Variation in fat lactose and protein in human milk over 24h and throughout the first year of lactation

Fat in human milk is extremely variable and can represent up to 50 % of infant energy intake. To accurately determine milk composition and infant intake at 1 (n 17), 2 (n 17), 4 (n 17), 6 (n 15), 9 (n 6) and 12 (n 5) months of lactation, samples of fore- and hind-milk were collected from each breast at each feed over 24 h periods from an initial group of seventeen women. The content of fat in milk varied over 24 h, with a mean CV of 47·6 (SE 2·1) % (N 76) AND 46·7 (se 1·7) % (n 76) for left and right breasts respectively. The 24 h amounts of fat, lactose and protein in milk differed between women (P=0·0001), but were consistent between left and right breasts. Daily milk production differed between breasts (P=0·0001) and women (P=0·0001). Accordingly, amounts of fat (P=0·0008), lactose (P=0·0385) and protein (P=0·0173) delivered to the infant over 24 h also differed between breasts and women (P=0·0001). The energy content of milk and the amount of energy delivered to the infant over 24 h were the same between breasts, but differed between women (P=0·0001). The growth rate of a group of only six infants in the present study was not related to either the concentrations or amounts of fat, lactose, protein and energy in milk over the first 6 months of life. These results show the individuality of milk composition and suggest that only a rigorous sampling routine that takes into account all levels of variation will allow the accurate determination of infant intake of fat, lactose, protein and energy