Effect of carbohydrate ingestion on glucose kinetics and muscle metabolismduring intense endurance exercise

There has been recent interest in the potential performance and metabolic e ffects of carbohydrate ingestion during exercise lasting similar to1 h. In this study, 13 well-trained men ingested in randomized order either a 6% gl ucose solution (CHO trial) or a placebo (Con trial) during exercise to exha ustion at 83 +/- 1% peak oxygen uptake. In six subjects, vastus lateralis m uscle was sampled at rest, at 32 min, and at exhaustion, and in six subject s, glucose kinetics was determined by infusion of [6,6-H-2]glucose in both trials and ingestion of [6-H-3]glucose in the CHO trial. Of the 84 g of glu cose ingested during exercise in the CHO trial, only 22 g appeared in the p eripheral circulation. This resulted in a small (12 g) but significant (P < 0.05) increase in glucose uptake without influencing carbohydrate oxidatio n, muscle glycogen use, or time to exhaustion (CHO: 68.1 +/- 4.1 min; Con: 69.6 +/- 5.5 min). Decreases in muscle phosphocreatine content and increase s in muscle inosine monophosphate and lactate content during exercise were similar in the two trials. Although endogenous glucose production during ex ercise was partially suppressed in the CHO trial, it remained significantly above preexercise levels throughout exercise. In conclusion, only 26% of t he ingested glucose appeared in the peripheral circulation. Glucose ingesti on increased glucose uptake and partially reduced endogenous glucose produc tion but had no effect on carbohydrate oxidation, muscle metabolism, or tim e to exhaustion during exercise at 83% peak oxygen uptake.