There is some semantic confusion, here. The body is always producing some ketones and some glucose, regardless of the metabolic milieu. Glucose metabolism is evolutionarily extremely ancient, and all cells can metabolize glucose when necessary.
The ability to metabolise fatty acids goes back only to the first eukaryotes, who formed a symbiotic relationship with certain other bacteria. These symbionts are now known as the mitochondria. They were originally considered to be just another organelle in cells, but the fact that they contain their own DNA, completely distinct from the DNA in the cell nucleus, together with certain fossil evidence, shows that they were once separate organisms.
(Another proof of their external origin is the fact that when mitochondria of different DNA are introduced into the same cell, they will fight one another to the death, possibly killing the cell in the process. This is why sperm cells carry no mitochondria and we thus inherit only our mother’s mitochondrial DNA.)
The mitochondria can handle a certain amount of glucose, but too much for too long (such as when we eat the standard Western diet) damages them. Fructose in quantity also damages them, as do an excess of alcohol (ethanol) and/or branched-chain amino acids.
“Fat-adapted,” or as Dr. Phinney and Prof. Volek prefer, “keto-adapted,” refers to a state in which the skeletal muscles are at the point where they are capable of routinely metabolising fatty acids again. This involve healing the mitochondria and fostering the growth of new mitochondria (mitogenesis) and the reactivation of certain cellular pathways involved in metabolising fatty acids. In this state, the skeletal muscles prefer to metabolise fatty acids over glucose and even ketones for their main needs. This means that “keto-adapted” athletes are using mostly fatty acids to fuel their endurance performance, reserving glucose to fuel explosive power.
None of this says, however, that carb-fueled athletes can’t use some amount of ketones, nor does it say that keto-adapted/fat-adapted athletes don’t use glucose. It should be viewed more as a continuum of shifting fuel sources to power the body’s current activity.
I don’t believe, however, that it has actually been shown that consuming carbohydrate after exercise actually builds muscle. It seems more likely, to me, at least, that protein and fat will do a better job of that, especially if the protein contains some BCAA’s (they won’t cause metabolic damage, if they are needed for building tissue). Carbohydrate may very well help to fuel performance if consumed before exercise, but that’s different.
