Yeah but what's causing the issue for the crossfitters /keto dieters? There has to be something quantifiable.
And its hard to quantify scientifically. Heart rate variability is the best known and widely available tool to gauge this. It's a way to measure the engagement delay of the CNS on a basic level.
It was explained to me as a "stress bucket" Take all of your known life stress's, add them to all of your exertion stress (exercise). Now you have a level of conscious stress. When you add unconscious stress, the bucket is way over filled. Its hard for me to define unconscious stress easily in this forum, but think of it as unexpressed fear, anger, hate, love...etc.. Basically unexpressed emotions.
It looks like low carbing and high fat burning utilizes the body's stress response system to fuel itself. Cortisol is raised, testosterone goes down..etc. Im not the authority on explaining this, but they reach a point where something changes hormonally. The symptoms are identical to initial crash stages of CFS. Blood sugar is then disregulated, sensitivity to light, digestion slows, speeds..etc, feeling crazy, disconnected..etc.
The common theme is high fat, very low carb while still exerting in a significant way.... IE stress. Someone who overtrains like this is generally disconnected from awareness of stress to the body. The ones that I see that have crashed are usually unaware of unconscious stress levels.
Generally speaking, carbs should not be eliminated when the CNS is over engaged. Having said that, if we are talking about severe anxiety, (and not CFS) a breakfast of high fat and high protein is usually the way to go. Jordan Peterson explains it pretty well that when you suffer from anxiety, eating a bunch of carbs will knock you back into an anxiety response due to a reactive hypoglycemic insulin response that will take until the next sleep period to resolve.