Based on a mass-balance model, a surrogate measure of the whole body leucine transport into and out of cells under steady-state conditions was calculated as u/TTR, where u is the infusion rate of (stable label) leucine tracer and TTR is the difference between the tracer-to-tracee ratio of extracellular and intracellular leucine. The approach was evaluated in ten healthy subjects [8 males and 2 females; age, 31 ± 9 (SD) yr; body mass index, 24.0 ± 1.6 kg/m²] who received a primed (7.58 µmol/kg) constant intravenous infusion (7.58 µmol · kg¹ · h¹) of L-[1-¹³C]leucine over 180 min (7 subjects) or 240 min (3 subjects). Five subjects were studied on two occasions 1 wk apart to assess reproducibility. Blood samples taken during the last 30 min of the leucine infusion were used to determine plasma leucine concentration (129 ± 35 µmol/l), TTR of leucine (9.0 ± 1.5%), and TTR of -ketoisocaproic acid (6.7 ± 0.8%). The latter TTR was taken as the measure of the free intracellular leucine TTR. The whole body inward and outward transport was 6.66 ± 3.82 µmol · kg¹ · min¹; the rate of leucine appearance due to proteolysis was 1.93 ± 0.24 µmol · kg¹ · min¹. A positive linear relationship between the inward transport and plasma leucine was observed (P < 0.01), indicating the presence of the mass effect of leucine on its own transport. The transport was highly variable between subjects (between-subject coefficient of variation 57%) but reproducible (within-subject coefficient of variation 17%). We conclude that reproducible estimates of whole body transport of leucine across the cell membrane can be obtained under steady-state conditions with existing experimental and analytical procedures.

mathematical model; stable-label isotope tracer; steady-state conditions; reproducibility