Fuel kinetics during intense running and cycling when fed carbohydrate

Oral [13C]glucose and endogenous energy substrate oxidation during prolonged treadmill running

“…Also, as already reported during cycling exercise with carbohydrate ingestion (2,5,10,14,20,21,26), a much higher amount of plasma glucose originated from exogenous glucose (0.92 and 1.22 g/min at minutes 60 and 120, respectively, representing 70-72% of plasma glucose and 36-54% of total glucose oxidation). The rate of plasma glucose oxidation, which was well above 1 g/min at minute 60, reached 1.69 g/min at the end of exercise.…”

“…During running exercise, with ingestion of 150 g of glucose, Derman et al (10), using [ 14 C]glucose, have shown that plasma glucose oxidation was 0.83 g/min (0.40 and 0.43 g/min from the liver and from exogenous glucose, respectively) at the end of a 63-min exercise period at 80% V O 2 max , providing 23% of the energy yield. A slightly higher value (1.4 g/min) can be computed from data reported by Tsintzas et al (35) during a 104-min running exercise at 70% V O 2 max , with ingestion of 0.6 g of glucose/min; total glucose oxidation computed from indirect calorimetry averaged 3.1 g/min, whereas muscle glycogen oxidation averaged 1.7 g/min (2.5 mmol ⅐ min Ϫ1 ⅐ kg Ϫ1 ϫ 3.8 kg of working muscles, dry weight).…”

“…The oxidation rate of muscle glycogen can be estimated from changes in muscle glycogen content (7,34,35) or could be computed by difference between the total oxidation rate of glucose (computed by using indirect respiratory calorimetry) and of plasma glucose (computed by using tracer techniques) (2,5,10,14,20,21,26). Using this method, Derman et al (10) and McConell et al (21) have shown at 80-83% V O 2 max that the muscle relies much more on muscle glycogen (3.1-3.4 g/min providing 70-82% of the energy yield) than on plasma glucose oxidation (0.8-0.9 g/min).…”

“…Accordingly, the oxidation rate of exogenous glucose remained in the lower range of values reported (0.3-0.7 g/min). In the study by Derman et al (10), exogenous glucose oxidation was compared in response to running and cycling exercise at a high workload (3.6 l O 2 /min; 80% V O 2 max ) with ingestion of a large amount of glucose (2.4 g/min). The oxidation rate of exogenous glucose was lower during running than cycling exercise (0.230 and 0.345 g/min, respectively), providing Ͻ10% of the total energy yield.…”

“…The ingested glucose was artificially labeled with 13 C to compute its oxidation rate from the production of 13 CO 2 (V 13 CO 2 ) at the mouth. In addition, oxidation of plasma glucose was computed from V 13 CO 2 and 13 C/ 12 C in plasma glucose, liver glucose output was estimated by difference between plasma and exogenous glucose oxidation, and the oxidation of muscle glycogen was computed from the difference between total glucose and plasma glucose oxidation (10,26).…”