Are Carbohydrates Really Needed During Exercise?

One study questions whether or not we should fuel our muscles during a run or let them use stored energy reserves.

One study questions whether or not we should fuel our muscles during a run or let them use stored energy reserves.

We all know that consuming carbohydrates during strenuous endurance exercise enhances performance. Given this fact, it would seem to be a good idea to consume carbs, in a sports drink or gels, during every long or hard run, as the resulting elevated performance in each workout would yield a stronger training effect that, over time, would add up to greater fitness.

However, some science has shown that the opposite is true. The body actually depends on the depletion of carbohydrate (that is, glycogen) stores in the muscles and liver to trigger some of the positive physiological adaptations to training, such as increased fat-burning capacity. When carbs are consumed during a workout, the body’s own carbohydrate stores become less depleted and there is a less pronounced training effect.

Even so, research has shown that while training in a glycogen-depleted state enhances low-intensity endurance in mice, it does not enhance high-intensity time trial performance in humans compared to training with normal glycogen levels. Thus it is still unclear whether the benefits of consuming carbs during workouts outweigh the benefits of withholding carbs during workouts or vice versa.

Researchers from the Australian Institute of Sport sought to put this question to rest one way or the other in 2010. They divided a pool of trained cyclists and triathletes into separate groups and had each of them go through a 28-day block of training, with one group consuming carbs during every workout and the other group abstaining from carbohydrate intake during workouts. Performance was measured before and at the end of the training block in a cycling time trial. The researchers found that performance improved equally — 7 percent on average — in both groups despite greater increases in carbohydrate-burning capacity in the carb group.

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Obviously, the improvement in time trial performance was a result of the training and not to the provision or withholding of carbs during training. So it appears that the benefits of training with carbs are indeed counterbalanced by those of training without, at least in relation to high-intensity, moderate-duration time trial performance. One wonders if there might be an additive effect associated with mixing carb-fueled and non-carb-fueled workouts in training. In other words, would athletes who consumed carbs in some workouts and did not consume carbs in others get fitter than athletes who did just one or the other? I humbly propose that this be the next study done in this area.


About The Author:

Matt Fitzgerald is the author of numerous books, including Racing Weight: How To Get Lean For Peak Performance (VeloPress, 2012). He is also a Training Intelligence Specialist for PEAR Sports. To learn more about Matt visit