5.1. Branched-chain amino acids

Studies suggesting that branched-amino acids (BCAAs) may promote tissue anabolism have led to considerable interest in the provision of BCAAs in total parenteral nutrition (TNP). Their metabolic role as precursors for the synthesis of muscle glutamine has also stimulated interest in their use. Interactions between BCAAs and aromatic amino acids in their transport into the brain, and the indication that they may decrease brain tryptophan uptake and serotonin production, has led to the suggestion that they may be of therapeutic value. For example, they may be able to stimulate the respiratory center and aid patients with sleep apnea.

Specific trials of TPN solutions enriched with BCAAs have shown the following relationships. In patients with hepatic-failure-induced encephalopathy, improvement of mental function is observed, but benefits over administration of similarly low amounts of regular amino acid mixtures are uncertain (NAYLOR et al., 1989). Trials of their influence on nitrogen balance in patients fed varying intakes of protein have shown:

(a) limited effects with intakes greater than 70 g,
(b) improvement at intakes between 40 and 70 g,
(c) no effect of intakes at less than 40 g, where other amino acids may be limiting.

In renal failure, where lowering blood urea levels is desirable, enriched mixtures containing 40 to 70 g total amino acids have been useful in treatment.