Athletes training for endurance competitions tend to eat a lot, especially carbohydrates, which produce glucose to fuel the muscles. Olympic swimmer Michael Phelps took in 12,000 calories a day during the 2008 Summer Olympics, for example. Regimented nutrition diets are also popular among athletes. The top Mixed Martial Arts fighters employ full-time nutritionists who prepare each meal for them.

But fasting?

More bodybuilders, professional cyclists and other athletes are turning up the