After validation of test-weighing procedures milk volumes produced by 13 multiparous Caucasian women were followed longitudinally through the first year of lactation. All practiced exclusive breast-feeding for at least 5 mo. Milk transfer to the infant was low on days 1 and 2 and increased rapidly to 498 +/- 129 g/d (means +/- SD) on day 5 and then more slowly to 753 +/- 89 g/d during months 3-5. There was a characteristic milk volume for each mother-infant pair that was significantly related neither to milk yield on days 4-6 nor to birth weight. It was, however, strongly related to infant weight at 1 mo, suggesting that infant and/or maternal factors coming into play during the first month of life are strong determinants of subsequent milk transfer to the infant.

Concentrations and secretion rates of macronutrients and major ions in human milk during lactogenesis (birth to 8 d) and late lactation (greater than 6 mo postpartum) are reported. Postpartum changes in lactose, sodium, and chloride concentrations signalled closure of the paracellular pathway during days 1-2. From days 2 to 4 postpartum, initiation of copious milk secretion was accompanied by significant increases in citrate, free phosphate, glucose, and calcium concentrations and a decrease in pH. During weaning, significant changes in milk protein, lactose, chloride, and sodium concentrations were observed only when milk volume fell below 400 mL/d; more than one feed per day was necessary to maintain milk secretion. Temporal changes in the concentration of other milk components, except glucose and magnesium, were not different in weaning and non-weaning women. Differences between the relation of milk volume and composition during lactogenesis and weaning suggest that volume is differently regulated in the two periods.

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