Changes in the nutrient composition of human milk during gradual weaning

Six lactating women were recruited at 5 to 7 months postpartum for a study of milk composition during gradual weaning. At recruitment all subjects were exclusively breast-feeding their infants. The subjects were asked to reduce the frequency or duration of breast-feeding episodes by approximately one-third each month for a period of 3 months. Milk was collected before weaning began and at 2-wk intervals for 12 wk. The concentrations of protein and Na were 142 and 220% of base-line values, respectively, at the end of this period. Changes in fat composition were difficult to interpret. A linear trend was observed through wk 10, but values for wk 12 were similar to those obtained before weaning was initiated. The concentration of Fe at wk 12 was 172% of the base-line value. The concentration of Ca did not change and that of Zn fell to 58% of the base-line value at wk 12. The significance of the changes in nutrient concentrations, their relationship to the involution of the mammary gland, and the suitability of milk from this stage of lactation for milk banking are evaluated.