Hormonal control of sodium content in human milk.

The possible role of prolactin (PRL) and aldosterone (Aldo) in regulation of Na content of human milk was investigated. Simultaneous determinations of Na and K in milk, PRL and Aldo in maternal plasma and milk were performed on postpartum days 2, 4 and 6, in samples from 15 mothers giving birth at term and from 8 mothers giving preterm birth. Na concentration and Na/K ratio in milk, Aldo concentration in milk and maternal plasma decreased significantly with advancing lactation. In preterm plasma and milk PRL concentration remained unchanged, whereas in term plasma PRL decreased, milk PRL increased significantly by the 6th day post partum. After preterm delivery milk Na and K levels were significantly higher, maternal plasma and milk Aldo slightly lower than after delivery at term. Daily Aldo and PRL excretion into the milk markedly increased during lactation irrespective of the length of gestation and showed a significant negative correlation with milk Na level and Na/K ratio. It is suggested that the increasing rate of Aldo and PRL excretion into the milk may be of importance in the regulation of Na concentration in human milk.