Something that differs between fat and carbohydrate metabolism is the rate at which energy can be supplied. Energy is supplied at a very slow rate while carbohydrate at a faster rate. When you have the bonk you slow down to the speed at which energy can be supplied by fat.Originally posted by Look381i
--This seems to be true only for those who are carb and sugar reliant. I have discovered that once my body adjusted to lipolysis or ketosis (fat-burning) as opposed to sugar or carb burning, I don't have to worry about the bonk, so long as I have either eaten sufficient fat and protein or have extra body fat stores to draw on. I eat virtually no carbs at this point and lack of endurance is not an issue. If I lack anything, and I am not sure that I do, it might be my overdrive gear for exceptionally long maximum efforts (at or near VO2max).
Obviously, if you ride slowly enough you could go on forever without eating carbohydrate as the rate of carbohydrate use would be quite slow and so can have endurance without being glycogen loaded. On the other hand, higher intensities require energy from carbohydrates and fat to fuel the muscular contractions so quickly lead to glycogen depletion and the knock.
Without carbohydrate in the diet the body simply can't perform at these higher intenisties for very long or on repeated days. The need for carbohydrate in exercise performance has never been questioned; unlike the questions about the role of CHO during weight loss and health (i.e. the basis of the Atkins diet).
Sorry for the simple reply (I have just come in from training) I can expand if you wish.