A key element defining lactation performance is the total amount of milk produced. The amount of milk transferred to the infant affects the infant's nutrient intake and the mother's nutrient requirements. In this report, the subcommittee distinguishes between milk intake by the infant (also referred to as milk volume) and milk production by the mother. Ordinarily, production is measured as intake, but it may exceed intake if extra milk is removed from the breast and is not consumed by the infant or the infant regurgitates milk.
The most widely accepted method for measuring milk intake is test weighing, a procedure in which the infant is weighed before and after each feeding, preferably using a balance scale accurate to ±1 g. In this method, milk intake is usually underestimated by approximately 1 to 5% (Brown et al., 1982; Woolridge et al., 1985) because of evaporative water loss from the infant between weighings. The procedure is potentially disruptive to the nursing patterns of the mother and infant, especially if nursing is very frequent or the infant nurses occasionally during the night while sleeping with the mother. Under conditions typical of breastfeeding mothers in the United States, the method is generally well accepted (Dewey and Heinig, 1987). Intake is usually reported in grams because they are the unit of measurement used in test weighing; the density of human milk is approximately 1.03 g/ml (Neville et al., 1988; Woolridge et al., 1985). Newer techniques for measuring breast milk intake based on the use of stable isotopes have been developed, but few data obtained with them have been published (Butte et al., 1988; Coward et al., 1982; Fjeld et al., 1988; Wong et al., 1990). Maternal milk production can be measured mechanically by extracting all the milk or by using a combination of test weighing and extraction of residual milk.
There is a very wide range in milk intake among healthy, exclusively breastfed infants. Figure 5-1 illustrates variability in infant milk intake during established lactation. In industrialized countries, milk intakes average approximately 750 to 800 g/day in the first 4 to 5 months, but range from approximately 450 to 1,200 g/day (Butte et al., 1984b; Chandra, 1981; Dewey and Lönnerdal, 1983; Hofvander et al., 1982; Lönnerdal et al., 1976; Neville et al., 1988; Pao et al., 1980; Picciano et al., 1981; Rattigan et al., 1981; Wallgren, 1944/1945; Whitehead and Paul, 1981). Recent data from developing countries indicate a similar mean level of intake when a rigorous methodology for measuring milk volume is used (Brown et al., 1986b; Prentice et al., 1986) (see Figure 5-2).
Milk intake after the first 4 to 5 months varies even more widely. In U.S. infants who were breastfed for at least 12 months and were given solid foods beginning at 4 to 7 months, milk intake averaged 769 g/day (range, 335 to