Changes in the concentration of cortisol in milk during different stages of human lactation.

A study was undertaken to determine by radioimmunoassay the changes in concentrations of cortisol in the mammary secretion of individual women during late pregnancy, lactogenesis, established lactation and after cessation of breast-feeding. The concentration of cortisol in colostrum averaged 7.5% of that found in serum during late pregnancy. The concentration of cortisol (mean +/- S.E.M.) was relatively high in the mammary secretions during late pregnancy (25.5 +/- 1.8 ng/ml) and decreased within 2 days after delivery (10.2 +/- 2.0 ng/ml) to reach low values by 10 days post partum (1.8 +/- 0.7 ng/ml). During advanced lactation the cortisol values varied between 0.2 to 32 ng/ml but the mean concentration was significantly (p less than 0.05) less (7.2 +/- 0.8 ng/ml, n = 75) than during late pregnancy. With the abrupt termination of breast-feeding, the concentration of cortisol generally increased above the values determined during established lactation, but, even during involution, the progressive changes in concentration varied markedly (range 0.5-40.0 ng/ml). The function of milk cortisol for the newborn is now known. However, it is possible that cortisol in breast milk may help to control the transport of fluids and salts from the gastrointestinal tract of infants.