Barbour, Krista A. PhD; Edenfield, Teresa M. PhD; Blumenthal, James A. PhD
There exists an abundance of evidence demonstrating the physical health benefits of exercise. Regular physical activity has been shown to be associated with improved glucose metabolism, lower blood pressure, and reduced risk of a variety of medical conditions ranging from hypertension, diabetes, and cardiovascular disease to cancer and osteoporosis.1 In addition, there are data to suggest the beneficial effect of physical activity on psychological symptoms and psychiatric disorders.
This review summarizes the available literature on the mental health benefits of physical exercise, describes potential mechanisms of action, and reviews practical considerations regarding adherence to exercise training. Finally, we offer recommendations regarding directions of future research of the study of exercise as a treatment for psychiatric disorders.
The term “depression” is commonly used to describe a continuum of psychological distress ranging from a dysphoric mood state to diagnosis of a clinical disorder, such as major depressive disorder (MDD). In the United States, psychiatric disorders currently are diagnosed using criteria outlined in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition.2 According to the manual, a diagnosis of MDD requires endorsement of either depressed mood or diminished interest/pleasure along with at least 4 other symptoms (eg, changes in appetite or weight, sleep disturbance, psychomotor agitation or retardation, fatigue, feelings of worthlessness, concentration difficulties, and recurrent thoughts of death or suicide). This constellation of symptoms must have been present during the same 2-week period and result in clinically significant distress and impairment of functioning. Most studies examining exercise as a treatment for psychological symptoms have focused on depression (subclinical depressive symptoms and MDD) and are featured prominently in this review.
Physical Activity and Depression: Epidemiologic Evidence
Large-scale, cross-sectional studies have demonstrated an inverse relationship between physical activity (which can include leisure and occupational activities) and depressive symptoms, such that higher levels of activity are associated with fewer depressive symptoms.3–5 Results of prospective studies have demonstrated that, relative to individuals who reported higher levels of physical activity, inactive persons are at increased risk for depression at the time of a subsequent follow-up assessment.3–6 While individuals who increased their physical activity level from baseline to follow-up were at no greater risk of developing subsequent depression than those who remained active throughout this same period, individuals who decreased their level of physical activity from baseline to follow-up assessment were more likely to have higher subsequent depression scores relative to those maintaining a high level of physical activity.6
Physical Activity and Depression: Evidence From Interventional Studies
Although cross-sectional studies provide important information regarding the relationship between physical activity and depression, the results of such studies do not offer information regarding causality: Low levels of physical activity may lead to greater depression or depression may result in lower levels of exercise. Thus, randomized controlled trials (RCTs) are necessary to establish a causal relationship between exercise training and reduced symptoms of depression. In the next section, we examine the evidence for an antidepressant effect of exercise.
Exercise training generally has been shown to improve depressive symptoms in nondepressed participants (ie, individuals who endorse symptoms of depression but would not meet criteria for a diagnosable disorder).7 Specifically, participants who undergo exercise training tend to demonstrate significantly greater improvement in depressive symptoms than do control participants. This effect has been shown in both middle-aged and older adult participants.8 However, some studies have failed to show an antidepressant effect of exercise in nondepressed participants,9,10 which may be due, in part, to the fact that there is little room for improvement in these participants (ie, a “floor effect”). Studies of individuals with clinical levels of depression (eg, MDD) may provide the opportunity to observe greater improvement in depressive symptoms from pretreatment to posttreatment.
Lawlor and Hopker11 conducted a meta-analysis including 14 RCTs that evaluated the efficacy of exercise as a treatment for depression. Results of the meta-analysis indicated that exercise led to greater reduction in depressive symptoms than did no treatment, and was similar in effectiveness in comparison with cognitive therapy. However, because of the poor design of many of the trials reviewed, they concluded that the extent of the effectiveness of exercise in reducing symptoms of depression was difficult to determine. In a recent update of the Lawlor and Hopker meta-analysis, Stathopoulou and colleagues12 examined 11 exercise treatment studies (4 of which were published since the Lawlor and Hopker review) and concluded that exercise can be an effective intervention for depression when compared with control conditions. Thus, while studies of exercise as a treatment for depression have suffered from design flaws, it does appear that exercise is a viable treatment option for individuals with depression.
