Exercise And Depression

exercise and depression 1
graphic exercise and depression 1

Exercise And Depression

PROPOSED MECHANISMS FOR THE EXERCISE-DEPRESSION RELATIONSHIPWhile the research is consistent and points to a relationship between exercise and depression, the mechanisms underlying the antidepressant effects of exercise remain unclear. Several credible physiologic and psychological mechanisms have been described, such as the thermogenic hypothesis,36 the endorphin hypothesis,37,38 the monoamine hypothesis,39–42 the distraction hypothesis,43 and the enhancement of self-efficacy.29,43–46 However, there is little research evidence to either support or refute most of these theories.Thermogenic HypothesisThe thermogenic hypothesis suggests that a rise in core body temperature following exercise is responsible for the reduction in symptoms of depression. DeVries36 explains that increases in temperature of specific brain regions, such as the brain stem, can lead to an overall feeling of relaxation and reduction in muscular tension. While this idea of increased body temperature has been proposed as a mechanism for the relationship between exercise and depression, the research conducted on the thermogenic hypothesis has examined the effect of exercise only on feelings of anxiety rather than depression.36,47,48Endorphin HypothesisThe endorphin hypothesis predicts that exercise has a positive effect on depression due to an increased release of β-endorphins following exercise. Endorphins are related to a positive mood and an overall enhanced sense of well-being. This line of research has not been without criticism. The debate remains as to whether plasma endorphins reflect endorphin activity in the brain. Some37,38 have argued that even if peripheral endorphin levels are not reflective of brain chemistry, they could still be associated with a change in mood or feelings of depression. Several studies have shown increases in plasma endorphins following acute and chronic exercise49–51; yet, it remains unclear if these elevations in plasma endorphins are directly linked to a reduction in depression. Lastly, the phenomenon of runner’s high, often attributed to endorphin release, is not blocked by naloxone injection, an opiate antagonist.52,53Monoamine HypothesisThe monoamine hypothesis appears to be the most promising of the proposed physiologic mechanisms. This hypothesis states that exercise leads to an increase in the availability of brain neurotransmitters (e.g., serotonin, dopamine, and norepinephrine) that are diminished with depression. These neurotransmitters increase in plasma and urine following exercise, but whether exercise leads to an increase in neurotransmitters in the brain remains unknown.40–42 Animal studies suggest that exercise increases serotonin and norepinephrine in various brain regions,39,54–56 but, to date, this relationship has not been studied in humans.Therefore, while several physiologic mechanisms remain plausible, methodological difficulties have prevented this line of research from advancing. Martinsen57 discusses how testing biochemical hypotheses is often difficult in humans due to the invasive procedures necessary to obtain samples (e.g., spinal taps for cerebrospinal fluid samples). Further, biochemical samples obtained from blood or other bodily fluids may not directly reflect the activity of these compounds in the brain.39 Hopefully, with the advent of new less invasive neuroimaging techniques, future researchers can examine whether exercise leads to the neurochemical changes in the brain predicted by these physiologic hypotheses.Distraction HypothesisSeveral psychological mechanisms have also been proposed. As was the case with the physiologic mechanisms, many of these theories have not been tested extensively. The distraction hypothesis suggests that physical activity serves as a distraction from worries and depressing thoughts.43 In general, the use of distracting activities as a means of coping with depression has been shown to have a more positive influence on the management of depression and to result in a greater reduction in depression than the use of more self-focused or introspective activities such as journal keeping or identifying positive and negative adjectives that describe one’s current mood.58,59Exercise has been compared with other distracting activities such as relaxation, assertiveness training, health education, and social contact.17,18,23,60 Results have been inconclusive, with exercise being more effective than some activities and similar to others in its ability to aid in the reduction of depression. However, exercise is known to increase positive affect, which is diminished in depressed patients and is not augmented by distraction activities. The diminished capacity to experience positive affect is an essential distinguishing symptom in clinical depression.Self-Efficacy HypothesisThe enhancement of self-efficacy through exercise involvement may be another way in which exercise exerts its antidepressant effects. Self-efficacy refers to the belief that one possesses the necessary skills to complete a task as well as the confidence that the task can actually be completed with the desired outcome obtained. Bandura36 describes how depressed people often feel inefficacious to bring about positive desired outcomes in their lives and have low efficacy to cope with the symptoms of their depression. This can lead to negative self-evaluation, negative ruminations, and faulty styles of thinking. It has been suggested that exercise may provide an effective mode through which self-efficacy can be enhanced based on its ability to provide the individual with a meaningful mastery experience. Research examining the association between physical activity and self-efficacy in the general population has focused predominantly on the enhancement of physical self-efficacy and efficacy to regulate exercise behaviors. The relationship between exercise and self-efficacy in the clinically depressed has received far less attention. The findings of the few studies that have examined this relationship have been equivocal as to whether exercise leads to an enhancement of generalized feelings of efficacy.23,61 However, 1 recent study62 has reported that involvement in an exercise program was associated with enhanced feelings of coping self-efficacy, which, in turn, were inversely related to feelings of depression.More research is needed to determine which, if any, of the mechanisms described herein are important moderators of the exercise effect. It is highly likely that a combination of biological, psychological, and sociological factors influence the relationship between exercise and depression. This is consistent with current treatment for depression in which the effects of pharmacotherapy and psychotherapy on depression are additive and address biological, psychological, and sociological aspects of the patient. There may also be individual variation in the mechanisms or combination of mechanisms mediating this relationship.32 Additionally, different mechanisms may be important at specific times in the natural course of depression. Until more is known about the possible mechanisms, this relationship may be best studied utilizing a biopsychosocial approach.
exercise and depression 1

