THE 24-H KINETICS OF LEUCINE OXIDATION IN HEALTHY-ADULTS RECEIVING A GENEROUS LEUCINE INTAKE VIA 3 DISCRETE MEALS

The significance of meal size and frequency for the 24-h leucine tracer-balance technique was examined. Continuous measurements of leucine oxidation throughout a 24-h d were performed in six healthy, young adults who were given a weight-maintaining diet (188 kJ . kg(-1). d(-1); 1 g protein . kg(-1). d(-1)) for 6 d followed by primed, continuous intravenous infusions of L-[1-C-13]leucine and [N-15-N-15]urea. The 24-h study was started at 1800 on day 6 and three equal discrete meals were given at 2000, 0600, and 1200. Leucine oxidation was assessed from plasma [C-13]alpha-ketaisocaproate enrichment and (CO2)-C-13 excretion. The mean (+/- SD) leucine oxidation after each meal (over 6 h) was not significantly different (P > 0.5) among the three discrete meals: 20.0 +/- 3.9, 20.2 +/- 1.9, and 20.3 +/- 2.4 mg . kg(-1). d(-1) for the meals given at 2000, 0600, and 1200, respectively. Twenty-four-hour leucine oxidation was 75.0 +/- 7.8 mg . kg(-1). d(-1) for a leucine dietary intake of 80 mg . kg(-1). d(-1) (and approximate to 9.7 mg tracer . kg(-1). d(-1)). The 24-h pattern in leucine oxidation was paralleled by plasma leucine concentrations. Further, leucine oxidation and urea excretion predicted relatively similar values for 24-h protein oxidation. These data are compared with results from our similar previous studies using a multiple-small-meal feeding protocol.