Fuel utilisation during prolonged low-to-moderate intensity exercise when ingesting water or carbohydrate

Previously, we examined the effects of carbohydrate (CHO) ingestion on glucose kinetics during exercise at 70% of maximum O2 uptake (VO2, max). Here we repeat those studies in heavier cyclists (n = 6 per group) cycling for 3 h at a similar absolute O2 uptake but at a lower (55% of VO2, max) relative exercise intensity. During exercise, the cyclists were infused with a 2-3H-glucose tracer and ingested U-14C glucose-labelled solutions of either flavoured water (H2O) or 10 g/100 ml glucose polymer, at a rate of 600 ml/h. Two subjects in the H2O trial fatigued after 2.5 h of exercise. Their rates of glucose appearance (Ra) declined from 2.9 +/- 0.6 to 2.0 +/- 0.1 mmol/min (mean +/- SEM) and, as their plasma glucose concentration [Glu] declined from 4.7 +/- 0.2 to below 3.5 +/- 0.2 mM, their rates of glucose oxidation (Rox) and fat oxidation plateaued at 2.7 +/- 0.4 and 1.7 +/- 0.1 mmol/min respectively. In contrast, all subjects completed the CHO trial. Although CHO ingestion during exercise reduced the final endogenous Ra from 3.4 +/- 0.6 to 0.9 +/- 0.3 mmol/min at the end of exercise, it increased total Ra to 5.5 +/- 0.5 mmol/min (P < 0.05). A higher total Ra with CHO ingestion raised [Glu] from 4.3 +/- 0.3 to 5.3 +/- 0.1 mM and accelerated Rox from 3.5 +/- 0.2 to 5.9 +/- 0.2 mmol/min after 180 min of exercise (P < 0.05). The increased contribution to total energy production from glucose oxidation (34 +/- 1 vs. 20 +/- 1%) decreased energy production from fat oxidation from 51 +/- 2 to 40 +/- 5% (P = 0.08) and produced patterns of glucose, muscle glycogen (plus lactate) and fat utilisation similar to those during exercise at 70% of (V O2, max). Thus, CHO ingestion is necessary to sustain even prolonged, low to moderate intensity exercise and when ingested, it suppresses the higher relative rates of fat oxidation usually observed at exercise intensities less than 60% of VO2, max.