General and specialized parenteral amino acid formulations for nutrition support

Advances in the understanding of amino acid metabolism and of the interaction of amino acids with skeletal muscle, liver, brain, and other tissues have led to refinements of parenteral amino acid solutions. Clinical situations may dictate the use of specific amino acid formulations. Branched-chain amino acid (BCAA) solutions may normalize aberrant amino acid profiles in patients with hepatic encephalopathy; however, controlled trials demonstrate little effect on clinical outcome, and the effectiveness in patients with acute liver failure or undergoing orthotopic liver transplantation is unproved. BCAA solutions have also been tried in septic and severely stressed patients with equivocal results. Renal failure has been treated with essential amino acid solutions, yet low-dose standard amino acid formulations are probably equally effective. Pediatric preparations have been tailored to "normalize" amino acid profiles to those of healthy term, breast-fed neonates. Recent studies suggest that premature infants receiving these formulations may achieve intrauterine growth rates, although the effect on long-term outcome is unknown. Glutamine may be essential for the preservation of intestinal mucosal structure and function; further study is indicated to determine the necessity of adding glutamine to parenteral amino acid solutions. Recently, amino acid infusions have been associated with enhanced ventilatory drive, possibly via stimulation of central ventilatory mechanisms. A variety of other side effects have been documented, including acidosis, hyperammonemia, hypercalciuria, and possibly bone disease and hepatotoxicity. Further understanding of the metabolism of intravenous infusion of amino acids is necessary to provide optimal nutritional protein support. Because full information regarding the complex effects of intravenous substrates is lacking, special amino acid formulations must be used with caution.