Sports Drinks: No Swallowing Necessary

There’s all sorts of mumbo-jumbo about how sports drinks boost athletes’ performance, especially in endurance events such as yesterday’s Boston Marathon. But according to an intriguing new study, it isn’t the sports drinks’ calories (athletes benefit even if they spit out the drink rather than swallow it) or their sweet taste (drinks with artificial sweeteners do not boost performance). Instead, suggest Ed Chambers of the University of Birmingham and colleagues in a paper in The Journal of Physiology, carbohydrates in the drinks fit into receptors in the mouth that in turn activate the brain’s pleasure and reward centers, spurring athletes to push themselves harder without realizing how hard they're working.

For their study, the scientists prepared drinks containing either glucose (a sugar), maltodextrin (a tasteless carbohydrate) or plain water, mixed with artificial sweeteners so they tasted identical. Eight endurance cyclists rinsed their mouths for 10 seconds with one of the three drinks, and then got on a stationary bike. Results: athletes who swished with the glucose or maltodextrin drinks outperformed those on sweetened water by 2 to 3 percent, raising their pulse and sustaining a higher average power output—even though they said they didn’t feel they were working harder.

Chambers explains it this way: “Much of the benefit from carbohydrate in sports drinks is provided by signalling directly from mouth to brain rather than providing energy for the working muscles.”

That was born out by neuroimaging. Using fMRI to monitor brain activity after the athletes rinsed their mouths with one of the three drinks, the scientists found that glucose and maltodextrin increased activity in regions associated with reward or pleasure (the anterior cingulate cortex and striatum). The artificially sweetened water did not. They propose that the sugar or carbohydrate glommed onto receptors in the mouth, causing a signaling cascade that activated these brain regions, with the result that the athletes felt they were not working as hard as they actually were—contributing to endurance and power output. Once again, it seems as if the brain, not the muscles, ultimately govern how well we do even on what seems to be a purely physical task. As the scientists put it, “carbohydrate in the human mouth activates regions of the brain that can enhance exercise performance.”