> A lack of physical exercise is going to put all cells, including brain cells, in a catabolic state the same as muscle atrophy will waste away muscles due to lack of physical activity.
Why is this? Using your brain cells won't lead to muscle atrophy.
From research we know physical activity is healthy for the brain, but there's no reason this should be the case from first principles. You can make the same argument in the other direction.
"The brain and body have existed in state with limited energy. So to maintain homeostasis when the muscles are used it puts the brain into a catabolic state to feed the new stronger muscles."
And if catabolism between muscles and the brain is linked then surely steroids will make people every smart by switching the entire body into anabolism from catabolism.
> Cells are cells, they are where energy is made, they are binary only existing in anabolic or catabolic states and never simultaneously existing in both states
This is 100% not true. All cells are simultaneously building stuff and tearing stuff down. Some hormones like insulin say "build more stuff, tear less down" and others like cortisol say "tear stuff down and stop building".
But the body can simultaneously build some stuff and tear other stuff down. Consider the transitions from different types of muscle fiber. From beefy body builder muscles to lean marathon running muscles. That will be tearing down a lot of proteins but building up a lot of mitochondria.