For endurance athletes the keto diet sounds ideal. By keeping carbohydrate consumption low, fat becomes the primary fuel for muscles during exercise. Increased fat burning translates into increased lean body mass. Like many propositions, it sounds too good to be true and it is.
 
Although your muscles primarily use carbohydrate and fat for fuel they are not necessarily interchangeable. Whether your muscles use fat or carbohydrate (glycogen) for fuel depends on both the duration and intensity of your exercise. At low intensity exercise your muscles can efficiently use a preponderance of fat as fuel however as exercise intensity increases glycogen becomes the mandatory fuel. And if you don't supplement carbohydrate during exercise you will quickly deplete your muscle's limited supply of glycogen. When muscles fatigue, endurance performance plummets.

There is also another excellent reason to consume carbohydrate during endurance exercise. Low carbohydrate availability has a detrimental effect of protein balance. Studies have shown that when carbohydrate is limited there is an increase in muscle protein breakdown. High carbohydrate consumption during exercise decreases protein breakdown and maintains net protein balance whereas low carbohydrate decreases protein synthesis and reduces net protein balance. Greater muscle protein loss translates into greater post-exercise muscle damage, soreness and a tougher recovery.