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
(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