free amino acid analysis.

The mean concentration of total FAA in human milk was significantly higher than any of the infant formulas (8139 micromol/L for pre-term human milk; 3462 micromol/L for full term human milk; TF-A, 720 micromol/L; TF-B, 697 micromol/L; PTF-A, 820 micromol/L; PTF-B, 789 micromol/L) (P <0.01). FAA concentration in term and premature human colostral milk was significantly higher than in human transitional and mature milks (P <0.01). In comparing individual FAAs, there were significant differences in concentrations between term human milk and preterm milk except for phosphoethanolamine, hydroxyproline, asparagine, and alpha-amino-eta-butyric acid. There were significant differences in all FAA concentrations between all human milks and infant formulas (P <0.05), but no significant differences were found among the study formulas.

The concentration of FAA is high in human colostral milk and decreases through the transitional and mature milk stages. FAA is higher in all human milks than in infant formulas.