Correction of metabolic acidosis to ameliorate wasting in chronic kidney disease: goals and strategies.

Metabolic acidosis is an important cause of protein-energy wasting, commonly observed in chronic kidney disease (CKD). This wasting is, in part, a result of the imbalance between protein degradation and synthesis induced by metabolic acidosis. The increase in protein degradation seen with metabolic acidosis is largely secondary to increased activities of the adenosine triphosphate-dependent, ubiquitin-proteasome system and branched-chain ketoacid dehydrogenase. Studies consistently have shown increased protein degradation with lower serum bicarbonate levels and/or arterial pH; however, the evidence for the anti-anabolic effects of metabolic acidosis is less consistent. In contrast to these metabolic studies, many cross-sectional studies have shown a direct relationship between the severity of metabolic acidosis and the adequacy of nutritional status in CKD patients. Moreover, lower serum bicarbonate levels have been associated with better survival in some epidemiologic studies of patients undergoing maintenance hemodialysis. It is likely that these relationships are confounded by the direct association of dietary protein intakes with metabolic acidosis-controlling the survival data for measures of dietary protein intakes, malnutrition, and inflammation shows a rather steep increase in the risk of death with lower serum bicarbonate levels. Two randomized controlled studies have shown that correction of metabolic acidosis is associated with reduction in risk for hospitalization in chronic peritoneal dialysis patients; the studies in maintenance hemodialysis patients have been small and inconsistent. For now, metabolic studies and data from clinical trials lend support to the recommendations made by the Nutrition Workgroup of the Kidney Disease Outcomes Quality Initiative to maintain serum bicarbonate levels of 22 mEq/L or greater in all CKD patients. Limited data suggest that a higher serum bicarbonate level (around 24 mEq/L) may be even more beneficial, particularly in chronic peritoneal dialysis patients.