Originally Posted by
FrenchFit
I'm intermittent fasting, no food during the day, month 4, age 60. The glycogen reserves are stunning, I'll often take a energy bar with me on a long ride and bring it back unopened. No bonking, no cramping. However, the very high output, red zone, capacity is simply gone. I'm speculating that carbo loading and fueling is akin to blood doping, there is so much of a glucose/insulin reaction in the blood you are always in an over-reved mode, and you have to keep supplementing it because once it depletes you go to bonktown. Hammer Syndrome would be a good name for it. But I'd agree with the argument: super-human efforts are not going to happen on metered glycogen reserves, the red-line gets pegged much lower.
I don't think you're right about "the glycogen reserves are stunning". I think exactly the opposite is happening. You are riding in a glycogen-depleted state but your threshold for fat-burning has risen, so you are able to fuel your effort direct from fat stores at higher intensities than before. This is why you don't need the energy bar - you are burning very little glycogen - and is also why the top-end has gone: when your muscles do have to resort to glycogen as their primary fuel, it isn't available.
I've tried this myself, going low-carb but continuing to train in an effort to lose weight. It was fine for long, steady rides, I could ride for hours without food. But once I moved onto the more intense stuff, intervals and so on, I simply couldn't hit the numbers, I wasn't able to sustain the very intense efforts.