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* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada, T6G 2P5;
The Department of Biochemistry, Memorial University of Newfoundland, St. John’s, NL, Canada, A1B 3X9;
** Department of Animal Science, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2;
The Research Institute, The Hospital for Sick Children, Toronto, Canada;
Department of Paediatrics and
Department of Nutritional Sciences, University of Toronto, Toronto, ON, Canada, M5G 1X8
3 To whom correspondence should be addressed. E-mail: ron.ball{at}ualberta.ca.
The sulfur amino acids (SAA), methionine and cysteine, are normally supplied in a 50:50 ratio in the oral diet of pigs. In contrast, cysteine is not included in any appreciable amounts in parenteral solutions due to its instability in solution. Cysteine can replace part of the methionine requirement, but is not required when methionine is supplied at a level that meets the entire SAA requirement. However, the role of the gut on cysteine sparing has not been investigated. In the present study, the enteral and parenteral methionine requirement was determined, with excess dietary cysteine, by using the indicator amino acid oxidation (IAAO) technique. Piglets [n = 28, 2 d, 1.65 ± 0.014 kg (SE)] were fed elemental diets containing adequate energy, phenylalanine and excess tyrosine, with varied methionine concentrations and excess cysteine [0.55 g/(kg · d)]. Diets were infused continuously via intravenous (parenteral) or gastric (enteral) catheters. Phenylalanine oxidation was determined during a primed, constant infusion of L-[1-14C]-phenylalanine, by measuring expired 14CO2 and plasma specific radioactivity (SRA) of phenylalanine. For both the parenteral and enteral groups, phenylalanine oxidation (% of dose) decreased linearly (P < 0.01) as methionine intake increased and then became low and unchanging. Using breakpoint analysis, the methionine requirement was estimated to be 0.25 and 0.18 g/(kg · d) for enteral and parenteral feeding, respectively. These data show that the parenteral methionine requirement is 
70% of the enteral requirement when measured in the presence of excess dietary cysteine (P < 0.05). A comparison with our previous studies in which methionine was the only source of sulfur amino acids shows that the addition of dietary cysteine reduces the methionine requirement by 40% in both enterally and parenterally fed neonatal piglets. Therefore, dietary cysteine is equally effective in sparing dietary methionine whether fed enterally or parenterally.
KEY WORDS: • cysteine • cysteine sparing • indicator amino acid oxidation • methionine • total parenteral nutrition
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