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

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Abstract
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 [I5N]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 postprandialy; 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.