I know that it is normal to experience some level of muscle cramping and or fatigue when transitioning to a low carb eating plan. I went through this, and it was relatively short term, a minor annoyance. The cramps disappeared, and I am able to undertake rather intense endurance exercise (biking, running) with no cramps or fatigue/ I am now experiencing something like the opposite. If I 'fall off the wagon' and start eating significant amounts of sugar and carbs for several days (say, around the holidays), I start to cramp very badly, especially in the legs, even under very light exertion (a trip *down* a set of stairs can trigger it). Endurance exercise is damned near impossible. I can't walk a mile without extreme pain. Does anyone else experience this? The simple solution is to stop eating the crap. And when I do, the cramps disappear very quickly. But my question is, what the hell is causing this? It seems something about low-carbing has changed the way my body works, because I never had this problem with sugar and carbs previous to adopting a low carb way of life. I have one theory that perhaps because I do endurance exercise while on low carb, my muscles have 'remodeled' to now be almost entirely slow-twitch (slow twitch fibers don't need carbs, fast twitch do). The proportion of slow-twitch has increased, replacing fast-twitch fibers. When on low carb, the now minority fast-twitchers lay dormant, and don't do much of anything, because they have no glycogen, their fuel of choice. When I give them enough fuel, they spring back to life. But now, there just aren't enough of them, and they have become inefficient through disuse, thus they fatigue very quickly, and send out pain signals telling my brain to stop whatever it is I am doing. So basically my theory is that by training while on low carb, my muscles have become so efficient at burning fat and ketones, they've lost the ability to effectively utilize glycogen.