Leucine is not a good choice as an indicator amino acid for determining amino acid requirements in men

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-[sup.13C]leucine isotope is an acceptable indicator by comparing it with an established tracer, L-[1-[sup.13]C]lysine. Healthy men (n = 7; 29.9 [+ or -] 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 [sup.13]C[O.sub.2] 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 [sup.13]C[O.sub.2], 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. KEYWORDS: * indicator amino acid oxidation * leucine * aromatic amino acids * phenylalanine * tyrosine