Actually this is a still unploughed furrow and one that needs a lot more attention. The reason for my enigmatic post is that there is a common belief fostered by energy food makers that a rider starts empty and like an old steam train needs to fuel up as they go and I don't thing this is true.
There's a few hundred grams of glycogen stored in muscle tissue in an average healthy person. This gets burned up quicker as we exercise harder.
I often do "fasted" commutes. I get on the train for two hours
without eating anything. I almost always do "fasted" working: going two, three maybe even four hours "fasted" before eating again.
Sometimes I do "fasted" walks to the shops - but only if I'm feeling lucky.
Even with a lot of people trying to use evidence and science, there's a lot of nonsense around.
When you are exercising hard, an average adult will burn through those stores calories pretty quickly. I burn ~900 calories per hour cycling, ~1,300 calories running. My performance, and any other human beings* performance, will tail off as the muscles run out of glycogen, and my body starts to find other sources of glycogen. With no glycogen immediately available my muscles start to slow, my performance reduces, and consequently the fitness benefits tail off. Muscular endurance is built by a series of stresses put on and recoveries of the muscles. I can no longer stress them as I am out of glycogen. The exercise benefits are evaporating.
As my body starts to convert fat to glycogen, at a much lower level of efficiency, I start to get a supply of glycogen to my muscles again - instead of shutting down completely, they can keep going at a much reduced stress and effectiveness.
None of this is affected by how thin or fat I am.
*Recent studies show some evidence that "fat adaptation" over a long period can increase metabolic efficiency of utilising fat as a glycogen source, this means that you can train your body to get more glycogen from a gram of fat, and that you don't need to have the cutover from glucose sources to fat sources untrained people would have. You cannot become metabolically efficient with fat by the occasional "fasted" ride.
There is no evidence that fat adaptation improves performance. There is tons of evidence that taking in carbs while exercising does improve performance.
There are potential benefits from less dependence on carbohydrates for athletes - dental health and diabetes being current concerns. this is not to say that a diet weighted towards fat is without health concerns.
