THE EFFECT OF CHRONIC ALCOHOL INGESTION ON WHOLE BODY AND MUSCLE PROTEIN SYNTHESIS — A STABLE ISOTOPE STUDY

Abstract

The cause of the proximal myopathy associated with chronic alcohol ingestion has yet to be established. The clinical feature of muscle wasting implies either inhibited skeletal muscle protein synthesis, stimulated breakdown or a combination of both. Previous data suggest that breakdown is reduced, rather than promoted. This provides evidence, albeit indirect, that the myopathy is the result of inhibited muscle protein synthesis, which has been demonstrated recently in the rat model. We have examined the influence of chronic alcohol intake on post-absorptive fractional skeletal muscle protein synthesis in man using a primed continuous (1 mg/kg/hr) infusion of L-[l- ¹³ C]leucine for 8 hr. Percutaneous quadriceps muscle biopsies (200 mg) were taken after 2 and 8 hr of the infusion for measurement of the incorporation of ¹³ C leucine into muscle protein. Plasma ¹³ C enrichment of a-ketoisocaproic acid, the deaminated product of leucine, was used to represent that of the precursor pool. We studied 6 fully ambulant alcoholics, who exhibited no overt evidence of skeletal muscle disease and who had consumed at least 100 g alcohol daily for a minimum of 10 years. Mean (± S.D.) fractional muscle protein synthesis was 0.0274 ± 0.0087 (95% confidence intervals 0.0204–0.0344%/hr). This value is significantly lower than recently published control values obtained using identical protocols which range from 0.046 to 0.055%/hr. In addition, whole body leucine oxidation was lower ( P < 0.05) in the chronic alcoholics than in healthy controls, whereas neither whole body protein synthesis nor breakdown was significantly reduced. This study confirms for the first time in man that chronic alcohol ingestion suppresses fractional quadriceps muscle protein synthesis which we suggest is the major causative factor for the proximal myopathy seen in such patients.