BACKGROUNDInfancy is a critical period of growth and physiological development, in which breast milk is the best source of nutrients. Compared to western countries, research on breast milk of Chinese population are limited. Thus, it is necessary to measure breast milk energy and macronutrient concentrations of healthy urban Chinese mothers at different lactation stages, to expand the database of milk composition of Chinese population, and to examine whether dietary or other maternal factors can affect the levels of macronutrients in breast milk.

METHODSBreast milk of full expression of one side breast from 436 urban Chinese lactating mothers at 5-11 days, 12-30 days, 31-60 days, 61-120 days, and 121-240 days postpartum was obtained at 9:00 a.m. to 11:00 a.m. Total energy, lactose, protein, and fat contents were measured. 24-hour dietary recall was surveyed, and maternal nutrient intakes were analyzed.

RESULTSMilk composition changed over the course of lactation and large individual variations were documented. The concentrations were 61.3 kcal/dl for total energy, 7.1 g/dl for lactose, 0.9 g/dl for protein, and 3.4 g/dl for fat in mature milk. Stage of lactation was a strong factor affecting milk composition. Minimal evidence was found for associations between maternal current dietary intake and milk macronutrient concentration, consistently with prior research. Maternal body mass index (BMI) was positively associated with milk fat content, to a greater extent than did dietary intake. All other maternal characteristics were not significant for milk composition.

CONCLUSIONThese findings suggest that milk composition is generally weakly associated with maternal factors except for stage of lactation, and is likely to be more susceptible to long-term maternal nutritional status than short-term dietary fluctuation.