Muscle protein turnover in fed critically ill patients and healthy volunteers. Muscle protein turnover data from fed critically ill patients (n = 7) and fed healthy volunteers (n = 8). Both groups were fed according to energy expenditure. Muscle protein turnover was measured and calculated using isotopically labelled phenylalanine (Phe) and employing a two-pool model of the leg. Bars demonstrate a similar muscle protein synthesis rate, but a different protein degradation rate (P < 0.05), resulting in a different protein balance (P < 0.05). Data illustrate the underlying mechanism behind the rapid development of sarcopenia in critical illness. Data with permission from . *Statistically different from healthy controls, P < 0.05.