[vanderZwan]

We only recently have figured out how exercise protects the brain against stress-induced depression[0]. In short: red muscle (the endurance kind) produces an enzyme that breaks down kynurenine, one of the stress hormones that is associated with triggering all kinds of mental illnesses when it stays at chronically elevated levels.

But there are many more possible causes of depression than stress - one could compare it to a fever: a high fever kills, but there are many possible illnesses leading to a fever. Expecting that exercise helps with depression every time is like assuming every fever is caused by the same illness. Not to mention the fact that while having more red muscle may help with coping with chronic stress, it does not actually fix the cause of chronic stress in and of itself (although it can of course help with providing someone with the mental fortitude needed to take on that particular problem).

There are all kinds of reasons to do more exercise, but touting them as a catch-all fix for depression is dangerous and ignorant.

[0] https://ki.se/en/news/how-physical-exercise-protects-the-bra...