Amino Acid Metabolism in Pediatric Patients

Abstract

As with energy requirements, protein requirements are relatively much greater in infants and decline progressively with age. Amino acid metabolism in pediatric patients is characterized by the following differences. The requirement for essential amino acids in neonates is larger than that in adults. Because of low activity of phenylalanine hydroxylase and cystathionase, hyperphenylalaninemia and hypermethioninemia tend to occur, whereas tyrosine and cysteine tend to be deficient. In addition to cysteine and tyrosine, histidine, lysine, arginine and taurine are considered as semiessential amino acids. Nowadays there are different kinds of amino acid formulas to satisfy these specific requirements, and most of these formulas are intended to normalize the plasma aminogram. However, the nutritional benefit of these formulas for growth and development is still not completely proven, and the pharmacological use for specific diseases is expected with some modification of these formulas.

Introduction

Protein stores can only be used to a limited degree for production of glucose, although glucose supply can be maintained through gluconeogenesis from amino acid precursors for days during fasting. By shifting to a fuel system of fat use and ketone-body production as starvation progresses, the body modifies its utilization of nitrogen (N) stores, grossly reducing gluconeogenesis and urinary N excretion.[1]This adaptation is mandatory for survival because loss of >25–30% of body protein results in death. Because maintenance of body N stores is critical to survival, it becomes a primary goal of nutritional support in critically ill children who already may be malnourished, may not tolerate enteral nutrition, or may be exposed to surgical stress. Even in the steady state, body proteins constantly undergo breakdown and resynthesis. During growth, there is a net deposition of protein, as well as increased rates of both synthesis and breakdown. The amino acids released by breakdown of protein are reused for protein synthesis. However, because this process of reuse is not completely efficient and some amino acids are lost by oxidative catabolism, both essential amino acids and a dietary source of nitrogen are needed. The daily turnover of body proteins is several-fold greater than the amino acid intake, showing that reuse of amino acids is a major contributory factor to the economy of protein metabolism. In addition to being needed for new tissue formation, protein precursors also are required for synthesis of enzymes, neurotransmitters, hormones, and bile salts.