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Total branched-chain amino acids requirement in patients with maple syrup urine disease by use of indicator amino acid oxidation with L-[1-¹³C]phenylalanine

¹Departments of Nutritional Sciences and ²Paediatrics, University of Toronto, Toronto M5S 3E2; ³The Research Institute, The Hospital for Sick Children, Toronto, Ontario M5G 1X8; ⁴School of Dietetics & Human Nutrition, McGill University, Ste-Anne-de-Bellevue, Quebec H9X 3V9; and ⁵Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Alberta T6G 2P5, Canada

Submitted 25 September 2003 ; accepted in final form 13 February 2004

Maple syrup urine disease (MSUD) is an autosomal recessive disorder caused by defects in the mitochondrial multienzyme complex branched-chain -keto acid dehydrogenase (BCKD; EC 1.2.4.4), responsible for the oxidative decarboxylation of the branched-chain ketoacids (BCKA) derived from the branched-chain amino acids (BCAA) leucine, valine, and isoleucine. Deficiency of the enzyme results in increased concentrations of the BCAA and BCKA in body cells and fluids. The treatment of the disease is aimed at keeping the concentration of BCAA below the toxic concentrations, primarily by dietary restriction of BCAA intake. The objective of this study was to determine the total BCAA requirements of patients with classical MSUD caused by marked deficiency of BCKD by use of the indicator amino acid oxidation (IAAO) technique. Five MSUD patients from the MSUD clinic of The Hospital for Sick Children participated in the study. Each was randomly assigned to different intakes of BCAA mixture (0, 20, 30, 50, 60, 70, 90, 110, and 130 mg·kg^–1·day^–1), in which the relative proportion of BCAA was the same as that in egg protein. Total BCAA requirement was determined by measuring the oxidation of L-[1-¹³C]phenylalanine to ¹³CO₂. The mean total BCAA requirement was estimated using a two-phase linear regression crossover analysis, which showed that the mean total BCAA requirement was 45 mg·kg^–1·day^–1, with the safe level of intake (upper 95% confidence interval) at 62 mg·kg^–1·day^–1. This is the first time BCAA requirements in patients with MSUD have been determined directly.

indicator amino acid oxidation; amino acid requirements

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				R. Elango, R. O. Ball, and P. B. Pencharz Indicator Amino Acid Oxidation: Concept and Application J. Nutr., February 1, 2008; 138(2): 243 - 246. [Abstract] [Full Text] [PDF]