RPE, blood glucose, and carbohydrate oxidation during exercise: effects of glucose feedings

This study examined the effects of glucose ingestion on differentiated and undifferentiated ratings of perceived exertion (RPE) during prolonged cycling exercise. On two occasions, seven trained males cycled for 180 min on a Monark cycle ergometer at 70% peak VO2 (VO2peak). Subjects consumed an 8% glucose/electrolyte drink (G) or a flavored water placebo (P) every 15 min throughout exercise. Measurement of RPE, ventilation (VE), oxygen uptake (VO2), respiration rate (RR), respiratory exchange ratio (RER), heart rate (HR), and venous blood sample collection preceded ingestion of the drink. Subjects were homogenous with respect to height, weight, and VO2peak. RPE for the legs and overall body were significantly attenuated (P less than 0.05) during the last 45 min of exercise. Plasma glucose and insulin were higher (P less than 0.05) in G than in P at virtually all time points. CHO oxidation and work rate were maintained throughout exercise in G but not during the last 30 min of exercise in P (P less than 0.05). Percent changes in plasma volume, plasma lactate, HR, VE, RR, and RPE for the chest were not different between conditions (P greater than 0.05). The data suggest that ingestion of carbohydrate beverages during endurance cycling can maintain plasma glucose and CHO oxidation during the latter stages of prolonged exercise. As a result, it appears that a relationship exists between attenuation of ratings of perceived exertion (especially in the legs), blood glucose, and CHO oxidation late in prolonged exercise. The mechanism for this probably involves the increased availability of blood-borne glucose to serve as substrate for brain and/or muscle energy metabolism during a time when endogenous stores of carbohydrate are low.