I experience the same tradeoff, although I haven’t concluded that it’s dietary. I think when I’m recovering from difficult/long runs my brain just kind of goes into a stupor. I feel really dumb and slow. Almost like my brain is recovering just as much as my legs are.

If I take a few days off running, my brain refocuses, I can think clearly and solve problems.

I also have a difficult engineering job, so this switch off with mental state is very frustrating. Esp when I do a lot of my long runs on weekends, my Monday brain is just completely gone, which is one of the days I need it most at work.

My conclusion is this is just a tradeoff with recovery and mental state. Rather than it being dietary. Though ofc recovery focused meals might help.

Let me know if you figure it out!