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Nutrient Requirements and Optimal Nutrition

Leucine Is Not a Good Choice as an Indicator Amino Acid for Determining Amino Acid Requirements in Men^1,2

Jean W-C. Hsu^*,, Wantanee Kriengsinyos, Linda J. Wykes^**, Mahroukh Rafii, Laksiri A. Goonewardene, Ronald O. Ball^*,, and Paul B. Pencharz^*,,,,3

^* Department of Nutritional Sciences, University of Toronto, Toronto, ON, Canada M5S 3E2; The Research Institute, The Hospital for Sick Children, Toronto, ON, Canada M5G 1X8; ^** School of Dietetics and Human Nutrition, McGill University, Ste Anne de Bellevue, QC, Canada H9X 3V9; Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, AB, Canada T6G 2P5; and Department of Paediatrics, University of Toronto, ON, Canada

³ To whom all correspondence and reprint requests should be addressed. E-mail: paul.pencharz{at}sickkids.ca.

Leucine tracer has been widely used for examining whole-body protein turnover in humans, but has not been evaluated as an indicator to be used in the indicator amino acid oxidation (IAAO) method. The goal of this study was to determine whether the L-[1-¹³C]leucine isotope is an acceptable indicator by comparing it with an established tracer, L-[1-¹³C]lysine. Healthy men (n = 7; 29.9 ± 4.8 y old) were fed in random order a diet with 7 graded intakes of phenylalanine without tyrosine. In the first study (n = 5), subjects were administered an excess leucine intake of 65 mg/(kg·d), and in the second study (n = 5), they were given the mean requirement of 45 mg/(kg·d) to determine whether leucine intake affected the pattern of response. Previous IAAO studies using lysine and phenylalanine demonstrated a clear pattern in ¹³CO₂ production, i.e., increasing test amino acid intake resulted in a linear decrease to plateau, with a readily discernable breakpoint indicating the requirement. This pattern of production of ¹³CO₂, indicates clear partitioning of the indicator amino acid between oxidation and protein synthesis. This was not observed with leucine at an intake of 65 mg/(kg·d). Conversely, at the lower leucine intake of 45 mg/(kg·d), a breakpoint was seen and a total aromatic amino acid requirement estimate that did not differ from that obtain using lysine as the indicator was obtained. In conclusion, leucine may be used as the indicator in the IAAO technique only when the daily intake leucine is given at its mean requirement level and the potential metabolic effects of other variables are taken into consideration.

KEY WORDS: • indicator amino acid oxidation • leucine • aromatic amino acids • phenylalanine • tyrosine

This article has been cited by other articles:

				P. B. Pencharz, J. W-C. Hsu, and R. O. Ball Aromatic Amino Acid Requirements in Healthy Human Subjects J. Nutr., June 1, 2007; 137(6): 1576S - 1578S. [Abstract] [Full Text] [PDF]