The description of it as well as one of the theories on the cause make me think of burnout:
> Kreher and others believe that the mind plays an equally important role. Because your central nervous system influences your brain as well as your physiology, he theorizes that true overtraining syndrome is triggered by a combination of psychological and physical stress.
> “That’s the multi-system nature of the condition,” Kreher says. “The interaction of the mind and body is incredibly influential.” To finish a 100-mile race requires a psychological thrashing, and some runners have described a kind of post-race depression. As they try to rally to train for the next goal, perhaps beating themselves up for their prior performance, that mental stress, in addition to the physical depletion, could bring on OTS.
I wonder if gut changes could arise as a result of endurance training and then that affects the mind? This seems to be an understudied area. They are just apparently looking into gut changes with regards to endurance training:
https://pmc.ncbi.nlm.nih.gov/articles/PMC12116556/