Similar to how our muscles adapt at the cellular level and how they utilize oxygen, our muscles also adapt to how it uses its fuel sources. Particularly carbohydrate and fat. The more efficient our muscles utilize its fuel sources the more resistant our muscles are to fatigue. As carbohydrate and fat are the two primary fuel sources for a working muscle.
Our bodies store glycogen in our muscle and liver and we use this glycogen to fuel our training sessions. As we rest and provide our bodies with sufficient carbohydrate we will store this glycogen in our muscles. It has been shown that trained muscle stores significantly more glycogen than untrained muscles. This allows us as athletes to tolerate increased training as we have a greater amount of glycogen stored that we can then utilize for energy. However, we can only store this glycogen if we make a conservative effort to. Therefore, we can not as athletes simply eat according to hunger, but we must incorporate nutrient timing to make sure we are restocking our body with the fuels it needs at the right times. Chronic fatigue and an overall feeling of tired is not simply a result of increased training, as much as it is a result of an imbalance between glycogen use and carbohydrate intake. It is important that we ingest carbohydrate both during training sessions and at the right times after training sessions to prevent an imbalance. Especially as we are training on back to back days and even more importantly for those of us who train twice a day.
Trained muscles have also been shown to increase the ability to oxidize free fatty acids. This allows us to burn fat more efficiently, and decrease the demands on our supply of muscle glycogen. Many at this point will start saying to themselves "I knew I should start trying a low carbohydrate, high fat diet to increase my performance." However, this is easier said than actually done by the body. You see, simply eating fat does not allow us to burn fat. This is because as we eat fat we tend to elevate our blood's triglyceride levels. This then has to be broken down to free fatty acids in order for our body to utilize it as fuel. For the most part we have been unsuccessful in dietary attempts to elevate free fatty acids.
It is important to be open to new ideas and modern practices and see what advantages and/or disadvantages they might provide. Everybody is individualized and what works for some, might not work for others. In my mind it is clear that by knowing how our muscle's adapt to utilizing it's fuel sources we should still be fueling our training sessions as well as recovering by ingesting carbohydrate as our primary fuel source.
Wilmore, Jack H. (1994). Physiology of Sport and Exercise. Illinois: Human Kinetics.
Burke, Louise M. (2015). Re-Examining High Fat Diets for Sports Performance: Did We Call the "Nail in the Coffin" Too Soon?. Sports Med, 45(Suppl 1): 33-49.