Daily leucine oxidation and derived values for whole-body leucine balance, obtained by continuous measurement throughout a 24-h period, were compared with those predicted from short-term measurements during fasted and fed states in five healthy adults studied during two 6-d experimental diet periods, each immediately followed by a 24-h continuous intravenous tracer infusion of L-[1-13C]leucine. Leucine intake was either 14 or 38.3 mg.kg-1.d-1. Mean measured daily leucine oxidation (mg leucine.kg-1.d-1) was 27.8 and 45.2 for the 14- and 38.3-mg intakes, respectively. Oxidation rates predicted by extrapolation of rates measured during the final hour of fasting (15 h after last meal) and the 5th h of feeding were approximately 12% higher (P < 0.01) than measured rates for both diets. For the prediction based on the 12th h of fasting and 5th h of feeding, it was 4% higher or 0.4% lower than measured rates for the 38.3- and 14- mg intakes, respectively. Hence, relatively small differences exist between measured vs predicted estimates of daily leucine oxidation and balance. These studies support previous conclusions that the current, international requirement value for leucine in healthy adults is far too low.