- There’s maybe an argument to be made for ultra endurance performance where you have a limited amount of stored glycogen, but virtually unlimited amount of stored fat you can use for fuel. And you’re not running close to lactate threshold or VO2max, nowhere close to VO2max, when you’re doing an ultra endurance race. So yeah, maybe there’s something there.
- But for anything remotely shorter than that you’re largely limited by oxygen utilization. And you get more ATP per molecule of oxygen used when you’re oxidizing carbohydrate versus fat. So if you’re super reliant on fat and you don’t have much carbohydrate in your diet, you’re not fueling with carbohydrate during exercise, you don’t have as much stored glycogen - that’s going to limit performance. Because ultimately it’s just a matter of how much energy can you produce per unit of oxygen your body is utilizing. And it produces more ATP when oxidizing carbohydrate rather than oxidizing fat. And that’s very very basic physiology. That’s something you would learn in exercise physiology 101 class. Day one stuff.