The effects of exercise on depressed individuals have been investigated extensively. Too often, the conclusions drawn from these studies have gone beyond what the actual data can defend. Consider a pair of meta-analyses of published studies from groups in Australia and the UK—a meta-analysis involves compiling results from other investigations.

The authors of the Australian study examined data from 97 studies that included over 128,000 participants. That sounds impressive; however, the devil is 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 with drug therapy since the studies were not head-to-head comparisons. The study also reported that exercise benefited many 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 is that the benefits of exercise described in many 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........

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