Factors influencing the infant's demand for milk have not been studied thoroughly. When the milk supply is ample, the infant's milk intake is positively associated with infant weight. Because the mean weight of boys is heavier than that of girls of the same age, intake is also associated with the sex of the infant. Illness of the infant may reduce appetite and therefore milk intake. In The Gambia, Prentice et al. (1986) observed that decreases in milk intake by infants during the wet season (a period of food scarcity) were usually associated with gastrointestinal or respiratory infections. As described later in this chapter, maternal supplementation did not prevent the seasonal decline in milk volume, indicating that this pattern was probably not due to maternal nutritional limitations but to either altered feeding practices or illness-induced anorexia among the infants. From the Gambian data, it is difficult to separate the influence of these factors. In contrast, Brown et al. (in press) found that milk intake among breastfed infants in Peru remained constant, whereas intake of other foods was reduced during illness.
Maternal variables such as age and parity have little or no relationship to milk production in most populations (as measured by the infant's intake of human milk). There have been few studies of the volume of milk produced by adolescent mothers. In one study, Lipsman et al. (1985) found that milk intake appeared adequate (based on measures of infant growth) for 22 of the 25 infants of well-nourished, lactating teenagers. Among women aged 21 to 37, no association was observed between maternal age and infant milk intake (Butte et al., 1984b; Dewey et al., 1986), despite Hytten's (1954) concerns that milk yield may decrease because of "disuse atrophy" after age 24.
There is some evidence that milk production on the fourth day post partum is higher among multiparous than it is among primiparous women (Zuppa et al., 1988); however, once lactation is established, there is no statistically significant association between parity and infant milk intake in well-nourished populations (Butte et al., 1984a; Dewey et al., 1986; Rattigan et al., 1981). In The Gambia, infants of mothers who had borne 10 or more children had low milk intakes (Prentice, 1986), but this level of parity is rarely seen in industrialized countries.