Depressed? Don't Rely on Exercise Alone

The effects of exercise on depressed individuals have been investigated extensively. Too often, the conclusions drawn from these studies in the popular press or social media have gone beyond what the actual data can defend.

Such hyperbolic media response happened recently in response to a meta-analysis of published studies from a group in Australia. A metanalysis is when someone compiles results from other investigations and attempts to draw a conclusion. The authors examined data from 97 studies that included over 128,000 participants. That sounds impressive; however, the devil awaits in the details that the popular media completely ignored. For example, only 11 of those studies were focused on depression.

The authors concluded that exercise had a medium effect on depression. It is impossible to know how a "medium" effect compares to drug therapy since the studies were not head-to-head comparisons. The study also reported that exercise benefited lots of other health conditions, including HIV or kidney disease, various mental disorders, and cancers. Furthermore, any level of exercise was effective! If this sounds too good to be true, it is.

One point often ignored in recent media accounts is that the benefits of exercise described in these studies decreased over time. The authors also noted that patients who exercised less often each week reported greater benefits than patients who exercised more often each week. This is the exact opposite of the typical dose-benefit relationship that should be observed. When a treatment works well for many conditions, and the benefits wear off over time or do not show a dose-response relationship, you are witnessing the placebo effect. The problem is that it is impossible to control for the placebo effect in exercise studies; the control subjects are well aware that they are not exercising.

Past studies of the benefits of exercise have numerous fatal flaws that undermine confidence in their results. Previous studies have varied widely in size, type of control group, methodological rigor, length of follow-up, and even the type of exercise modality. Randomized exercise trials have generally ranged in length from six weeks to four months and typically emphasized aerobic exercise, although some studies on resistance training have also been conducted. The many differences in study design have contributed to the current level of confusion and misunderstandings about the benefits of exercise on depression.

Although many trials have been conducted on adults with major depressive disorder, only a few used high-quality methodologies in which the treatment allocation was concealed. Many of the other studies failed to comply with the standard intention-to-treat analyses. This means that the final analysis included every subject assigned a randomized treatment. This type of analysis ignores noncompliance (the subjects exercised too much or not at all), protocol deviations (they performed the wrong exercise), withdrawal from the study, or any of a number of potential things that might have happened to the subjects after they had been assigned to their study groups.

The general problem is it is very difficult to convince human subjects to exercise consistently without introducing their own creative changes, which often undermines the integrity of the study. Many past studies' results were difficult to analyze because they failed to include a control group in the design. Sometimes the most important variable, the degree of depression experienced by the subjects, was assessed by someone not blinded to treatment. Thus, the studies were vulnerable to experimenter bias and are thus unreliable. Unfortunately, that has not stopped some pundits from referencing them.

For these reasons, the initial enthusiasm and some outrageous claims about the effectiveness of exercise in combating depression have not been supported by newer studies. Recent well-controlled studies have found only a “modest to moderate” antidepressant effect due to daily long-term exercising.

In studies that used a standard head-to-head comparison, exercise was no better or worse than standard cognitive behavioral therapy for treating depression—that is, just talking to someone, hopefully from a safe distance, was as effective as exercising. When researchers compared the effectiveness of exercise on adults who were given a standard anti-depressant drug therapy, they found no significant benefit of any one approach.

Overall, the benefits of physical exercise for depression are subtle but real. Our most recent and largest studies, which are not metanalyses, clearly demonstrate that although exercise is not better than drugs, it should be considered an important adjunct therapy, combined with talk therapy, to any treatment plan for depressed patients.

A recent well-controlled study just published in Nature has added to this conclusion. Exercise was shown to provide no significant benefits to mental health. Muscles did not evolve to directly benefit brain function.

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