A recent study has examined the association between physical exercise and mental health in 1.2 million adults in the USA . The key finding of the study was that people who exercised had approximately 40% fewer days of poor mental health than those who did not exercise, even after controlling for a range of physical and sociodemographic factors.
Exercise is known to have many benefits for physical health – reducing the risk of cardiovascular disease, stroke and diabetes – but the association with mental health is not fully understood. Chekroud and colleagues aimed to explore the association between physical exercise and mental health in a large, representative sample. They analysed data from 1,237,194 American adults using the 2011, 2013 and 2015 Centres for Disease Control and Prevention Behavioural Risk Factors Surveillance System survey, to compare the number of days of self-reported poor mental health between those who exercised and those who did not. The two groups were matched on age, race, gender, marital status, income, education, BMI, self-reported physical health, and previous diagnosis of depression.
They found that those who exercised had an average of 1.49 (43.2%) fewer days of poor mental health in the past month than those who did not exercise but were matched for several physical and sociodemographic characteristics. Although all exercise types were associated with fewer days of poor mental health (with a minimum reduction of 11.8% and a maximum reduction of 22.3%), the largest reductions were seen for team sports (22.3%), cycling (21.6%), and aerobic and gym activities (20.1%). Nevertheless, even low-impact exercises such as walking and doing household chores were associated with better mental health when compared to not exercising at all. The authors also found that 45 minutes of exercise appeared to be the optimum amount of time to exercise, with those exercising for 45 minutes having better mental health than those who exercised for less than 30 minutes or more than 60 minutes. Similar findings were observed for the number of times people exercised – people who exercised 3 to 5 times per week reported fewer days of poor mental health than those who exercised outside of this.
This study has received a lot of attention in the media, with many news outlets using headlines such as “Exercising too much could actually be damaging your mental health”  or “Exercising for 90 minutes or more could make mental health worse, study suggests” . It seems that the associative nature of this cross-sectional study has been lost in translation in the media. The authors clearly state that their study does not show that exercise improves your mental health or that exercising too much worsens your mental health, but simply shows that people who exercised regularly had better mental health than those who did not.
Though this study does not show a causal link between physical exercise and mental health, several randomised control trials have identified a causal relationship, showing positive effects of exercise on mental health outcomes such as depression [3, 4], anxiety  and post-traumatic stress disorder . However, it is also plausible that this association is bi-directional, with a longitudinal observational study suggesting that not exercising may be both a symptom of and contributor to poor mental health, whereas exercising may indicate and contribute to resilience against poor mental health .
Despite this study not identifying the causal relationship between exercise and mental health, the findings are still extremely interesting. The study itself is highly creditable as it had a large sample and controlled for a range of physical and sociodemographic confounding variables, with the differences in mental health existing regardless of age, race, gender, income, etc. Finally, the impact of regular exercise on mental health was larger than the effects of wealth and obesity.
The findings of this study could have implications for designing future clinical trials, by identifying a non-linear U-shaped relationship between exercise and mental health, which may be indicative of an optimum amount of exercise for improving mental health. Further, they showed that the benefits of exercising on mental health could be easily accessible, as even 30 minutes of walking per day was associated with better mental health.
References Chekroud, S. R., Gueorguieva, R., Zheutlin, A. B., Paulus, M., Krumholz, H. M., Krystal, J. H., & Chekroud, A. M. (2018). Association between physical exercise and mental health in 1· 2 million individuals in the USA between 2011 and 2015: a cross-sectional study. The Lancet Psychiatry.  The Independent (2018, August 9). Exercising too much could actually be damaging your mental health. Retrieved from https://www.independent.co.uk/life-style/health-and-families/exercising-too-much-worsen-mental-health-study-a8484126.html  The Telegraph (2018, August 8). Exercising for 90 minutes or more could make mental health worse, study suggests. Retrieved from https://www.telegraph.co.uk/science/2018/08/08/exercising-90-minutes-could-make-mental-health-worse-study-suggests/  Harvey, S. B., Øverland, S., Hatch, S. L., Wessely, S., Mykletun, A., & Hotopf, M. (2017). Exercise and the prevention of depression: results of the HUNT Cohort Study. American Journal of Psychiatry, 175(1), 28-36.  Schuch, F. B., Vancampfort, D., Firth, J., Rosenbaum, S., Ward, P. B., Silva, E. S., … & Fleck, M. P. (2018). Physical activity and incident depression: a meta-analysis of prospective cohort studies. American Journal of Psychiatry, appi-ajp.  Carek, P. J., Laibstain, S. E., & Carek, S. M. (2011). Exercise for the treatment of depression and anxiety. The International Journal of Psychiatry in Medicine, 41(1), 15-28.  Rosenbaum, S., Sherrington, C., & Tiedemann, A. (2015). Exercise augmentation compared with usual care for post‐traumatic stress disorder: a randomized controlled trial. Acta Psychiatrica Scandinavica, 131(5), 350-359.  Pereira, S. M. P., Geoffroy, M. C., & Power, C. (2014). Depressive symptoms and physical activity during 3 decades in adult life: bidirectional associations in a prospective cohort study. JAMA psychiatry, 71(12), 1373-1380.