Reduction in muscle glycogen and protein utilization with glucose feeding during exercise.

Effects of feeding glucose on substrate metabolism during cycling were studied. Trained (60.0 +/- 1.9 mL x kg(-1) x min(-1)) males (N = 5) completed two 75 min, 80% VO(2max) trials: 125 g 13(C)-glucose CHO); 13(C)-glucose tracer, 10 g (C). During warm-up (30 min 30% VO2max) 2 . 2 g 13(C)-glucose was given as bicarbonate pool primer. Breath samples and blood glucose were analyzed for 13(C/12)C with IRMS. Protein oxidation was estimated from urine and sweat urea. Indirect calorimetry (protein corrected) and 13(C/12)C enrichment in expired CO(2)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 glycogen depletion.