Volume 88, Issue 1, July (2002), pp. 29-37 © The Author 2002

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

Leon R. Mitoulas^1*, Jacqueline C. Kent¹, David B. Cox¹, Robyn A. Owens², Jillian L. Sherriff³ and Peter E. Hartmann¹
Departments of ¹Biochemistry and ²Computer Science, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
³School of Public Health, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

 (Received 30 July 2001–Revised 2 January 2002–Accepted 30 January 2002)

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.

Corresponding author: Leon R. Mitoulas, fax +61 8 9380 1148, email Leon.Mitoulas@uwa.edu.au