Fitness - Taking a Bite Out of Sports Stress - NYTimes.com Editors' Note Appended AFTER a night that included several beers and not enough sleep, Keith Gillis, a 31-year-old cyclist in Truro, Nova Scotia, set out on a 74-mile road ride with the caveat that he was feeling fatigued. Yet two-thirds of the way through the ride, Mr. Gillis said, he was setting the pace, riding ahead of his cycling partner. "Even though I'd felt tired at the start, I had the energy to lead, and I wasn't out of breath," he said. Rich Cruse/richcruse.com UP TO SPEED Chris McCormack, a triathlete sponsored by Under Armour, wore its mouth guard during the bike portion of a competition. Lars Klove for The New York Times Under Armour's Performance Mouthwear (top) and Makkar Pure Power Mouthguard. To what did he attribute his stamina? A flexible mouth guard by a Canadian company called Makkar that he has been biting on while riding since April. When fellow cyclists ask him why he isn't winded when they are, Mr. Gillis tells them, "because I have my Superman guard in." Mr. Gillis is among a small but growing number of athletes wearing what manufacturers like to call "performance mouthpieces" while cycling, running or weight training. One of the newest tools in a performance-enhancement arsenal, these mouthpieces are light, flexible pieces of molded plastic that fit over the teeth - and are only vaguely reminiscent of that retainer from junior high school or the bulky mouth guards worn by football players. Dentists say these high-end mouth guards can open up the airways, prevent teeth-clenching and align the jaw. Being able to take in more air while exercising has obvious benefits - more oxygen for working muscles - while a relaxed jaw can decrease stress and help an athlete's body function more efficiently. "There is research to support improved breathing mechanics and reduced jaw fatigue," said Fabio Comana, an exercise physiologist with the American Council on Exercise. Depending on how you look at it, there is some truth to the claims. Unlike regular mouth guards, which are available off the shelf and at modest prices, performance mouthpieces cost hundreds or even thousands of dollars and must be custom-fitted by a dentist. Ordinary mouth guards are usually dropped in boiling water and fitted to the wearer's teeth to protect against injury. With performance mouth guards, the idea is to reposition the jaw, anywhere from a few millimeters to, in the case of Mr. Gillis, a quarter of an inch sideways. Two main brands are on the market - Makkar and Under Armour - and each makes the claim that it can increase an athlete's strength, reduce stress and improve overall performance. Professional athletes have taken note: during the World Series this year, television viewers could see Derek Jeter wearing a mouthpiece made by Bite Tech, the company that developed the technology that Under Armour uses. In early November, Jon Gruden of ESPN said on "Monday Night Football" that many of the New Orleans Saints wore Makkar mouth guards. The Makkar Pure Power Mouthguard (or PPM, as the company calls it) was introduced in 2006 and costs from $595 to $2,250, not counting the dentist's fee. Makkar's Web site touts the mouth guard's use in diverse sports, including golf, soccer, swimming and tennis, and includes endorsements by the basketball player Shaquille O'Neal and the football player Terrell Owens. Under Armour's line of Performance Mouthwear was introduced in September with a basic price of $495. Among the professionals who wear them are Adrian Peterson of the Minnesota Vikings and Marian Gaborik of New York Rangers. There is a big difference between the two brands: While the Makkar product must be gripped between the upper and lower teeth, the Under Armour one sits only on the lower teeth. But both are meant to set the jaw at ease. "When you have the jaw in relaxed position, the rest of the body can be more relaxed - it's a domino effect," said Kathrina Agatep, a dentist in San Diego who sells both brands. Repositioning the jaw is not the same as keeping the jaw slack while exercising. "Even if you have your mouth open when you run, that doesn't necessarily mean the joint and rest of your body is in the maximal alignment," Dr. Agatep said. While the products" potential benefits may sound good, it isn't clear how much of an edge they actually confer. A study sponsored by Makkar in 2008 at Rutgers University found that athletes wearing Pure Power Mouthguards could jump higher and perform better at their peak, but it did not find that their endurance was any better. "There wasn't a huge difference," said Shawn Arent, an assistant professor in the department of exercise science at Rutgers who led the study. It's not the greatest thing since sliced bread. It's not magic. But for an elite athlete who has been training for a long time, even a 3, 4 or 5 percent increase in performance is a hard thing to come by. Similar research by Under Armour and Bite Tech with athletes at the Citadel, a military college, showed that using the mouth guards helped improve endurance and air flow. Dena Garner, an assistant professor at the Citadel who has studied athletes using Bite Tech devices since 2005, said she thought some of her original findings were "a fluke." But "every time I've done lactate studies with this mouthpiece, I'm finding there is a difference," she said. This year Captain Garner used an Under Armour mouthpiece while training for a marathon. Previously she "had been happy with running 10-minute miles," she said, but wearing the mouthpiece, she consistently ran a mile in as little as 8 minutes. "It was pretty astounding to me," she said. I didn't feel as tired as when I ran the 10-minute-per-mile pace. Clenching the teeth can lead to the release of the stress hormone cortisol, which, at excessive levels, can impede athletic performance. Having the Under Armour product in place "interrupts that flight-or-fight response," said Bob Molhoek, chief executive of Bite Tech. Editors' Note: December 24, 2009 The Fitness column last Thursday, about mouth guards that supposedly increase athletic performance, included a recommendation of Under Armour's mouthpiece by Chris McCormack, a triathlete, and a picture showing him in competition. Mr. McCormack is sponsored by Under Armour; the article should have either mentioned this or omitted his comments and the picture.