BRIEF: Why Athletes Hit the Wall

(Inside Science) – You’ve probably seen videos of marathon runners collapsing just short of the finish line. Despite their brains urging them to push on, their bodies simply stop listening. Sometimes, even the best athletes hit the wall.

Scientists have long known that a lack of sugar in the bloodstream is at least partially responsible. While muscles consume primarily sugar and fat, the brain only burns sugar. So when your sugar supply is exhausted, your brain-body connection goes awry. Training builds endurance, however, since your muscles become more adept at burning fat and therefore leave more sugar in the blood for the brain.

But endurance isn’t just about burning fat more efficiently, according to a new study published today in Cell Metabolism . It’s also about suppressing the muscle’s ability to consume sugar at all.

Through experiments with mice, the researchers identified the genetic and molecular processes behind building endurance. The key, they found, is a molecule called PPAR-delta, which is known to be important for controlling how cells burn energy.

Activating the molecule, the researchers show, boosts the genes that allow muscles to burn fat. But at the same time, it also hinders the genes that enable muscles to consume sugar. And these molecular changes have real effects on stamina.

Mice treated with a drug that activates PPAR-delta were able to run about 100 minutes longer than untreated mice. That’s an increase of about 70 percent -- without any training at all.

This hints that researchers might be able to develop a similar human drug that delivers some of the benefits of exercising without having to lift a finger, said Ronald Evans of the Salk Institute for Biological Studies in La Jolla, California. The idea would be to confer the health benefits of exercise to those with limited mobility, such as the elderly or people with diseases like muscular dystrophy.

“We’re not saying that normal, healthy people shouldn’t exercise,” Evans said. “But we are saying that for people who cannot exercise, you still would gain a benefit.”