It has to be real carbohydrates, though; the scientists used a solution of water and a flavorless starch derivative called maltodextrin. Artificial sweeteners have no effect.
And the scientists think they have figured out why it works. It appears that the brain can sense carbohydrates in the mouth, even tasteless ones. The sensors are different from the ones for sweetness, and they prompt the brain to respond, spurring on the athlete.
Many athletes depend on sugary beverages to keep them going. But often, when blood is diverted from the stomach to working muscles during intense exercise, drinks or foods cause stomach cramps. So a carbohydrate rinse can be a way to get the same effect.
“You can get an advantage from tricking your brain,” said a discoverer of the effect, Matt Bridge, a senior lecturer in coaching and sports science at the University of Birmingham in England. “Your brain tells your body, ‘Carbohydrates are on the way.’ ” And with that message, muscles and nerves are prompted to work harder and longer.”
It’s a relatively small effect, said George A. Brooks, an exercise researcher at the University of California, Berkeley, who was not involved with the research. But a small difference, he added, “can make a big difference in competition.”
The discovery began with some puzzling findings dating to the 1990s.
Until then, exercise scientists thought they knew why it could help to eat or drink carbohydrates during a long endurance event like a marathon. Muscles can use up their glycogen, the storage form of glucose, during long exercise sessions. But if athletes consume carbohydrates, they can provide a new source of fuel for their starving muscles.
That theory predicts that carbohydrates should have no effect on performance in shorter races, an hour or less. Muscles can’t use up their glycogen that fast, and by the time the body metabolizes the carbohydrates for fuel, the race is almost over.
Then came a handful of studies showing that carbohydrates did have an effect in short exercise sessions. Athletes, often trained cyclists, rode hard and fast for an hour or so after drinking either a beverage containing carbohydrates or one that tasted the same but contained an artificial sweetener.
In intense exercise sessions lasting more than half an hour, the athletes were able to go faster or keep going longer when they had the drink with carbohydrates. Their performance improved as much as 14 percent.
Some studies, though, did not find an effect. And the difference seemed to be that athletes who were hungry showed improved performance.
It made no sense. Could the body somehow have metabolized the carbohydrates in the drinks and put them to use in such a short time? Did the muscles even need carbohydrates in such short bouts of exercise?
Asker Jeukendrup, an exercise physiologist at the University of Birmingham, and his colleagues put that idea to the test. They were among the first researchers to discover a carbohydrate effect in cyclists riding hard for an hour, and they had been puzzling over what could account for it.
So they gave trained cyclists intravenous infusions of glucose or, as a control, intravenous salt water, before asking them to ride as fast as they could for about 24 miles, about an hour. The intravenous glucose meant the athletes had large amounts of sugar available right away — no digestion required. But it had no effect on their performance.
Next they tried what seemed like a crazy idea. They asked the cyclists to do the same ride, but first to rinse their mouths with the maltodextrin solution (or, as a control, with water).
“The results were remarkable,” the researchers wrote. Just rinsing with a carbohydrate had the same effect as drinking it.
Other scientists repeated the experiment. One group used runners, asking them to run for 30 minutes or, in another study, 60 minutes. Rinsing the mouth with carbohydrates consistently led them to run farther, as compared with rinsing with placebos.
Dr. Jeukendrup and his colleagues continued to tweak the study conditions. What happened, they asked, if athletes ate breakfast before rinsing with carbohydrates, or drinking a carbohydrate solution? Then, they found, carbohydrates had no effect.
Meanwhile, neuroscientists found that rodent brains, at least, responded to carbohydrates in the mouth independently of their response to sweetness. It is carbohydrates that matter, and so artificial sweeteners do not stimulate these pathways that go from the mouth to the brain.
Then Dr. Bridge and his colleagues in Birmingham used functional magnetic-resonance imaging to determine whether glucose, which tastes sweet, has the same effect on the brain as the tasteless carbohydrate maltodextrin. They also tested artificial sweeteners for comparison. The brain scan results confirmed the exercise study results: Carbohydrates activated brain areas involved with rewards and muscle activity. Artificial sweeteners did not.
Is rinsing worthwhile for most athletes? Scott J. Montain, an exercise researcher at the United States Army Research Institute of Environmental Medicine, thinks not. The effect is real, he said, but added, “Endurance competitors are better off just consuming the calories.” That way they get real fuel, instead of “sipping and then spitting out expensive, sticky spit.”
Dr. Jeukendrup and Dr. Bridge, though, say they use the mouth-rinsing trick themselves.
“You do notice a benefit,” Dr. Bridge said. But he noted that in a study, the athletes don’t know if they are getting carbohydrates or not. “If you know you are doing it,” he said, “then there’s a chance it’s a placebo effect.”