But dietary fat is not as readily available as carbohydrates, which are stored in muscles in a form known as glycogen. Muscles can take up and burn glycogen without many intermediate metabolic steps. Glycogen provides a fast sugar buzz and that buzz can fuel most exercise just fine.

Fat, on the other hand, must first be broken down into fatty acids and other components before it can be used by the muscles, an intermediate step that makes dietary fat less immediately available and efficient as a fuel, especially during intense exercise.

However, exercise scientists long ago established that endurance training makes athletes better able to use fat as a fuel. And that metabolic adaptation prompted many scientists and coaches in recent years to wonder what would happen if you extended that ability to its farthest extreme and trained an athletes body to rely almost exclusively on fat, by removing almost all carbohydrates from the diet and ramping up grease intake?

"I'm sorry, Dave. I'm afraid I can't do that."