Human milk: comparison of the nitrogen composition in milk from mothers of premature and full-term infants

Abstract

Total nitrogen concentration of human milk during the first 4 weeks of lactation is higher in milk obtained from mothers giving birth prematurely than in milk from mothers giving birth at term. In the present study, the relative composition of total nitrogen was found to be similar in premature and full-term milk except for some minor differences in small molecule components. Total nitrogen is comprised of an average of 82% protein, 8% of the nonprotein nitrogen components--urea, free amino acids, uric acid, and ammonia--and the remaining 10% is predicted to be peptide nitrogen.

PIP: The purpose of this study is to determine and compare the composition of total nitrogen (N) in milk produced during the 1st month of lactation by mothers giving birth either prematurely or at term. Total N was divided into protein, nonprotein, and its components: urea, free amino acids, uric acid, ammonia, and creatinine. At specific day intervals from 2-29 days postpartum, complete 24 hour milk collections were obtained from 7 mothers giving birth at 38-40 weeks of gestation. At the completion of each 24-hour period the total daily voume of milk was well mixed and measured before an aliquot was removed and stored at -20 degrees celsius for analysis. Linear regression analyses of the pooled data for total N demonstrated a decrease in N concentration at a similar rate in both the full term (FT) and premature (PT) milks over the 1st 29 days of lactation. The PT milk was shown to be significantly higher (P0.001) in total N. In the PT milk, protein N accounted for 82% of the total, 8% was nonprotein N components, and 10% is predicted to be peptide N. The absolute concentration of protein N was higher in the PT milk over the 4 weeks of lactation observed (P0.005) and the concentration of N derived from urea, free amino acids, lysine, and theorine were present in highest concentrations but showed no differences with lactational stage or gestational age. Valine, leucine, isoleucine, and histidine were higher in the PT milk than FT milk at the 15-29 day period only (P0.05). The data provide evidence that the N composition of milk of mothers giving birth prematurely is similar to milk of mothers at full term. However the higher N content of PT mothers' milk will provide the premature infant with higher amounts of protein N, nonprotein N, urea, and fre amino acids. During the 1st month of lactation expressed PT milk is 10-20% higher in energy density than FT milk due to a 30% higher fat concentration in PT milk, also advantageous for the premature infant.