In one of the larger RCTs, researchers at Duke University13 examined exercise training in comparison with antidepressant medication (the selective serotonin reuptake inhibiter, sertraline) as a treatment for MDD in middle-aged and older adults. Participants were 156 men and women diagnosed with MDD who were randomized to supervised exercise, sertraline, or a combination of exercise and sertraline. The 16-week exercise treatment consisted of 3 weekly sessions of aerobic activity. By the end of the treatment period, each of the 3 treatment groups experienced a significant reduction in their levels of depression. The treatments did not differ significantly from one another in efficacy, suggesting that exercise may be a viable alternative to medication in the treatment of MDD.
A follow-up assessment completed 6 months after treatment14 showed that patients who were in remission after treatment who were assigned to the exercise- alone group experienced lower relapse rates of depression than did those receiving medication or a combination of exercise and medication; only 9% of remitted participants in the exercise group experienced relapse. In comparison, more than 30% of participants in the medication and combination groups experienced relapse. Examination of predictors of recovery (ie, factors associated with the likelihood of not having MDD 6 months after treatment) revealed that greater symptoms of depression after 4 months of treatment were associated with increased likelihood of MDD at follow-up. In addition, patients who reported regular exercise had a 50% reduction in risk of being diagnosed with MDD 6 months after study completion. In summary, the results from this trial indicate that exercise training may be as effective as antidepressant medication in treating depression, and that maintenance of exercise may be associated with reduced risk of relapse for patients with MDD.
Considerations of Exercise Dose and Modality
One question that remains unanswered is the dose of exercise necessary to obtain an antidepressant effect. There is uncertainty regarding the frequency, intensity, and duration that are necessary for treating patients with depression. Dunn and colleagues15 reported the results of the DOSE trial, which was designed to address the dose-response issue. In this trial, 80 previously sedentary adults diagnosed with MDD were randomized to undergo 12 weeks of 1 of 5 aerobic exercise training treatment conditions: low-energy expenditure/3 days per week, high-energy expenditure/3 days per week, low-energy expenditure/5 days per week, high-energy expenditure/5 days per week, or stretching/flexibility control.
Results of the trial demonstrated that exercise conducted at the high-energy expenditure dose (ie, consistent with public health recommendations) was effective in reducing depressive symptoms over the 12-week treatment period. Interestingly, while participants randomized to the low-energy expenditure dose experienced some reduction in depressive symptoms over the treatment period, they did not respond significantly better than did participants randomized to the control condition. Regarding frequency of training, no significant difference in treatment response was found between participants who exercised 3 days a week and those who exercised 5 days per week. Thus, total energy expenditure was considered to be the key aspect of exercise dose related to remission of MDD, regardless of days per week exercised. Thus, individuals with depression should be encouraged to achieve at least the minimum recommended levels of energy expenditure.
A second issue relates to the optimal exercise modality to achieve reductions in depressive symptoms. Although most research has focused on aerobic type activities (eg, walking, jogging, cycling, and swimming), there are data to suggest that resistance training may also be effective. A community sample of older adults with depressive symptoms was randomized to either progressive resistance training or a health education control.16 At the end of the 20-week treatment period, participants in the resistance training group demonstrated a significant reduction in depressive symptoms in comparison with those in the control group. Furthermore, this antidepressant effect for exercisers remained at 26-month follow-up, with 33% of participants randomized to the exercise condition continuing regular weightlifting. Thus, resistance training might be considered in lieu of aerobic exercise for patients not capable of or who lack motivation for aerobic activity.12 Interestingly, this finding suggests that the improvement in depression associated with exercise may not require an increase in cardiovascular fitness.12
Bipolar disorder (BD) is a recurrent and debilitating psychiatric disorder characterized by a chronic dysregulation of mood, with fluctuations between low mood (eg, depression) and elevated mood (eg, mania). Elevated mood is typically associated with behavioral activation (eg, increased goal-directed activity, decreased need for sleep) and altered cognitive functioning (eg, racing thoughts, distractibility, grandiose delusions) for an extended period of time (eg, ≥1 week).2 These mood fluctuations often lead to impulsive, risky behaviors (eg, substance abuse, sexual indiscretion, excessive spending), and interpersonal difficulties. Individuals with a diagnosis of BD are at risk for increased mortality resulting from suicide, homicide, accidents, and natural causes (eg, including coronary heart disease).17,18
Regarding the treatment of BD, pharmacologic interventions (eg, lithium, Depakote) are considered the standard of care, as medication is often effective and the etiology of the disorder is considered to be primarily biological in nature. Therefore, researchers have historically concentrated on investigations of biological mechanisms (eg, endocrine system dysregulation) and psychopharmacologic treatment (eg, mood stabilizers, anticonvulsants) of the disorder. However, adjunctive therapies that target important aspects of the disorder (eg, medication adherence) and reduce risk of relapse are also beneficial.
