Exercise improves mental health? What you need to know

Last week, HealthDay News (and many other news sites) covered a study from The Lancet Psychiatry looking at the association between exercise and depression. People who said they exercised tended to report fewer days of poor mental health compared to those who said they didn’t exercise, according to the study.

That finding was unfortunately misreported as showing that “exercise really can chase away the blues, to a point.” Why was this misleading? The observational study wasn’t capable of showing cause and effect (or, as statisticians might say, that it wasn’t a “causal” relationship).

It’s the classic chicken-or-egg dilemma. We don’t know, for example, if exercise led to better mental health or better mental health led people to exercise. It’s feasible that both are true, too.

Even though the data set had 1.2 million people, observational findings like these–that can only detect patterns–are reversed all the time when a true experimental, randomized controlled trial (RCT) is conducted. Just last week we wrote about this happening when savvy researchers used both an observational study and an RCT to study workplace wellness programs. While observational data showed the workplace wellness programs got employees to exercise more, the RCT found that wasn’t the case.

That’s not to say there’s isn’t some RCT evidence indicating positive effects of exercise on mental health. There is. However, that research isn’t definitive and generally doesn’t speak to the claims generated by this study, which purports to show specific durations and types of exercise that have the greatest beneficial effects. Such claims simply aren’t supported by the evidence on offer.

In a must-read Twitter thread, PhD student Anne Scheel did an excellent job of making this point and picking apart the multiple problems with this Lancet study and the ensuing news coverage:

So there's a new N = 1.2 MILLION study finding a correlation between physical exercise and mental health (h/t @peder_isager).
Everybody freaking out because…. it's causal, right? Surely it's causal.
And "causal" = exercise –> mental health.
Thread +https://t.co/AIF2H1bcgE

— Anne Scheel (@annemscheel) August 13, 2018

Deep into her thread, she revealed one telling detail many journalists didn’t: Adam Chekroud, the lead study author on the Lancet paper, has a significant conflict of interest. He’s the founder of the startup Spring Health, which Chekroud said via a tweet thread is “using these findings to develop personalized exercise plans.” So, in other words, he stands to gain financially by selling plans that are underpinned by these results.

Spring Health, as Scheel points out, retweeted news headlines that overstate the study’s findings. The Lancet also retweeted some misleading news stories.

This wasn’t at all surprising to Darren L Dahly, PhD, an epidemiologist at University College Cork, Ireland.

“And there it is. There’s the bullshit everyone. Noise mine a giant observational dataset. Act like you understand causes, while hand-waving about associations. Then move right onto selling personalized exercise and wellness plans,” he tweeted.