Parenteral Nutrition: Amino Acids

Abstract

There is growing interest in nutrition therapies that deliver a generous amount of protein, but not a toxic amount of energy, to protein-catabolic critically ill patients. Parenteral amino acids can achieve this goal. This article summarizes the biochemical and nutritional principles that guide parenteral amino acid therapy, explains how parenteral amino acid solutions are formulated, and compares the advantages and disadvantages of different parenteral amino acid products with enterally-delivered whole protein products in the context of protein-catabolic critical illness.

High‐Protein Hypocaloric Nutrition for Non‐Obese Critically Ill Patients

“…Protein‐catabolic critical illness increases body protein loss and correspondingly increases the protein requirement. Total urinary N is not commonly measured in hospital laboratories, but urea N excretion is easily determined, and has been used for decades to evaluate the severity of protein catabolism …”

“…The appropriate amount of protein to provide protein‐catabolic, non‐obese critically ill patients is not yet known; it urgently deserves to be investigated . The dose may range from 1.5 to 2.5 g protein (1.8–3.0 g free amino acids)/kg dry body weight per day, as guided by an evaluation of the intensity of each patient's protein‐catabolic process. The carbohydrate dose should be dictated by the severity of each patient's energy substrate resistance and aim to avoid or limit insulin therapy as much as possible.…”

“…(The published article overstates the protein dose in the active treatment group by failing to appreciate that the molecular weights of peptide‐bound amino acids are 1 molecule of water less than the hydrated amino acids in PN mixtures. As a consequence, the free amino acids in PN solutions provide 17% less protein substrate than a similar mass of formed protein …”

“…Most EN products are formulated to meet normal protein and full energy requirements; consequently, unless supplemented by protein modules, they provide too little protein and too much energy for critically ill patients . Some EN products are now available that are capable of providing relatively generous amounts of protein without serious energy overfeeding, although in some instances the fluid volume required can be large, and the number of days required to reach the nutrition goal with these products remains an open question . Premixed PN solutions also deliver too little protein and too much energy, volume, or both to be safe or practical in critical illness.…”

“…On the other hand, individually compounded PN solutions, as used by Doig et al, can easily and almost immediately provide target amounts of protein substrate with few calories. This appears to be the most suitable vehicle for investigating prompt high–amino acid, low‐energy SNS …”