Protein metabolism in critical illness: methodologies and their problems underlying in kinetic studies using isotope tracers in vivo.

The response to critical illness involves alterations in all aspects of metabolic control, favoring catabolism of body protein. In particular, as the result of the alteration of protein metabolism, body protein loss occurs, which has been reported to be inversely correlated with the survival of critically ill patients. Despite the availability of various therapeutic modalities aiming to prevent loss of body protein pool such as total parenteral nutrition, and enteral nutrition that has made to provide excessive calories as a form of energy substrate and protein itself, the loss of body protein could not be prevented. Loss of body protein store occurs as a consequence of the alteration of intermediate metabolism that works for the production of energy substrate. This alteration of substrate metabolism may link to the alteration of protein metabolism. However, no specific factors regulating protein metabolism have been identified. Thus, further investigations evaluating amino acid and protein metabolism are required to obtain a better understanding of the metabolic regulation in the body, which may lead to the development of novel and more effective therapeutic modalities for nutrition in the future.