Studies in human lactation: milk composition and daily secretion rates of macronutrients in the first year of lactation.

Time-dependent changes in milk composition and secretion from pregnancy through � 6 mo of exclusive breast-feeding were studied in 13 multiparous women. Concen- trations and secretion rates of lipid, lactose, protein, sodium, chloride, potassium, total calcium, ionized calcium, magnesium, glucose, citrate, inorganic phosphate, creatinine, and urea and pH were analyzed longitudinally from day 6 until weaning corn- menced. The composition of the antepartum secretion was re- lated to the permeability of the junctional complexes between mammary cells. Significant increases in lactose, glucose, pH, and ionized calcium and significant decreases in protein, sodium, potassium, chloride, and calcium concentrations were observed between 1 and 6 mo. Significant differences among individuals, which persisted through lactation, were observed for the con- centrations ofsodium, potassium, chloride, and inorganic phos- phate. The amount oflactose, ionized calcium, and magnesium transferred to the infant was also characteristic of each mother- infant pair. Our data imply that lactation performance is deter- mined in the first month postpartum. Am J Clin Nutr I99 1;54:69-80.