The thing is, since a few we actually know now how exercise helps: it builds resilience against depression[0][1]. The press release from the Karolinska Instituted website[2] explains it fairly well, but in short: when you're stressed, you produce kynurenine. How kynurenine works precisely is unknown, but what is clear is that having chronically elevated levels of it can induce all kinds of mental illnesses. Skeletal muscle creates kynurenine aminotransferase (KAT), an enzyme which breaks it down.

The subtlety here is that this building resilience is not the same as fixing the causes of depression[3].

(I'm a bit surprised this research isn't mentioned in these papers)

http://science.sciencemag.org/content/357/6349/eaaf9794

[0] Skeletal Muscle PGC-1α1 Modulates Kynurenine Metabolism and Mediates Resilience to Stress-Induced Depression, https://www.cell.com/cell/abstract/S0092-8674(14)01049-6?cod...

[1] Kynurenines: Tryptophan’s metabolites in exercise, inflammation, and mental health, http://science.sciencemag.org/content/357/6349/eaaf9794

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

[3] http://pbfcomics.com/comics/treatment/

That theory would seem to contradict the study I cited, no?

Both papers you cite are by the same group, and I don't think the results represent a consensus of the experts.

Not necessarily: in another study it was shown that kynurenine levels went back to original levels in patients who only exercised[0].

> Conclusions: Physical exercise positively affects mood and cardiovascular fitness, but does not lead to long-lasting changes in plasma levels of kynurenine and KYNA in patients affected by mild-to-moderate depression.

So that would suggest that the body just reaches a new equilibrium. Again, the point is that resilience does not fix causes - it could be that this is similar to trying to fix traffic jams with more roads; the only real fix is to get rid of the cars as much as you can (after which having an extra lane can make traffic go smoother).

> Both papers you cite are by the same group, and I don't think the results represent a consensus of the experts.

IIRC, that kynurenine does something is apparently well-established, and not just by this group. Also, that first paper has quite a few citations, you can check for follow-up research on Google Scholar[1]

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5384718/

[1] https://scholar.google.de/scholar?hl=en&as_sdt=0%2C5&q=Skele...

> So that would suggest that the body just reaches a new equilibrium.

This is another degree of freedom in the theory, right?

I mean, this could all be true for all I know, but as an outsider I think this falls into the category of stuff I can't judge, and (like a thousand other theories investigated by experts and published in impressive places like Science) hasn't yet risen to the level of consensus.

That and I wonder if there is a spiraling sort of effect where if you have mental health issues you might feel tired as it is, so you don't exercise.... but I'm not sure that means that the exercise is the issue... just the result.

Let alone populations like the working poor who have little time in their lives for things like exercise, but growing mental illness issues.

The working poor certainly don't have a lot of time... that's not a myth.

Pretending it must be like some other issue is not helpful, just as you describe the other issue.

> Mood-changing exercise can take as little as 3 minutes. If I do 2 minutes of jumping jacks and 30 pushups in the morning, my day is dramatically different.

You know this is what homeopaths claim, right?

They're not magically wrong about everything, if they were then that'd be as useful as being right.

They have a section discussing why this paper might seem to contradict other studies showing a causal link:

> This outcome seems at odds with findings from randomized controlled trials in clinical and population samples indicating that regular exercise can relieve symptoms in subclinical individuals and in patients diagnosed as having an anxiety or depressive disorder. To understand the different outcomes of both types of study, it is crucial to make a distinction between the effects of prescribed and externally monitored exercise in selected subgroups and the effects of voluntary leisure-time exercise at the population level. Only voluntary leisure-time exercise is influenced by genetic factors, whereas the other type of exercise is environment driven. The absence of causal effects of voluntary exercise on symptoms of anxiety and depression does not imply that manipulation of exercise cannot be used to change such symptoms. It means that a population association, cross-sectional or longitudinal, cannot be used to justify exercise as a treatment without an actual randomized controlled trial. The possible difference in the antidepressant effects of prescribed vs voluntary exercise is consistent with findings from a recent study suggesting that the therapeutic effects of exercise are nonspecific to exercise. The antidepressant effects of exercise may only occur if the exercise is monitored and part of a therapeutic program.

So it sounds like among twins, if one twin naturally exercises more by their own volition, that twin doesn't in general show less depression/anxiety than the other twin, which is evidence against exercise causally reducing depression/anxiety. However it sounds like it may still be that prescribed exercise as an intervention, beyond what an individual would naturally choose to do, and maybe only in conjunction with therapy, could still benefit an individual (and perhaps then only individuals with clinical symptoms).

This study also does help by showing how the associative inverse relationship between exercise and depression in the general population seems to be from genetics influencing exercise, and not the other way around, and the important of a randomized control trial on exercise as an intervention.

Few academics stake their careers on papers saying that X does fuck all (unless everyone already things X is amazingly effective)

Cochrane Collaborations.

https://www.cochrane.org/CD004366/DEPRESSN_exercise-for-depr...

> Exercise is moderately more effective than no therapy for reducing symptoms of depression.

[...]

> The reviewers also note that when only high-quality studies were included, the difference between exercise and no therapy is less conclusive.

Thanks for providing this study.

This reminds me of the Iron Law of Evaluation:

https://www.gwern.net/docs/sociology/1987-rossi

The Iron Law refers social program interventions but I think it is applicable to a lot of health interventions too.

https://www.gwern.net/docs/sociology/1987-rossi