Milk protein quality in mothers delivering prematurely: implications for infants in the intensive care unit nursery setting.

Because reports comparing the nitrogen content of breast milk from mothers delivering prematurely and at term have conflicted, the present study sought to compare these milks during the first month of lactation with respect to total protein and the distribution of protein among individual protein species and amino acids. No significant differences in total protein concentration were established. However, immunoglobulin comprised a greater proportion of protein in preterm milk than in term milk. The major component of the milk casein fraction, beta-casein, was present in similar proportions in early term and preterm milk, yet increased with time in term milk but not preterm milk. Lactoferrin and lactalbumin were similar in both milks. Differences in amino acid composition were found for serine, arginine, and tyrosine. With respect to protein composition, preterm milk and term milk thus differ during the first month of lactation, and the different patterns of change in individual components suggest that factors other than milk volume contribute to protein content.