The 24-h kinetics of leucine oxidation in healthy adults receiving a generous leucine intake via three 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- 13 C]leucine and [ 15 N- 15 N]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 [ 13 C]α-ketoisocaproate enrichment and 13 CO 2 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 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.