Athletes – elite and weekend warrior alike – are always looking for a competitive edge. Even legal ones.

They'll fiddle with their diets. Take supplements. And take in sports drinks.

Two years ago, the American College of Sports Medicine issued revised guidelines on how much fluid you need to take in while exercising.

The bottom line was: it varies. But it did note that if you're exercising for more than an hour, you need to do more than just replace the fluid you're losing. You also need to replace electrolytes and take in carbohydrates to fuel your muscles. Don't do it and you run the risk of running out of gas or – as marathon runners will say – hitting the wall.

It's big business selling sports drinks. Worldwide sales approached $30 billion in 2007, according to a report from Zenith International, a company that provides research on the beverage industry. That's 1.8 litres of sports drink for every person on the planet.

The market is expected to grow by a third by 2012, taking per capita annual consumption to 2.3 litres.

There have been concerns that the stuff is contributing a growing obesity epidemic. Sports drinks can pack a lot of sugar and a lot of calories — and they can be rough on your teeth.

You see vats of the stuff from the comfort of your couch, while you're watching pro football, hockey and baseball. You'll see thousands of discarded cups of some kind of "ade" whenever a marathon or other road race winds its way through your neighbourhood.

While scientists have known for a long time that these drinks do help athletes, they weren't sure how they worked. A new study suggests the key might be mind games.

The research – published in the April 2009 issue of the Journal of Physiology – found that performance in endurance athletes was boosted not only from sugary drinks but also from drinks that contained a tasteless carbohydrate. And it didn't matter whether athletes swallowed the drink or spat it out.

The researchers prepared drinks that contained either glucose (a sugar), maltodextrin (a tasteless carbohydrate) or neither. They added artificial sweeteners until they all tasted the same. They asked groups of endurance athletes to complete a challenging time-trial, during which they rinsed their mouths with one of the three drinks.

Athletes given the glucose or maltodextrin drinks outperformed those on "disguised" water by up to three per cent. They were able to work harder even though they didn't feel like they were.

The researchers suggest that as-yet unidentified receptors in the mouth send signals to your brain that then tell your body that you can keep working hard.

The researchers monitored the athletes' brain activity and found that both glucose and maltodextrin triggered specific areas of the brain associated with reward or pleasure, while the artificial sweetener did not.

The brain was telling the body "you're not working as hard as you think, so keep going."

The study is the latest in a string that suggest it is not the muscles, heart or lungs that do you in over the long run, but the brain — based on the information it receives from the body.

Kind of like what Yogi Berra once said about baseball, although it's especially relevant to endurance sports: [It's] 90 per cent mental — the other half is physical.