METABOLITES OF LACTOSE SYNTHESIS IN MILK FROM WOMEN DURING ESTABLISHED LACTATION

Metabolites in the lactose synthesis pathway were measured in human milk during established lactation. The mean concentrations (range) were: lactose, 198 mM (175-233); glucose, 1.5 mM (0.1-2.4); glucose 6-phosphate, 11-mu-M (4-23); glucose 1-phosphate, 2.7-mu-M (1.6-6.8); UDP-glucose, 2.7-mu-M (0.6-6.0); UDP-galactose, 0.7-mu-M ( < 0.1-2.5); inorganic phosphate, 2.2 mM (0.8-2.9). During storage of milk within the breast for 2 h, glucose 6-phosphate concentration increased by 1.4 +/- 0.5-mu-M, while UDP-glucose decreased by 1.0 +/- 0.4-mu-M. None of the other metabolites showed significant changes in concentration during this period. Only glucose showed a circadian rhythm in concentration, with the highest concentrations between 1400 and 1800 h. The volumes of milk produced by the mothers ranged from 178 to 1,739 ml/24 h. Despite this variation, there were no significant relationships between the 24-h average concentrations of glucose, glucose 6-phosphate, glucose 1-phosphate, UDP-glucose, UDP-galactose, or inorganic phosphate and the amount of lactose produced over 24 h.