Muscle Protein Metabolism in Critically Illness

Abstract

36 SMN SURGICAL METABOLISM AND NUTRITION be obtained by subtracting the MPB from the MPS value. In healthy adults, the turnover rate of muscle protein is reported to be 1~2% per day.[14] There are alternating periods of positive and negative net protein balance throughout the day. [15] Protein breakdown exceeds synthesis in the fasting state, even in healthy adults.[3] Obligatory oxidation of essential amino acids released into the intracellular space during the fasting period contributes to the negative balance. The amino acids supplied through the following meals replete the loss during the fasting period. As this cycle repeats, the total net protein balance approaches zero and the muscle mass remains constant. STABLE ISOTOPE TRACER METHOD In vivo protein turnover in humans can be measured using the stable isotope tracer method.[13] One or more trac-ers (e.g., stable isotope-labeled amino acids) are infused intravenously, and the plasma levels of tracers are measured repeatedly to calculate the turnover rate of the tracee (e.g., amino acids of interest). In the steady state, after the continuous infusion of a stable isotope tracer at a constant rate, the blood concentration of the tracer and tracee reaches a certain plateau level. The ratio of tracer to tracee (i.e., isotope enrichment) in the steady state is inversely correlated with the rate of appearance of the tracee in plasma. If the rate of appearance of essential amino acids in the fasting state is determined, we can calculate the rate of degradation of whole-body proteins. Isotope enrichment was determined using gas chromatography and mass spectrometry. Kim et al. [13] reviewed the method of metabolic research with stable isotopes in their article. CRITICAL ILLNESS Catabolism dominates metabolism in critically ill patients. Increased MPB, rather than decreased synthesis, leads to muscle wasting. Essential amino acids derived from muscle breakdown are utilized to meet the increased demands for acute-phase protein synthesis, immune function, and wound healing. For example, more than 3 g/kg/day of protein, four times the normal daily intake, is required for wound healing of burns affecting 50% of the body surface area.[16] Contrary to common expectations, MPS is also increased in critical illness. Accelerated MPB provides excess intracellular free amino acids and, in turn, these free amino acids serve as sub-strates and facilitate MPS.[3] However, even with aggressive nutritional therapy, accelerated MPB is difficult to offset by protein synthesis due to anabolic resistance.[17] The change in each component of muscle protein metabolism in various clinical conditions is summarized in Table 1.