The process of ketosis is one of the physiological effects of fasting in which the brain (and some other bodily processes) uses ketones produced from fatty tissues as a fuel instead of the usual glucose. This is called “muscle sparing”.
When glucose isn’t readily available via the diet (in the form of carbohydrates) and the glycogen stores in the liver become depleted, the body could break down muscle to get it. But ketosis is an adaptation that will spare muscle during times of shortage by instead breaking down fat stores and manufacturing ketones for brain fuel. It is said this state is attained at approximately 48 hours of a water fast for women and closer to 72 hours for men.
http://www.allaboutfasting.com/effects-of-fasting-ketosis.html
“Ketosis doesn’t IMPROVE endurance performance”
“Ketosis is physiologically limiting”
“Ketosis is very disruptive to training”
“Ketosis is a Competitive Disadvantage”