Originally Posted by MoAlpha

There are very fit people who lack the “metabolic flexibility” to switch efficiently to fat burning during exercise or fasting. Metabolic inflexibility is also associated with obesity, type 2 diabetes and other aspects of metabolic syndrome, so it’s probably not a wonderful thing even in athletes. The good news is that metabolic flexibility is trainable to some degree in everyone, by diet, low-intensity training, fasting, etc. Elite road cyclists are probably the most metabolically flexible individuals in existence and they train specifically for it.