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 greater than or equal to 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.
Variation in the contents of nutrients of breast milk during one feeding.

 The concentrations of various nutrients in breast milk were studied in relation to the quantity of milk expressed from 1 breast on 1 occasion in 4 Swedish women. The contents of nutrients in breast milk showed considerable variation. Concentrations of fat, lactose, total nitrogen, nonprotein nitrogen, individual milk proteins (lactoferrin, alpha-lactalbumin, human serum albumin), and urea were measured. The results are graphed. Generally, the concentrations of fat and water-soluble nutrients of breast milk showed 2 distinct patterns during emptying of a breast. The fat content of fore milk was less than that of hind milk, the milk at the end of the feeding. Such water-soluble nutrients as lactose and protein did not vary during the feeding in this way. Although each of the 4 subjects showed the same pattern of rising concentration of fat with duration of the feeding, there was a definite range of fat concentrations among the women studied.

SOME OBSERVATIONS ON THE SODIUM AND POTASSIUM CONTENT OF HUMAN MILK
IT has been found by Miyamoto et al. (1957) that the sodium content of human colostrum expressed during pregnancy is high (35122 m.eq./l.). It is also known (Castellanos and Lizarralde, 1943; Macy, 1949) that the sodium content of colostrum obtained during the first few days of the puerperium is still considerably higher (1 1-59 m.eq./l.) than that of mature milk (1-19 m.eq./l.). Miyamoto et al. (1957) have shown that the decrease which occurs after delivery takes place over the first few days of the puerperium. No clear trend in the potassium content of human milk has been recorded, though the level of this ion is known to be higher on average in the puerperium than it is in pregnancy (Miyamoto et al., 1957). No systematic studies of the changes in sodium and potassium content of milk in individual patients in the puerperium seem to have been made. The present work gives some details of these changes. The data presented are also relevant to the electrolyte balance of the mother and the sodium and potassium intake of the newborn infant.