Postprandial changes in the content and composition of nonprotein nitrogen in human milk

The effect of a meal on human milk (HM) total nitrogen (TN) and nonprotein nitrogen (NPN) content and composition was examined. Two studies were performed in which milk and blood samples were collected 2-3 h after subjects consumed either a test breakfast or lunch. To monitor the rate of transfer of plasma urea into milk, two women were given [¹⁵N]₂-urea with the meal. Milk TN concentrations were not significantly different from premeal values. However, concentrations of milk NPN, urea nitrogen, and alanine were increased by ≥ 20% over premeal values. [¹⁵N]₂-Urea appeared in plasma and milk within 15 min and reached maximum enrichments of 10% and 5.5% in plasma and milk, respectively. Several HM NPN components increase in concentration postprandially; however, these concentrations were not always correlated with changes in plasma concentrations, suggesting that milk NPN may also reflect metabolic activities within the mammary gland.