HORIZONS IN NUTRITION
CSNS
LA NUTRITION DE DEMAIN
N C
Canadian Society for Nutritional Sciences/La Societe Canadienne des Sciences de la Nutrition
SCSN
Tailoring total parenteral nutrition Howard Parsons, MD hen enteral feeding is insufficient, parenteral administration of amino acid solution is an alternative. New knowledge has led to the tailoring of these solutions to meet specific metabolic requirements.' The requirements of neonates and infants differ from those of adults because of rapid growth and immature metabolic pathways, and preparations based on the amino acid composition of breast milk have been developed. The two available in North America contain less phenylalanine, methionine and glycine and more branchedchain amino acids, tyrosine, histidine, aspartic acid, taurine, cysteine and glutamic acid than standard solutions for adults. This article reviews the role of taurine, cysteine and glutamine. Taurine is included in solutions for infants because of its presence in breast milk and the high concentrations in retina, muscle, liver and heart.2 It is the most abundant free amino acid in the retina, and low plasma levels in infants receiving long-term total parenteral nutrition (TPN) devoid of taurine are associated with abnormal electroretinograms, though not with abnormalities in visual acuity. Although taurine conjugates with bile salts, reduction of TPN-associated cholestasis has not been confirmed. In adults, taurine is produced from cysteine by cysteine sulfonic acid decarboxylase. Cysteine, although a nonessential amino acid in adults, has, like taurine, been classified as conditionally indispensable in neonates. It is unstable in solution and, in the presence of oxygen, forms cystine, which can easily precipitate. Compounds of cysteine that are nutritionally acceptable and chemically stable - N-acetylcysteine, N,N-diacylcysteine, thiazolidine-4-carboxylic acid and L-2-oxo-thiazolidine-4-carboxylic acid - are converted to intracellular cysteine. Some investigators have been unable to show any advantage of cysteine supplementation W
other than increased plasma levels of sulfur amino acids.3 However, there are several theoretical advantages, such as promoting the synthesis of glutathione (L--y-glutamyl-L-cysteinylglycine), which participates in the formation and maintenance of sulfhydryl groups of molecules involved, for example, in detoxification of peroxides and free radicals. Glutamine is an important energy-yielding substrate for the mucosal cells of the intestinal tract and pancreas, for other rapidly growing cells and for hepatic gluconeogenesis. Several studies have shown a net release of glutamine from muscle in vivo and in vitro. During catabolic stress there is a 50% reduction in intracellular muscle glutamine, which is not reversed by conventional nutritional means. Glutamine has not been included in standard TPN solutions because of its relative insolubility and potential toxicity. In vivo studies have demonstrated that a synthetic glutamine-containing dipeptide, Lalanyl-L-glutamine, is readily hydrolyzed and improves overall nitrogen balance compared with isonitrogenous conventional solutions.4 Glutamine-containing solutions may be useful in patients with inflammatory bowel disease or the stress of trauma and in those undergoing bone marrow transplantation. Controlled prospective studies are warranted, but general use at this time should be restricted.
