Enteral amino acid and protein digestion, absorption, and metabolism

The amino acid requirements of neonates continue to be an area of investigation despite numerous studies, due to the complexity of determining these requirements within the ever-changing biochemical environment of the developing infant. In the following discussion some of the issues related to digestibility and absorption, special aspects of development as they impact on requirements-including aminoacid metabolismandinfant responses toaminoacid variations in the diet-will be addressed. Protein is often assessed for its function in infant nutrition based on its role in supporting growth. However, protein is responsible for supplying some 20 individual amino acids with a variety of specific functions, as well as serving as precursors for a number of biologically active proteins (for example enzymes, cytokines, and immunoglobulins) that have more complex roles than to merely supply the amino acid building blocks for the body’s proteins.¹The major conundrum in determining protein requirements for infants is that human milk proteins are fundamentally different than the proteins supplied in various substitution formulas. Human milk is perfectly satisfactory nutrition for the healthy term infant (and is also usually satisfactory for preterm infants when supplemented with additional nutrients to support the greater requirements of these infants). Thus, the determination of infant protein requirements is often an exercise in comparing human milk protein nutriture to various possible substitutes (adapted cow milk or soy proteins, usually).