Comparison of two methods for computing exogenous substrate oxidation using 13C-labeling.

With 13C stable isotope as tracer, the purposes of this study were to measure the oxidation rates of exogenous glucose by using two computation procedures that take into account changes in isotopic composition of CO2 arising from oxidation of endogenous substrates (Rendo) and compare these results with studies using 14C-glucose. Two different low levels of 13C-enrichment were used in the first procedure, while a very high level of enrichment was used in the second one. Each of the eight subjects completed four exercises (68 +/- 5% VO2max; 90 min) on cycle ergometer, at 7-d intervals. After 30 min of exercise, the subjects ingested in a single bolus of 30 g of 13C-glucose, dissolved in 300 ml of water, enriched at three different levels (trials A and B = -10.9 and +2.5; trial C = +291.9/1000 delta 13C-PDB-1), or water only. The metabolic and endocrine state at rest and its response to exercise with or without glucose ingestion were similar in the four trials, with the exception of FFA and glycerol, which were blunted by the ingestion of glucose. As expected, Rendo significantly increased from rest (-22.7 +/- 0.7/1000 delta 13C-PDB-1) to the beginning of exercise without glucose ingestion (-21.2 +/- 0.5/1000 delta 13C-PDB-1). The amounts of exogenous glucose oxidized over the last hour of exercise and computed from trials A and B and from trial C were 14.9 +/- 4.4 and 13.0 +/- 4.2 g, representing 7.4 and 6.3% of the total energy requirement, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)