Although research has shown that regular exercise may alleviate depressed mood in individuals diagnosed with MDD, the impact that exercise may have on mood fluctuation and stress adaptation responses in BD has not been widely studied. Preliminary data suggest that walking 30 minutes, 4 times per week for 4 weeks, followed by subsequent independent maintenance of this program resulted in an increase in the use of adaptive coping strategies and that longer-term follow-up suggested that decreased frequency of exercise participation resulted in a greater level of depressive symptoms reporting, particularly for individuals who participated in 4 weeks of prescribed exercise during treatment.19 These findings suggest that including a prescription for mild to moderate intensity exercise may have an important impact on an individual's perception of, and response to, stressful life events. In addition, self-maintained exercise, following participation in a 4-week program of prescribed exercise, may be beneficial in preventing exacerbations of depressive symptoms over time.
In summary, psychopharmacologic intervention, paired with empirically supported psychotherapeutic interventions (eg, cognitive behavioral therapy [CBT]), remains the standard treatment for BD. However, a more fully integrated treatment approach pairing these supported treatments with regular exercise may offer a more comprehensive intervention.
Although most studies of mental health and physical activity have focused on depression (in clinical samples) and depressive symptoms (in nonclinical samples), there are data to show that exercise also can be an effective treatment for anxiety symptoms and anxiety disorders. For example, acute bouts of exercise have been demonstrated to result in a reduction in state anxiety20,21 and panic symptoms.22
Several studies have examined the effect of exercise on anxiety sensitivity, conceptualized as a fear of anxiety-related physical sensations (eg, increased heart rate, rapid breathing). Anxiety sensitivity, in turn, has been found to be a precursor of panic attacks.23 For example, Broman-Fulks and colleagues24 examined the effects of exercise-induced physiological arousal on anxiety sensitivity. Participants were randomized to 6 training sessions of either low-intensity (treadmill walking at 1 mph) or high-intensity (treadmill walking or jogging to achieve heart rates between 60% and 90% age-adjusted predicted maximal heart rate). Results showed that both low- and high-intensity exercise reduced anxiety sensitivity at posttesting, with participation in high-intensity exercise associated with a more rapid reduction in anxiety sensitivity scores. It is theorized that exercise training serves as an exposure technique (a common component of behavioral therapies) by “exposing” the individual to feared bodily sensations in the absence of negative consequences (ie, panic attack). Thus, exercise training may treat individuals prone to panic attacks via reduction in anxiety sensitivity.25
Other studies have focused on individuals diagnosed with panic disorder. Such individuals experience recurrent, unexpected panic attacks. In addition, this disorder is accompanied by anticipatory anxiety regarding the occurrence of future episodes of panic, worry regarding the possible implications of panic attacks, and associated behavioral change such as avoidance of anxiety-provoking stimuli.2 Broocks and colleagues26 compared pharmacotherapy (clomipramine) with exercise training in 46 outpatients with moderate to severe panic disorder. By the end of the 10-week treatment period, both exercise training and medication significantly reduced panic symptoms relative to placebo (though clomipramine was superior to exercise). Further studies are needed to determine how to best utilize exercise training in the treatment of panic disorder. For example, the value of exercise training alone or combined with more established treatments has yet to be studied.
In addition, the intensity of exercise necessary to produce an anxiolytic effect is unclear. Although some research has shown that state anxiety is reduced in response to exercise regardless of duration or intensity,27 other findings suggest a positive relationship between anxiety symptoms and exercise intensity.28 That is, as the intensity of exercise increased, the anxiety also increased (perhaps because of an increase in arousal). Further research is needed to clarify the dose-response relationship between exercise training and anxiety.
