Membrane distribution in human milks throughout lactation as revealed by phospholipid and cholesterol analyses.

Membrane distribution in human milk was investigated. Milk samples from six women taken at intervals throughout 9 months of lactation were fractionated into fat globules, skim milk, fluff, and cells. These fractions and the intact milks were analyzed for protein, total lipid, and membrane material as revealed by phospholipid and cholesterol contents. All fractions showed initial levels of membrane that dropped by 1 month and then remained relatively unchanged thereafter. Total lipid (fat globules) in the milks was the primary factor determining membrane content. Distribution of membrane in mature milks was: fat globules, 80%; skim milk, 20% (including fluff, 5%); and cells, less than 1%. Mature milk assures the infants a relatively steady supply of membrane material. However, there appears to be approximately twice as high a concentration of it in milk during the first several weeks of lactation. At that time, the skim milk phase accounts for approximately 30% of milk membrane as compared to 20% during subsequent lactation.