Maternal Nutritional Status and Breastmilk Composition

BACKGROUND: Estimation of the nutrient requirements of young infants is often based on the population average intake from breastmilk on the assumption that milk must have evolved as the ideal food. This is a contestable assumption for some nutrients (especially iron). Additionally, whilst an average intake might suffice for an average infant there is considerable variation in milk composition. SUMMARY: This review considers how levels of the major nutritional components of breastmilk vary according to the mother's diet, her own nutritional status and whether she takes supplements. Several broad statements can be made: (a) the proximate composition of milk (protein, fat, and carbohydrate) is relatively insensitive to maternal nutrition although the fatty acid profile of milk does vary with maternal diet; (b) most water-soluble vitamins in milk closely follow maternal intake and status at least over the mid-range and are readily increased by supplementing the mother; folate is an exception; (c) the fat-soluble vitamins (A, D, K) are moderately sensitive to diet and can be altered by high-dose supplements; vitamin E is an exception; (d) milk iodine and selenium levels are sensitive to diet and hence show large geographical variations according to soil levels; the other minerals are virtually unaffected by diet or supplements. New estimates of milk composition have recently been published based on analysis of milk from healthy, well-nourished mothers in 4 counties and these will act as reference "target" values into the future. KEY MESSAGES: Broadly speaking, human milk has evolved to meet an infant's requirements but this might not always be the case as breastmilk composition can vary widely between mothers for some nutrients. Protein and carbohydrate (mostly in the form of lactose) in breastmilk tend to be unaffected by the mother's nutritional status. Fat content is very variable and is somewhat influenced by the mother's BMI. The fatty acid profile is strongly influenced by the mother's diet. Water-soluble vitamins reflect the mother's diet and can be increased by supplementation of the mother. Most minerals in milk are not related to the mother's status; the exceptions are iodine and selenium whose levels vary by geography according to the soil content of each mineral. New reference values for breastmilk nutrient levels and intakes from the MILQ Study have been published in 2025.