Amino acid needs and metabolism in preterm and term infants

Organic substances that consist of both an amino group (−NH2) and an acidic carboxyl group (−COOH) are named amino acids (AA). Over 300 amino acids are known to exist in nature but only 20 amino acids serve as the building blocks of protein in humans and animals. Proteins represent an important part of all cell membranes and intracellular matrix but also enzymes, blood transport molecules and hormones are proteins. All 20 amino acids must be present in the body in appropriate amounts to allow for protein synthesis and thus normal cell function and body growth. Disturbances in the metabolism of any amino acid, as seen in inborn errors of metabolism, will inevitably lead to disease, impairment or even death when left untreated. Of the 20 amino acids, 8 are called essential amino acids, as those cannot be synthesized by the human body and must therefore be provided in sufficient amounts by the diet. AA that can be synthesized by the body in sufficient amounts are called non-essential AA. Some amino acids can be sufficiently synthesized by the body under normal conditions, but fall short under conditions where the utilization of a specific AA exceeds synthesis (e.g., sepsis) or when synthesis pathways are underdeveloped (e.g., after premature birth). These amino acids are called conditionally essential or semi-essential AA (see Table 26.1). In this chapter, various methods to study AA and protein metabolism are reviewed. This is followed by discussions on the general consequences of AA under- and overnutrition in preterm infants and differences in AA metabolism during parenteral and enteral nutrition. Lastly, parenteral and enteral AA administration is discussed. Some of the non-protein AA, such as taurine or citrulline, play important roles in cell signaling and metabolism, but this is outside the scope of this chapter.