The impact of leucine infusion on skeletal muscle amino acid and energy metabolism in severely traumatized patients

Therapeutic use of branched-chain amino acids in burn, trauma, and sepsis

Various experimental studies conducted in the 1970s demonstrated, at least in the physiological situation, the anabolic and/or anticatabolic properties of branched-chain amino acids (leucine, valine, isoleucine) or their ketoacid derivatives. This led to several clinical studies in the late 1970s and early 1980s that aimed to evaluate the potential benefits of BCAA supplementation in nutritional support of the critically ill. The data on burn, trauma, and sepsis are, however, far from convincing. Besides significant discrepancies in their results and the fact that most of these studies involved very small populations of patients, few of them meet the current standards of therapeutic evaluation. However, some positive results in specific studies suggest that the underlying concept may be correct but that interpretation has been faulty. Indeed, we know now that while the BCAAs possess regulatory properties on protein metabolism, leucine is by far the most potent, while isoleucine and valine are inefficient. However, in the above-mentioned studies, BCAA-supplemented nutrition very frequently supplied almost equivalent amounts of all 3 BCAAs. Moreover, several studies were performed without adequate basal nutritional support, which most probably hampered the correct metabolic utilization of these amino acids. Taken together, these factors mean that the demonstrations of BCAA efficacy were fortunate in the least. In contrast, more recently, leucine was demonstrated to positively affect protein synthesis in an experimental model of sepsis or burn. In parallel, 2 prospective controlled trials of BCAA supplementation in septic patients also demonstrated an improvement in patients' nutritional status and outcome. Thus, we should abandon the concept of BCAA-supplemented nutrition for a more promising leucine-supplemented nutrition that requires further evaluation.

The separate and combined effect of leucine and insulin on muscle free amino acids

The effect of insulin and leucine on amino acid and protein metabolism in muscle is not fully understood. To characterize their separate and combined effects on free amino acids in muscle and plasma, 11 volunteers received an infusion of either leucine (1 g h-1, Group 1) or glucose (20 g h-1, Group 2) for 2 h followed by a combination of the two infusions for an additional 2-h period. In muscle both the leucine infusion and the leucine plus glucose infusion increased the concentration of free leucine significantly, while the sum of the other branched chain amino acids (BCAA), of the aromatic amino acids and of the basic amino acids decreased. Glucose infusion alone decreased the sum of the essential amino acids, the BCAA and the aromatic amino acids. The combination of leucine and glucose augmented the decreases, while the concentrations of glutamate, glutamine and alanine were unaffected. In plasma the leucine infusion doubled the leucine concentration and decreased alanine, valine, methionine, tyrosine, phenylalanine and the sum of the aromatic amino acids. Glucose infusion decreased methionine, serine, isoleucine and the sum of the essential amino acids and of the BCAA. The combination of leucine infusion and hyperinsulinaemia augmented the decreases. The plasma concentrations of the keto acids of valine and isoleucine decreased by the leucine infusion while the concentrations of the keto acid of leucine and isoleucine decreased by glucose infusion. The combination of leucine and glucose had an additive effect. These effects are attributed to a specific effect of leucine on the other two BCAA and a depression of muscle proteolysis by both leucine and insulin, resulting from glucose infusion.