These considerations continue today as manufacturers attempt to alter infant formulas to imitate human milk in either composition or performance and to address the nutritional needs of specific infant populations (e.g., those with cow-milk allergy, metabolic abnormalities, and prematurity) (Benson and Masor, 1994). This chapter is concerned with infant formulas that are being altered to mimic composition or performance of human milk; it does not address the nutritional needs of specific infant populations.
Soy-based formulas were developed for infants perceived to be intolerant of cow-milk protein. The first soy formulas were commercially available in 1929 (Abt, 1965). These formulas were made with soy flour and were not well accepted by parents, who complained of loose, malodorous stools, diaper rash, and stained clothing. In the mid-1960s isolated soy protein was introduced into formulas. These formulas were much more like milk-based formulas in appearance and acceptance. However the preparation of isolated soy protein resulted in the elimination of most of the vitamin K in the soy, and a few cases of vitamin K deficiency were reported. The occurrence of nutrient deficiencies in infants fed milk-free formulas contributed to the development of federal regulations concerning the nutrient content of formulas (Fomon, 1993). Soy formulas now account for about 40 percent of formula sales in the United States. Some parents want to avoid cow-milk protein in the diet and thus wean directly to soy without any reported intolerance to cow-milk formulas. While formulas containing extensively hydrolyzed protein have long been available for infants with allergy to intact cow-milk protein, formulas with protein that is not as completely hydrolyzed have recently been introduced for normal-term infants.
Infant formula manufacturers have made changes to formulas in order to match either human milk composition or breastfeeding performance (Benson and Masor, 1994). The term “breastfeeding performance” is used because, with the exception of one study of preterm infants (Lucas et al., 1994), all other studies comparing human milk with formulas involved breastfeeding—not providing human milk from a bottle.
Historically one approach to match human-milk composition is to add new ingredients (see Appendix B for the composition of formulas and human milk). This turns out to be a quixotic quest since human milk is a complex body fluid that is variable not only among individuals, but within an individual over time. In addition, it contains components, such as live cells and bioactive compounds, that either cannot be added to formulas or cannot survive a shelf life. Finally, not all human-milk constituents are essential; some, like LC-PUFAs, docosahexaenoic acid (DHA), and arachidonic acid (ARA), can be synthesized by term and preterm infants born at 33 weeks gestation (Uauy et al., 2000).
Manufacturers who wish to add some, but not all, ingredients found in human milk may defeat the purpose of the added nutrients or may potentiate negative interactions. Examples include the deleterious effect on growth when eicosapentaenoic acid is added without adequate DHA (Carlson et al., 1996) and the potential negative effect of adding polyunsatu-