Exercise And Depression

META-ANALYTIC FINDINGSWhile relatively few studies have been described here for illustrative purposes, there are now a large number of studies that support the efficacy of exercise in reducing symptoms of depression. Further, while the early research in this area suffered from a variety of methodological limitations (e.g., small samples, lack of random assignment, lack of control groups), current researchers have addressed these design issues, and presently there are multiple studies that have utilized experimental designs or employed a randomized clinical trial approach. Meta-analysis provides one means of summarizing this growing body of primary research and identifying variables that may moderate the effect of exercise on depression. Effect sizes (ESs) are calculated for each study and weighted to correct for positive bias that can result from small sample sizes. The statistical analyses of these ESs allow the researcher to investigate study and subject characteristics that may moderate the exercise-depression relationship as well as compare subsets of samples from the original studies and make comparisons (e.g., male vs. female, mildly vs. moderately depressed) that were not directly made within each of the individual primary research articles.There have been several meta-analyses conducted on the literature examining the relationship between exercise and depression. North et al.28 included 80 studies in their meta-analysis and reported an overall mean ES of −0.53, indicating that exercise reduced depression scores by approximately one half a standard deviation compared with those in comparison groups. That meta-analysis included primary research studies that had examined the exercise-depression relationship in a variety of samples (e.g., college students, medical patients, depressed and mentally ill patients, and normal adults). When those authors analyzed the subgroup of studies that had utilized clinical populations (e.g., substance abusers, post–myocardial infarction patients, hemodialysis patients), they reported an even larger ES of −0.94. However, patients suffering from depression secondary to a medical condition may be qualitatively different from those who suffer from clinical depression as the primary illness. As such, exercise interventions that are effective for medical populations may not be as effective in treating depression when it is the primary disorder.29Craft and Landers30 attempted to further clarify this relationship and included only studies in which individuals diagnosed with clinical depression served as participants. Thirty studies were included in that meta-analysis, and an overall mean ES of −0.72 was reported. Several variables related to study quality, subject characteristics, and exercise program characteristics were coded and examined in an attempt to determine potential moderators of this relationship. Interestingly, exercise program characteristics such as duration, intensity, frequency, and mode of exercise did not moderate the effect. In fact, only the length of the exercise program was a significant moderator, with programs 9 weeks or longer being associated with larger reductions in depression. Further, exercise was equally effective across a variety of patient subgroups. For example, subject characteristics such as age, gender, and severity of depression did not emerge as significant moderators. Finally, when compared with other traditional treatments for depression, exercise was just as beneficial and not significantly different from psychotherapy, pharmacologic therapy, and other behavioral interventions.30Lawlor and Hopker31 more recently conducted a meta-analysis that included only randomized controlled trials of clinically depressed patients. Fourteen studies were included in their meta-analysis. They reported an overall mean ES of −1.1 for exercise interventions compared with no-treatment control groups. Exercise was also as effective as cognitive therapy in alleviating symptoms of depression, with a nonsignificant ES of −0.3 emerging.Analyses of subsamples within these meta-analyses indicate that exercise is effective for both men and women of all ages as well as those who are initially more severely depressed. Further, comparison of exercise program characteristics across the studies yielded nonsignificant ESs, indicating that exercise need not be lengthy or intense and that fitness gains are not necessary for patients to experience positive benefits. Therefore, in light of this additional meta-analytic support, researchers have begun to recommend that depressed individuals should adopt physically active lifestyles to help manage their symptoms.30,32,33 Table 1 provides basic characteristics of the studies utilizing clinical populations that were included in the 3 meta-analyses discussed above.28,30,31 Readers are referred to each of the meta-analyses for additional information regarding individual primary research articles. Further, a review by Dunn and colleagues34 includes a summary of study and participant characteristics for quasiexperimental and experimental designs.Table 1.Characteristics of 37 Studies Included in Exercise and Depression Meta-AnalysesaThe data from the 3 meta-analyses discussed previously28,30,31 resulted in overall ESs of −0.72, −0.94, and −1.1. Converting these ESs to a binomial effect size display allows one to examine the practical clinical significance of these effect sizes.35 These values reflect an increase in success rate due to treatment (i.e., exercise) of 67%, 71%, and 74%, respectively, and such promising treatment outcomes are notable. In medical settings, clinical guidelines suggest that a 50% reduction in symptoms during the treatment phase is considered a treatment response. Researchers have argued that this criterion is more clinically relevant than statistically significant results, which can occur even if the participants have not experienced a 50% reduction in the symptoms of their depression.34 Therefore, even when using this more clinically relevant guideline, the meta-analytic data provide support for the efficacy of exercise in reducing symptoms of depression. While this discussion is not meant to argue against the use of antidepressant medication or psychological therapies, there is strong evidence to advocate the use of exercise as a potentially powerful adjunct to existing treatments.

Exercise And Depression

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