Priming the pump: carb levels for endurance exercise

The ingestion of carbohydrates during exercise has been well established^[1] to improve performance. Some of the reasons are seen in Figure 1: it sustains a high rate of carb oxidation^[2] (more carbs per minute can be burned), maintains blood glucose^[3] levels (it prevents exercise-induced blood-sugar dips), spares muscle glycogen^[4] (one of the muscle’s fuel sources), and improves the central nervous system’s^[5] response to fatigue. Not surprisingly, carbs and exercise have become like peas and carrots … always together!

Figure 1: Mechanisms for carbs improving performance

While there is little doubt that carb intake will bolster performance during extended exercise, questions remain as to the optimal amount of carbohydrate that should be ingested. Guidelines from the American College of Sports Medicine (ACSM) suggest intakes of 30-60 grams per hour during exercise, although benefits have been seen from as low as 16 grams^[6] per hour and as high as 105 grams^[7] per hour. Interestingly, carbs may even be of benefit without swallowing them. Simply using a carbohydrate mouth rinse^[8] can improve performance^[9] in a one-hour time trial! This means that part of the performance boost is mediated by the brain: when the brain thinks there's carbohydrate coming, it lets the muscles work a bit harder.

While a dose-response relationship between performance and carb intake has been observed^[1] up to 78 grams per hour, other studies^[10] have seen no improvements at 47 grams per hour compared with 27 grams per hour. A number of reasons could explain these differences in the published research: the gender of the participants, their body size, their level of fitness (trained athletes have greater glycogen stores), their sport (e.g. endurance athletes vs. sprinters), their diet (their usual carb intake, notably), their hydration status and fed or fasted state before the test, the type of carbohydrate consumed for the test, the duration of the test, etc. The sample size of the study is also important, as small studies may not have the statistical power to detect small, yet meaningful, differences.

Because optimal carb intake is not yet known for certain, the authors of this new study set out to examine the dose-response effect of an hourly ingestion of zero to 64 grams of carbohydrate during exercise in trained male cyclists.

Ingesting carbs during extended endurance exercise is a well-known way of improving performance. However, the optimal amount is up for debate. Guidelines state that 30-60 grams per hour is effective, but many other studies have found effects at both lower and higher doses. The goal of this study was to see how performance was affected by varying carbohydrate intake during exercise in trained male cyclists.