I have done exactly this, and it's been fine. I regularly ride centuries around the Bay Area on a sugar free diet. Your body definitely needs time to adapt to

I have done exactly this, and it's been fine. I regularly ride centuries around the Bay Area on a sugar free diet. Your body definitely needs time to adapt to a fat based diet, but it can satisfy all your energy needs. I've also done centuries with sugary 'energy' foods. I frequently found myself having to pull over or dangerously balance in order to keep my blood sugar high by dosing myself every 45 minutes. Your stomach churns, your teeth feel awful, being constantly in service to your legs. Burning the fat is a much more comfortable proposition.