Effects of feeding glucose on substrate metabolism during cycling were studied. Trained (60.0 ± 1.9 mL · kg−1 · min−1) males (N = 5) completed two 75 min, 80% VO2max trials: 125 g 13C-glucose (CHO); 13C-glucose tracer, 10 g (C). During warm-up (30 min 30% VO2max) 2 ⋅ 2 g 13C-glucose was given as bicarbonate pool primer. Breath samples and blood glucose were analyzed for 13C/ 12C with IRMS. Protein oxidation was estimated from urine and sweat urea. Indirect calorimetry (protein corrected) and 13C/ 12C enrichment in expired CO2 and blood glucose allowed exogenous (Gexo), endogenous (Gendo), muscle (Gmuscle), and liver glucose oxidation calculations. During exercise (75 min) in CHO versus C (respectively): protein oxidation was lower (6.8 ± 2.7, 18.8 ± 5.9 g; P = 0.01); Gendo was reduced (71.2 ± 3.8, 80.7 ± 5.7%; P = 0.01); Gmuscle was reduced (55.3 ± 6.1, 65.9 ± 6.0%; P = 0.01) compensated by increased Gexo (58.3 ± 2.1, 3.87 ± 0.85 g; P = 0.000002). Glucose ingestion during exercise can spare endogenous protein and carbohydrate, in fed cyclists, without gly-cogen depletion.
Van Hamont was with the Dept of Movement Sciences, Maastricht University, P.O. Box 616, 6200MD Maastricht, The Netherlands. Harvey is with the School of Physical Education, University of Otago, P.O. Box 56, Dunedin, New Zealand. Massicotte is with the Dépt de Kinanthropologie, Université du Québec à Montréal, Montréal, Canada. Frew is with the Dept of Chemistry, University of Otago, Dunedin, New Zealand. Peronnet is with the Dépt de Kinésiologie, Université de Montréal, Montréal, Québec, Canada. Rehrer is with the School of Physical Education and Dept. of Human Nutrition, University of Otago, P.O. Box 56, Dunedin, New Zealand.