between the type of infant feeding and the social status and functioning of the family. In the United States, breastfeeding rates increase with an increase in socioeconomic status. The favorable environment of these women and their infants is associated with a lower risk of many illnesses. In addition, since surveys indicate that the breastfeeding mother is less likely to smoke, her infant is at lower risk of respiratory problems from exposure to passive cigarette smoke. Further, the young infant in day care—often because the mother is working (and therefore less likely to breastfeed)—may be exposed to communicable diseases more often than the infant cared for exclusively at home. Thus, the lower risks of morbidity reported for breastfed infants may be in part due to factors other than breastfeeding. Other potential sources of bias are reviewed by Kramer (1987).
Human milk serves as the nutritional standard for infants. Certain nutrients (vitamins A, D, K, B12, riboflavin, and folate; iron; copper; zinc; and fluoride) are reviewed in this section to illustrate the uniqueness of human milk and relationships, if any, of the infant's nutritional status to maternal nutrient stores and maternal diet. Other essential nutrients are of no less importance to the infant; information about them is presented in Chapter 6.
Three major factors contribute to the nutritional status of the exclusively breastfed infant: nutrient stores, especially those accumulated in utero; the amount and bioavailability of nutrients supplied by human milk; and environmental and genetic factors that influence the efficiency of nutrient utilization. Nutrient stores at birth are determined by the rate of placental nutrient transfer and by the duration of gestation. The stores of many nutrients increase substantially during the last trimester of pregnancy and tend to be higher in infants with higher birth weight or greater gestational age. The infant's total nutrient intake is determined by nutrient concentrations in human milk and by the volume of milk consumed. The amount of nutrient absorbed by the infant is further influenced by the bioavailability of that nutrient in human milk.
Providing the breastfed infant with supplemental foods has a complex effect on the total amount of nutrient absorbed. For example, infants consuming such foods as formula or infant cereal generally decrease their intake of human milk (see Chapter 5) and, thus, the nutrients and other specialized components it supplies. Thus, the intake of supplementary foods may add nutrients in a less bioavailable form, decrease the bioavailability of nutrients in human milk, and decrease the intake of other important factors in human milk.
Growth, infections, and differences in the efficiency of nutrient utilization affect the infant's rate of nutrient utilization, which in turn can influence the infant's nutritional status. Birth weight is inversely associated with the rate of nutrient utilization. For example, infants who are small at birth usually