Exercise has long been advocated as a treatment for depression. A 2000 study published in "Psychosomatic Medicine" found that moderate aerobic exercise was as effective as medication in decreasing symptoms of major depression and that subjects who exercised had lower occurrence of relapse than medicated subjects. However, exercise has a point of diminishing returns; increasing your exercise levels too quickly without enough rest can negatively affect your emotional state and cause depressive symptoms.
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Overtraining Syndrome and Depression
Overtraining syndrome is the state you reach if you exercise to this point of diminishing return. You are unable to improve your performance despite increases in training intensity or duration and you may develop overuse injuries, changes in your blood chemistry, decreased immune function, increased resting blood pressure and heart rate, and negative changes in your mood. Overtraining occurs if you engage in prolonged, intense exercise without allowing enough time for rest and recovery. Depression and chronic fatigue are the two most common symptoms recognized in overtraining syndrome.
Causes of Exercise-Induced Depression
The causes of overtraining syndrome and resulting depression are unknown, but "The Physiology of Sports and Exercise" states that the symptoms of overtraining are so similar to those of clinical depression that the etiology -- or physiological causes -- may be the same for both conditions. In both overtraining syndrome and clinical depression, there are changes in hormone levels in the bloodstream.
Fortunately, depression caused by overtraining is easy to treat. Mood disturbances caused by overtraining subside when you reduce your training intensity and duration, and allow more recovery between training sessions.
According to an article published in 2013 by "Sports Health," there are no evidence-based ways to prevent exercise-induced depression. The risk of exercise-induced depression may be decreased with gradual increases in exercise intensity and duration, and allowing adequate recovery. When increasing your training level, try to change only one aspect at a time. For instance, if you are increasing the intensity, you should combine it with a decrease in duration and vice versa. You should always pay attention to how your body feels during and after your workouts, and back off the intensity and/or duration if you begin feeling excessively tired or moody.