1. The urinary excretion of 3-methylhistidine and creatinine was measured in 15 controls and in two groups of 15 patients with liver cirrhosis, with and without severe muscle wasting. All subjects were on a meat-free diet. The values obtained were used to calculate the fractional catabolic rate of myofibrillar protein.
2. In patients without muscle wasting 3-methylhistidine excretion was high in the presence of normal urinary creatinine. The fractional breakdown rate was significantly increased as compared with that of controls.
3. In patients with severe muscle wasting 3-methylhistidine excretion was normal and urinary creatinine was remarkably reduced. The myofibrillar catabolic rate was further increased compared with that of controls and of the other group of patients.
4. 3-Methylhistidine and creatinine excretion allow a complete evaluation of myofibrillar protein degradation, which appears to be remarkably increased in cirrhotic patients. The relevance of increased myofibrillar protein turnover in muscle wasting of subjects with advanced cirrhosis remains to be determined.