Exercise has long been cited to help improve our mood and combat depression. It is said so regularly, it has almost become gospel. But how effective really is exercise at preventing depression in everyday people? This blog intends to examine the results of a recent systematic review and meta-analysis looking at whether exercise-based interventions are effective at reducing depressive symptoms in people without clinical depression.
The prevalence of depression is growing worldwide, and growing with it is the burden it places on people’s lives (Liu et al., 2020). By 2030, depression is estimated to be the main cause of disease burden in high-income countries (Chisholm et al., 2003). To put this into perspective, the cost of days lost at work due to depression and anxiety worldwide is $1.15 trillion per year, while this figure is expected to double by 2030 (Chisholm et al., 2016).
The current focus of treatment for people with depression is mostly antidepressants and/or psychotherapies. Antidepressants have been proven to be more effective than placebo, but it’s well established that they are not for everyone and greater treatment choice should be a priority. Psychotherapies are comparatively expensive, time-consuming and offer only a small-to-medium effect on depressive symptoms (Schuch & Stubbs, 2019).
Due to the considerable burden depression has on the individual and society, there has been a push to identify modifiable lifestyle factors that can reduce depressive symptoms. Physical exercise has been suggested to buffer against developing depression by reducing depressive symptoms, improving quality of life and improving physical health outcomes (Schuch & Stubbs, 2019).
Preventative approaches aim to target people with prodromal depressive symptoms before they meet the diagnostic criteria of a depressive disorder; this would indicate prevention rather than exercise-based intervention being used as treatment. The overall aim of these approaches is to reduce the occurrence of new cases of depression. It can be hard to establish whether these preventative approaches have been effective as it would mean trying to predict whether these cases will have developed depressive episodes.
This researchers conducted a thorough and robust review of exercise interventions on depressive symptoms (Bellón et al, 2021). The review used a range of electronic databases and appropriate search criteria. Studies screened as suitable were assessed by pairs of independent reviewers.
The review only included randomised controlled trials (RCTs), considered the gold standard of clinical trials. Trials that identified participants with established depression were excluded as the study was interested in participants with sub-threshold levels of depression.
The review used a broad definition of exercise. They used exercise and physical activity interchangeably and defined it as “bodily movement generated by skeletal muscles that resulted in energy expenditure above resting levels.”
The studies were measured by a reduction in depressive symptoms or a change in incidence in depressive studies.
The review clearly addressed a well thought-through question. Consideration was given to the population being studied to ensure participants had sub-clinical levels of depression. The intervention used a broad definition of exercise consequently; the review had a wide range of exercise interventions.
The initial literature search yielded a total of 7,640 articles. Duplicates were eliminated and a full-text review was conducted on 418 articles. This resulted in 14 different RCTs meeting the inclusion criteria, which included 18 comparisons for the meta-analysis. When the authors completed their risk of bias model, only two RCTs achieved a low risk of bias.
The RCTs were conducted in North America, Europe, Asia and South America. All papers were published between 1999-2019 and had a total of 1,737 participants. The population across the RCTs included adult populations, the elderly, only women and patients with chronic conditions. The interventions included aerobic exercise (not further defined), walking and yoga.
The review concluded that exercise-based interventions had a small but significant effect on reducing depressive symptoms in people without clinical depression. The standard difference in mean scores from pre- to post-intervention was -0.34 (95% CI -0.51 to -0.17; P<0.001).
The subgroup analyses showed that exercise was more effective at reducing depressive symptoms when interventions verification were objective. Exercise interventions were less effective in the elderly, selective prevention, low intensity of exercise and with larger sample size.
The authors concluded:
Exercise-based interventions had a small effect on the reduction of depressive symptoms in participants without clinical depression and this finding was robust in a sensitivity analysis.
Strengths and limitations
The reviewers only selected randomised controlled trials (RCTs) for this review. By using RCTs they are better able to compare the studies. The reviewers can establish the severity of the participants’ depression and can develop a clear understanding of the true effect that exercise-based interventions had on depressive symptoms.
In addition, the studies included in the review covered a large population of varying ages in the adult population. Furthermore, the studies spanned over eight countries and four continents. This increases the generalisability of the findings.
This systematic review was a very well-designed study, applying vigorous guidelines (PRISMA and GRADE). Bias was assessed by using the Cochrane risk-of-bias tool. Bias was further reduced in the review using independent reviewers who selected the papers for inclusion and assessed bias. By using depressive symptoms as an outcome variable, the paper can understand implications for the general public.
Despite the strengths presented above, limitations do apply. Let’s start with the dose-response relationship. Previous literature has suggested exercise has a dose-response relationship with developing depression (Martinsen, 2008). This review is vague when reporting the duration and intensity of the exercise, or how much exercise is needed to achieve that small effect.
Moreover, the definition of exercise-based intervention was broad. Consequently, the range of exercise-based interventions varied. The variation in exercises can lead to differing effects on depressive symptoms. For example, yoga was used in some of the RCTs. Yoga is a low intensity exercise but also encompasses aspects of relaxation, meditation and mindfulness. These factors could have differing effects on depressive symptoms. Furthermore, despite being a broad definition, many types of physical activity were not included. The review does not add to the understanding of how resistance training (weight training) or team sports may also prevent depression.
Lastly, this review did not highlight any in-depth demographic details about the populations being studied. It would have been beneficial to know whether the populations studied previously engaged in exercise or if this was a new intervention to them. Individuals already engaging in exercise may have other personality traits and resources that make them more resilient to developing depression.
Implications for practice
There is potential for exercise to be prescribed for depressive symptoms or for people to be educated to use exercise to prevent depressive symptoms. The cost-effectiveness and desirability of exercise could make it an attractive preventative measure. Whilst the effect found in the review is small, it could be used in conjunction with other preventative measures, such as app-based mindfulness techniques.
Further research could explore the specifics of exercise-based interventions in reducing depressive symptoms. Future research could compare if different models of exercise-based interventions are more effective in reducing depressive symptoms.
In my opinion, we may see further benefits of exercise-based intervention in naturalistic settings. Whilst RCTs allow for stringent comparison of variables, exercise-based interventions offer a plethora of other benefits. Exercise offers individuals a chance to socialise, explore the outdoors, learn a new skill and offer a component of social desirability.
Statement of interests
Bellón, J. Á., Conejo-Cerón, S., Sánchez-Calderón, A., Rodríguez-Martín, B., Bellón, D., Rodríguez-Sánchez, E., Mendive, J. M., Ara, I., & Moreno-Peral, P. (2021). Effectiveness of exercise-based interventions in reducing depressive symptoms in people without clinical depression: systematic review and meta-analysis of randomised controlled trials. The British Journal of Psychiatry, 1–10.
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Martinsen, E. W. (2008). Physical activity in the prevention and treatment of anxiety and depression. Nordic Journal of Psychiatry, 62(SUPPL. 47), 25–29.
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