Changes in the concentrations of nutrient components of human milk during lactation

: The total protein, lipid, lactose and calcium concentrations in breast milk were measured monthly from 1 month to 8-12 months postpartum in 26 lactating women. In order to determine whether changes in the milk composition were due to changes in the duration of lactation or the decline in the milk yield, or both, a linear regression analysis of the change rate of the milk composition against that of the milk volume was performed for the period from 5 to 12 months in 12 women for whom milk yield per day was measured in addition to the milk composition. Protein concentration decreased by about 30% during the first 6 months. Thereafter it increased significantly when the decrease in the milk yield was rapid but declined significantly when the decrease in the milk yield was gradual. The lactose concentration did not change during the first 10 months but somewhat increased thereafter. The calcium concentration remained steady during the 5 months but decreased significantly thereafter. The rate of change for calcium was not affected by the extent of decline in milk yield. No significant changes were observed in lipid concentration and calculated energy content during the 12 months of lactation.