Studies on human lactation. I. Within-feed and between-breast variation in selected components of human milk.

Within-feed and between-breast differences in the concentrations of sodium, potassium, chloride, calcium, magnesium, lipid, protein, lactose, glucose, urea nitrogen, creatinine, zinc, and copper were examined in milk samples from 10 women. The average lipid content doubled in the interval from the beginning of the feed to the end. The composition of the aqueous phase of milk, as determined by the major osmotically active constituents, did not vary significantly within the feed. For these components as well as for lipid a small mid-feed sample of milk gave the same mean composition as the pooled, pumped contents of one breast suggesting that such a sample is adequate for determination of milk composition in population studies. Sporadic, inconsistent differences in the composition of the milk from the right and left breasts were observed. It is suggested that mastitis may contribute to these differences. It is recommended that samples routinely be taken from both breasts and analyzed for sodium and chloride to rule out episodes of mastitis or other local phenomena which sporadically alter milk composition.