Originally Posted by burritos

This is true if one has relied on eating carbohydrates as a primary macronutrient, which most humans do. But if you don't eat carbs, your body will upregulate liver gluconeogenesis enzymes that converts glycerol(the 3 carbon backbone molecule of triglycerides(fat molecule)) in to glucose. It will also up regulate lactate shuttle enzymes that can convert lactate back into sugar when your lactate production outpaces glucose burn. Glucose is vital for high intensity physical effort. The thing is the body does an amazing job producing glucose in the liver. You don't have to consume it.

But look no further at the amazing physical feats that a cheetah(or any animal in nature for that matter) can do without carb loading. They aren't sluggish.