Multiple health organizations, including the World Health Organization (WHO, 2002), the American Academy of Pediatrics (AAP, 1997), the American Academy of Family Physicians (AAFP, 2003), the American Dietetic Association (ADA, 2001), the Institute of Medicine (IOM, 1991), the Life Sciences Research Organization (LSRO, 1998), the U.S. Department of Health and Human Services (HHS/OWH, 2000), Health Canada, and the Canadian Pediatric Society (Canadian Paediatric Society, 1998) endorse breastfeeding as the optimal form of nutrition for infants for the first year of life. Nevertheless the vast majority of infants in the United States are fed human milk substitutes by 6 months of age (Ryan et al., 2002). This food source, although inferior to human milk in multiple respects, promotes more efficient growth, development, and nutrient balance than commercially available cow milk. The American Academy of Pediatrics recommends that infants who are not breastfed should consume iron-fortified infant formulas rather than cow or goat milk until 12 months of age (AAP, 1997).

Human-milk substitutes existed before the modern age of formulas. Because some infants could not be fed by their mothers, humans adopted two methods for substitute feedings. The most obvious was the utilization of a surrogate mother (e.g., wet nurse), who would feed the child human milk. The alternative was to feed the child milk obtained from another mammal. The most frequently used sources were the cow, sheep, and goat (Fomon, 1993). Until the end of the nineteenth century, the use of a wet nurse was by far the safest way to feed infants who could not be breastfed by their mothers. As general sanitation measures improved during the latter part of the nineteenth century, and as differences in composition between human milk and that of other mammals were defined, feeding animal milk became more successful. However few infants survived until infant formulas based on cow milk with added water and carbohydrate were introduced. Box 3-1 lists the main landmarks in the