Some players may want to avoid feeling bloated and are simply refreshing their mouths in the heat. But others appear more deliberate and purposeful. Players are notoriously cloistered during the World Cup and are especially loath to speak about their fitness secrets, so the contents of their bottles are not known. But they may be employing a technique called "carb rinsing" or "mouth washing", some exercise and nutrition scientists say. For more than a decade, research in endurance sports such as cycling and running has shown that athletes can gain a performance boost during intense bouts of exercise by rinsing their mouths with a carbohydrate solution, then spitting it out without swallowing. Essentially, scientists say, receptors in the mouth send signals to the pleasure and reward centres of the brain, suggesting that there is more energy on the way, so the muscles can push a little harder and there is no reason to feel so fatigued. Loading Replay Replay video Play video Play video "You're sort of tricking the brain a little bit; that's what we think the mechanism is," said Asker Jeukendrup, an exercise physiologist and sports nutritionist who, with colleagues at the University of Birmingham in England, detected in 2004 that carb rinsing made cyclists about a minute faster in 40-kilometre cycling time trials.

England's national team declined to discuss its nutritional tactics at the World Cup. But a person familiar with the team's regimen acknowledged that carb rinsing was something the squad "has been seen to do in the past" and was considered a "fairly standard practice". A fitness official in the English Premier League said that carb rinsing was used to boost energy, to avoid a feeling of heaviness in the stomach and to try to prevent cramps. Scientists also say that the technique can be beneficial in avoiding stomach distress that can occur from swallowing carbohydrate fluids. The research indicates that carb rinsing can enhance performance when fluids are swished around the mouth for five to 10 seconds, the longer the better as more oral receptors come into contact with carbohydrates. "I wouldn't say it's widespread yet" in the World Cup, though players seem to be carb rinsing more often than in the past, said Jeukendrup, who is now a professor of exercise metabolism at Loughborough University in England. "I hope it's all deliberate," he said. "It's good to see science making its way into real sport."

Of course, carb rinsing alone cannot sustain players for an indefinite period. Carbohydrate fuel must also be ingested as the body's muscles become depleted of glycogen, a stored form of glucose used to provide energy during exercise. Otherwise, players will eventually run out of gas. Most research indicates that carb rinsing is optimal for intense exercise lasting between 30 minutes and an hour. Given that a soccer match lasts at least 90 minutes and can stretch to two hours in the knockout rounds of the World Cup, the technique might be "the wrong thing to do", said Lindsay Bottoms, an exercise physiologist who is the lead researcher for sport, health and science at the University of Hertfordshire in England. She has found that mouth washing can improve the lunging accuracy of fencers. And one of her colleagues working with England's World Cup squad said the team's nutritionist had informed players of the potential benefits of carb rinsing in soccer. But, Bottoms added: "They need to ingest it rather than spit it out because of the duration of football. Ideally, I would say rinse it in your mouth and actually swallow it [to get a dual benefit]." Research into the potential benefits of carbohydrate ingestion for soccer performance is in its relative infancy. There are few chances to drink fluids beyond pregame and half-time. No one has figured out yet the optimal amount of carbs that must be swallowed to gain a boost. Some studies suggest it is more fluid than is usually ingested during a match.

Benefits to running endurance are fairly clear in the research, Jeukendrup said. But what is less certain, he has written, is whether drinking carbohydrate solutions improves soccer-specific skills such as dribbling, passing, shooting and heading, either by postponing fatigue or by affecting processes in the brain. There are studies that show positive effects on dribbling, passing and timing, and others that show no effects. This is not surprising, Jeukendrup said. Measuring soccer skills is extremely complicated and subject to many variables. With carb rinsing, many tantalising questions remain unanswered, such as whether it affects cognitive function, decision-making, reaction time. Does mouth washing allow a player to get to the ball a step quicker as fatigue sets in during the final 15 minutes of a match, providing an extra split-second to make a decision? Or to take a shot slightly harder and more accurately? Could carb rinsing have partly accounted for England overcoming their traumatic history of failure on penalty kicks in the shootout victory against Colombia last week?

"I would not go that far," Ian Rollo, the principal scientist for the Gatorade Sports Science Institute in Britain, said. "No way do we take credit for any penalty successes at the moment." And yet, carb rinsing will do no harm, Rollo and other scientists say. (The joke is that the only possible risk is getting cavities from sugary drinks.) And there might be some benefit to mouth washing, Rollo said, "So why wouldn't you do it?" As the World Cup nears its end, a carbohydrate rinse, manufactured by a New York company called Unit Nutrition, has become commercially available. Researchers at Michigan State University are measuring brain activity in college-age students to try to determine how long a potential performance boost from mouth washing lasts and whether Unit Nutrition's glucose rinse might improve the focus of athletes and other users. The results are "very preliminary" but suggest the performance boost lasts about 15 minutes. The rinse seems to counteract fatigue and enhances attention to a task, said David Ferguson, an assistant professor of exercise physiology at Michigan State. For players in the World Cup semi-finals, tired after pressure-filled matches played in the heat for nearly a month in Russia after an entire season with their club teams, carb rinsing does not seem likely to "make them run faster or kick the ball harder", Ferguson said.

Instead, he continued, "it's simply going to maximise their focus so that they are not succumbing to fatigue, so they can put themselves in the right position to make the right play." As England's match with Colombia moved towards penalty kicks, it was too late to get any carbohydrates into the blood stream but perhaps not too late for a "brain boost" from mouth washing, said Trent Stellingwerff, the director of performance solutions at the Pacific division of the Canadian Sport Institute, who watched the game and has studied carb rinsing. "You're going to do every trick in the book to try to maximise cognitive focus after two hours of a pretty intense match," he said. "Is there science behind it in a soccer model? Not that I'm aware of yet. Is it going to hurt? Absolutely not. If the athletes believe in it and it's part of their mojo, will that work? You betcha it will." The New York Times