Effects of cachexia due to cancer on whole body and skeletal muscle protein turnover

BACKGROUND. Data available in the literature regarding whole body protein (WBP) kinetics in patients with cachexia due to cancer are conflicting. Some authors have reported an increase of WBP synthesis and breakdown, whereas others have not found any significant changes; only a few researchers have investigated more compartments simultaneously. The main purpose of this study was to investigate WBP and skeletal muscle protein (SMP) turnover simultaneously in cachectic patients to understand better the mechanisms underlying the general wasting of the host present in cancer cachexia. METHODS. WBP and SMP synthesis and breakdown were studied in malnourished patients with advanced gastric carcinoma and in healthy volunteers. Protein turnover was evaluated in a postabsorptive state, using a model based on a primed constant infusion of L- [ ²H ₅] phenylalanine and L- [ ²H ₄] tyrosine, and by determining the isotopic enrichment and concentration in plasma during a plateau phase by gas chromatography and mass spectrometry. RESULTS. Rates of WBP synthesis and breakdown did not differ significantly between the two groups (whole body synthesis [WBS] of 4.35 ± 0.2 g/kg/day and whole body breakdown [WBB] of 4.77 ± 0.2 g/kg/day in the control group and WBS of 3.34 ± 0.7 g/kg/day and WBB of 4.5 ± 0.4 g/kg/day in the patient group). The skeletal muscle compartment of the patients showed a significantly lower synthesis compared with controls (patients, 9.6 ± 1.8 nmol/100 mL/minute and control, 25.9 ± 7.6 nmol/100 mL/minute; P <0.05), whereas the breakdown was similar in the two groups. Such reduction in SMP synthesis in the gastric carcinoma patients resulted in a more negative net balance. CONCLUSIONS. Conflicting data in the literature may be accounted for by the different selection of patients and controls. Furthermore, WBP kinetics is the result of the metabolism of at least two compartments, the muscle and the nonmuscle compartments (including the tumor), which can change in opposite ways. In patients with cachexia due to cancer, the skeletal compartment appears to be the more compromised, with a significant decrease in SMP synthesis.