It’s a well known fact that drinking carbohydrate-rich beverages during high-intensity exercise improves performance even if it’s relatively short, which made researchers suspect that direct metabolic effect could hardly be the reason since there’s simply not enough time to digest the carbs and deliver the energy to the muscles. The only alternative seemed to be “all in the brain”. To check this hypothesis, the metabolic input excluded completely: the athletes didn’t swallow the drink but only rinsed the mouth with it  - and performance also improved!

Now, the discussion is going on, which brain structures are responsible and how they overrule the chemical senses inside the body that tells the truth: no energy has been ingested, The brain for some reason believe the sense of sweetness rather than the qualitative report from the blood.

Why any sweet taste, coming with any sweetener, raises glucose concentration in the blood *before* the food has a chance to be digested? Because your body knows that eventually, it will have all the carbs you’ve swallowed and it doesn’t wait until it that happens and borrows real carbohydrates from carbohydrate depots. In the case of physical performance, the brain recruits muscles in anticipation of real energy coming into the blood soon, and this always happened in the past, before artificial sweeteners and wicked experimental protocols were invented.

Source: Jeukendrup, Chambers. Oral carbohydrate sensing and exercise performance. Current Opinion in Clinical Nutrition and Metabolic Care 2010, 13:447-451