A comparison between high carb (energy resources stocked in the liver and muscles, as glycogen) versus high fat (energy resources stocked as triglycerides, in muscles and tissues) in endurance exercise performance: High fat diets and endurance exercise performance. (I'm talking about essential fatty acids.)
In the end carbohydrates can supply more energy per time unit. It can supply energy even at high intensities, but the depletion of glycogen is when exhaustion sets. Fat on the other hand is burned slowly. It cannot supply energy during high intensities, and eating a high fat diet may lead to smaller glycogen stores. However, the amount of fat stored is never a limiting factor for performance.
When we start eating high fat we gradually get better and better at burning fat. If we are well trained, we also burn more fat at higher intensities. Fat is released faster and in greater amounts from the fat tissue, it is also transported faster into the muscles and the mitochondria. The muscles also store more energy as fat and get better at using this fuel as well. Full adaptation takes time tough, and it is likely that optimal adaptation need >4 weeks. High fat diets cause a shift in the expression of our genes into coding for several of proteins involved in fat metabolism.I don't know if these tests are reliable. Why do researchers prefers a 4 weeks, or 1 week trial ? Why not more ? Of course, it's costly, and ethically dubious. But I'm not convince a complete metabolic change can happen in a week.
After adapted to high fat diets we can oxidize more fat during exercise and we get a higher maximal fat oxidation. Endurance trained athletes are particularly adept at burning fat. An endurance athlete adapted to a high fat diet is a fat burning machine. Still, endurance trained athletes are not a homogenous group and may differ greatly in their fat burning capacity.
Achten and Jeukendrup tested 55 endurance trained athletes to find maximal fat oxidation and at what level of intensity it occurred. Maximal fat oxidation averaged 0,52g/min at an intensity level of 63% VO2max. But there were large variations in both maximal fat oxidation and the rate of fat oxidation. Although the average intensity for maximal fat oxidation was at 63% VO2max some athletes cycled at more than 85% VO2max before fat oxidation declined, while in others it went downhill when reaching 50% VO2max. This study used a high carb diet and the exercise was performed after an overnight fast.