References 1. Furst P, Alders S, Stehle P: Glutamine-containing dipeptides in parenteral nutrition. JPEN 1990; 14: 1185-1245 2. Gaull GE. Taurine in pediatric nutrition: review and update. Pediatrics 1989; 83: 433-442 3. Heyman MB: General and specialized parenteral amino acid formulations for nutritional support. JAm Diet Assoc 1990; 90:
401-408,411 4. Zlotkin H, Bryan MG, Anderson GH: Cysteine supplementation to cysteine-free intravenous feeding regimens in newborn infants. Am J Clin Nutr 1981; 34: 914-923
This article was made possible by an educational grant to the Canadian Society,for Nutritional Sciences (CSNS) from Clintec Nutrition Company, but the author and the content of the article were determined solely by the CSNS. The opinions expressed herein are those of the author and not necessarily those of the CSNS. Reprint requests to: Dr. Howard Parsons, Department of Pediatrics, Health Sciences Centre, University ofCalgary, 3330 Hospital Dr. NW, Calgar,y, AB T2N 4NI MAY 1, 1991
CAN MED ASSOC J 1991; 144 (9)
1141
CSNS
LA NUTRITION DE DEMAIN
N C
Canadian Society for Nutritional Sciences/La Societe Canadienne des Sciences de la Nutrition
SCSN
Tailoring total parenteral nutrition Howard Parsons, MD hen enteral feeding is insufficient, parenteral administration of amino acid solution is an alternative. New knowledge has led to the tailoring of these solutions to meet specific metabolic requirements.' The requirements of neonates and infants differ from those of adults because of rapid growth and immature metabolic pathways, and preparations based on the amino acid composition of breast milk have been developed. The two available in North America contain less phenylalanine, methionine and glycine and more branchedchain amino acids, tyrosine, histidine, aspartic acid, taurine, cysteine and glutamic acid than standard solutions for adults. This article reviews the role of taurine, cysteine and glutamine. Taurine is included in solutions for infants because of its presence in breast milk and the high concentrations in retina, muscle, liver and heart.2 It is the most abundant free amino acid in the retina, and low plasma levels in infants receiving long-term total parenteral nutrition (TPN) devoid of taurine are associated with abnormal electroretinograms, though not with abnormalities in visual acuity. Although taurine conjugates with bile salts, reduction of TPN-associated cholestasis has not been confirmed. In adults, taurine is produced from cysteine by cysteine sulfonic acid decarboxylase. Cysteine, although a nonessential amino acid in adults, has, like taurine, been classified as conditionally indispensable in neonates. It is unstable in solution and, in the presence of oxygen, forms cystine, which can easily precipitate. Compounds of cysteine that are nutritionally acceptable and chemically stable - N-acetylcysteine, N,N-diacylcysteine, thiazolidine-4-carboxylic acid and L-2-oxo-thiazolidine-4-carboxylic acid - are converted to intracellular cysteine. Some investigators have been unable to show any advantage of cysteine supplementation W
other than increased plasma levels of sulfur amino acids.3 However, there are several theoretical advantages, such as promoting the synthesis of glutathione (L--y-glutamyl-L-cysteinylglycine), which participates in the formation and maintenance of sulfhydryl groups of molecules involved, for example, in detoxification of peroxides and free radicals. Glutamine is an important energy-yielding substrate for the mucosal cells of the intestinal tract and pancreas, for other rapidly growing cells and for hepatic gluconeogenesis. Several studies have shown a net release of glutamine from muscle in vivo and in vitro. During catabolic stress there is a 50% reduction in intracellular muscle glutamine, which is not reversed by conventional nutritional means. Glutamine has not been included in standard TPN solutions because of its relative insolubility and potential toxicity. In vivo studies have demonstrated that a synthetic glutamine-containing dipeptide, Lalanyl-L-glutamine, is readily hydrolyzed and improves overall nitrogen balance compared with isonitrogenous conventional solutions.4 Glutamine-containing solutions may be useful in patients with inflammatory bowel disease or the stress of trauma and in those undergoing bone marrow transplantation. Controlled prospective studies are warranted, but general use at this time should be restricted.
References 1. Furst P, Alders S, Stehle P: Glutamine-containing dipeptides in parenteral nutrition. JPEN 1990; 14: 1185-1245 2. Gaull GE. Taurine in pediatric nutrition: review and update. Pediatrics 1989; 83: 433-442 3. Heyman MB: General and specialized parenteral amino acid formulations for nutritional support. JAm Diet Assoc 1990; 90:
401-408,411 4. Zlotkin H, Bryan MG, Anderson GH: Cysteine supplementation to cysteine-free intravenous feeding regimens in newborn infants. Am J Clin Nutr 1981; 34: 914-923
This article was made possible by an educational grant to the Canadian Society,for Nutritional Sciences (CSNS) from Clintec Nutrition Company, but the author and the content of the article were determined solely by the CSNS. The opinions expressed herein are those of the author and not necessarily those of the CSNS. Reprint requests to: Dr. Howard Parsons, Department of Pediatrics, Health Sciences Centre, University ofCalgary, 3330 Hospital Dr. NW, Calgar,y, AB T2N 4NI MAY 1, 1991
CAN MED ASSOC J 1991; 144 (9)
1141
