Protein Metabolism in Critical Illness: Methodologies and Their Problems Underlying in Kinetic Studies Using Isotope Tracers In Vivo

Abstract

It has been long time since the alterations of protein kinetics in critical illness was reported, and various attempts in administering energy substrates and/or nutrients have been made to improve negative protein balance. However, none of the nutritional supports available so far have completely curtailed negative protein balance. Administration of anabolic hormone associated with energy substrates seems to be the most effective means available that efficiently improve protein kinetics. Although the mechanisms of alteration of protein kinetics have not been fully understood and none of the factors that directly regulate protein kinetics in critical illness have been identified, recent studies using tracer method have enable us to elucidate the mechanism involved in the alterations seen in critical illness. The impairment of amino acid transport in skeletal muscle may explain some aspects of the unresponsiveness of amino acid and protein kinetics to the administration of energy substrates and/or amino acids. Although it may not be conceivable to explain the alteration of protein kinetics by a single factor, several mechanisms and factors that are mainly responsible for the alterations of protein kinetics will be clarified in the future. Metabolic study using stable isotope tracers is an essential tool for in vivo quantitative evaluation of protein and amino acid kinetics.