Alterations in protein and amino acid metabolism have been postulated to explain the frequent observations of muscle wasting and decreased plasma branchedchain amino acid concentrations in cirrhosis. In order to investigate the changes in protein metabolism, we have measured the rates of leucine turnover and oxidation in six stable, biopsy-proven cirrhotics and six age and sex-matched healthy control subjects after an overnight fast, using [l-13C]leucine tracer. Following a primed constant-rate infusion of [ l-'3C]leucine, the I3C enrichments of plasma leucine and expired COz were used to estimate leucine turnover and oxidation, respectively. Fat-free body mass was estimated from the measurements of total body water as quantified by HZ['BO] tracer dilution. The rates of COz production and oxygen consumption were measured hourly during the study period, using open-circuit respiratory calorimetry. Urinary urea, ammonia and total nitrogen excretion rates were quantified from timed urine samples.
Even though the plasma leucine levels were lower in cirrhotics as compared with controls (100.5 f 17.1 vs. 138.3 f 20.4 pmoles per liter, mean f S.D., p < 0.001), the rates of leucine turnover were not significantly different in the two groups (89.4 f 19.0 vs. 87.8 f 19.0 pmoles per kg.hr). In contrast, the rates of leucine oxidation were significantly reduced in cirrhosis (8.1 f 2.5 vs. 12.7 f 3.1 pmoles per kg-hr, p c 0.01). When all subjects were considered, the leucine oxidation rate was correlated with plasma leucine concentration (r = 0.62, p < 0.03). The plasma clearance rate of leucine was significantly increased in cirrhosis (903.5 f 232.0 vs. 650.8 f 164.4 ml per kg.hr, p c 0.001), as was the fractional turnover rate of the free leucine pool (2.3 f 0.5 vs. 1.6 f 0.2% per min, p c 0.02). Calculated rates of protein degradation and protein synthesis were sim-