John, good questions and a good subject. I agree with Michael @amwassil that it deserves its own thread. I’d say go ahead and make a new topic. Replies as of now are few and can be moved or copied and pasted.
Approximately - we have about 100 grams of glycogen in the liver and 400 grams in muscles when the storage is full. Bigger people will have more. Glycogen is a carbohydrate and has ~4 calories per gram, so that’s around 2000 calories. Those with a lot of muscle mass could have a considerably higher figure.
Things get ‘fuzzy’ pretty fast, I think. Even when eating no carbs, the liver makes some glucose, and I’m wondering if some does not get stored as glycogen. And on the “empty tank” part, I think it really takes a while to get there.
We don’t just use glycogen and then completely shut that off and switch over to burning fat. During prolonged exercise, as glycogen stores get depleted, the body starts burning fat, and that gradually increases as glycogen gets less and less. I don’t know if we get to zero glycogen or how close to it, if not.
Even as those energy sources get mixed somewhat toward the end of glycogen consumption, it’s still common to feel a severe energy decline as the body substantially moves into burning fat. This is the “hitting the wall” phenomenon for marathoners at about mile 20 or 30+ kilometers. If one is well fat-adapted, I’d think this would be lessened, but it’s still an individual thing, and for the given individual, long training sessions while burning only fat usually help - the body gets better at burning fat.
I’ve never felt any changes in my brain or head when going from ketosis back to substantial carbs, but I think some people do. For me more just a general sluggishness and “loaded-down” feeling overall, like I’m struggling once again with high insulin.
EDIT:
I see the new thread is already in place.