Changes in the content of N-acetylneuraminic acid in human milk during lactation

The concentration of N-acetylneuraminic acid (NeuAc) in human milk was determined in 2, 727 samples collected from 2, 434 mothers living in various districts of Japan from 3 to 482 days after delivery, in the winter and summer of 1989. The total amount of NeuAc decreased from about 150mg/100ml (3-5 days postpartum) to about 32mg/100ml (121-240 days postpartum), and thereafter it remained constant. NeuAc concentrations in samples taken in summer were lower than those found in samples collected in winter; however, there was no significant difference in the NeuAc concentrations in human milk collected at different locations. The ratio of NeuAc in defatted milk to that in whole milk decreased from 96-99% to 88% during lactation. The ratio of NeuAc in the 12% TCA-insoluble fraction to that in whole milk decreased from 24% to 15% during lactation, whereas, in the 12% TCA-soluble fraction the ratio of NeuAc remained constant at 72-77% during lactation. The present data suggest that it is necessary to distinguish the forms of NeuAc when attempting to determine the physiological role of human milk NeuAc in infants.