Leucine Oxidationin vivo: Inter-and Intraindividual Variation in Healthy Subjects as Assessed by Oral L-[1-¹³C]Leucine Loads

Interindividual variability of leucine catabolism was studied in eight overnight fasted healthy volunteers by means of a controlled oral L-[1-13C]leucine loading test (5 mg/kg body weight). With the exception of total CO2 expiration (CV = 5%),a considerable variation (CV generally < or ≪10%)was observed with all other metabolic parameters: increase of leucine/2-oxoisocaproate in serum, 13C-enrichment in serum metabolites and 13CO2 in expired air, and estimated oxidation rates. The degree of variation was only partially referable to individual differences. A comparable high degree of variability was found when two probands underwent multiple loading tests (n = 4). Possible reasons and implications are discussed.