EXERCISE AND STRESS REACTIVITY
Exaggerated cardiovascular responses to stress have been suggested to be associated with increased risk of developing cardiovascular disease and hypertension. Individuals displaying the “type A” (coronary-disease prone) behavior pattern have been shown to exhibit elevated cardiovascular responses in response to laboratory stressors. Previous studies have suggested that aerobic exercise training may reduce type A behaviors and attenuate cardiovascular stress responses.29
In a RCT investigating the impact of exercise and weight loss on blood pressure in a sample of overweight, hypertensive individuals, Georgiades and colleagues30 found that exercise was effective in reducing resting and stress-induced blood pressure levels, particularly when combined with a behavioral weight loss program. In addition, exercise combined with a weight loss program was found to decrease hemodynamic stress responses for this population to those levels targeted by pharmacologic antihypertensive treatments. These findings suggest that exercise training may play an important role in reducing resting blood pressure and in blunting the rise in blood pressure associated with psychosocial stress. In addition, these data are important, as they further highlight the potential role of nonpharmacologic intervention (ie, supervised exercise participation) in long-term maintenance therapy for individuals at risk for cardiovascular disease.
Although current research is supportive of a possible stress-attenuation role of exercise participation, findings in the past have been mixed regarding intensity, duration, and type of exercise needed to produce the most optimal effects. Additional research is needed to address these issues, as well as to determine whether exercise-related fitness changes are necessary for individuals to experience antidepressant and stress attenuation benefits of exercise participation.
EXERCISE IN MEDICAL POPULATIONS
Exercise participation has received increasing attention as a means to improve depressive symptoms in patients with cardiac and pulmonary diseases as well as dementia and deficits in cognitive function. These areas are discussed here.
There is growing evidence that clinical depression and elevated depressive symptoms are associated with worse outcomes in a variety of cardiac patient populations, including patients with coronary artery disease, acute post–myocardial infarction, heart failure, and post–coronary artery bypass surgery.31,32 Depression is especially common among cardiac populations; prevalence estimates range from 15% to more than 40%.32 Less is known about the prevalence of depression in patients enrolled in cardiac rehabilitation programs, but a recent survey of patients entering a phase II program found that approximately 26% of these individuals met diagnostic criteria for a depressive disorder.33
Several studies have examined the effect of exercise training on depression in cardiac patients.16,34 Generally, results of these studies show significant improvements in depressive symptoms upon completion of exercise training programs. For example, in a sample of more than 300 patients enrolled in cardiac rehabilitation after suffering a cardiac event, 20% of patients reported elevated depressive symptoms. At the end of a 3-month aerobic exercise training period, two thirds of the initially depressed patients reported resolution of their depressive symptoms. In addition, the depressed group demonstrated significant improvements in other quality-of-life variables.34
In the Enhancing Recovery in Coronary Heart Disease (ENRICHD) clinical trial, 2481 patients with acute post–myocardial infarction were identified as having symptoms of either depression or social isolation and were randomized to either CBT or usual care to determine the efficacy of CBT for ameliorating depression and social isolation and improving survival. Results demonstrated that although depression and social isolation improved in patients receiving CBT in comparison with usual care controls, there was no difference in overall survival.35 In an ENRICHD substudy, however, 2078 participants were available for 6-month follow-up assessment of their exercise behavior. Individuals who reported engaging in regular physical activity (47.2% of the sample) were more likely to experience a decrease in depressive symptoms over the 6 months following their myocardial infarction. In addition, regular physical activity was shown to increase survival in this sample; specifically, participants reporting regular exercise experienced a 50% reduction in risk of subsequent death in comparison with participants who reported no regular exercise.36 Taken together, these results suggest that exercise is associated with reduced depressive symptoms in cardiac patients, and that exercise may also improve survival in depressed cardiac patients.
In general, results of studies including patients enrolled in pulmonary rehabilitation programs suggest that exercise training may improve depressive symptoms in this population.37,38 For example, Emery and colleagues39 found that in a sample of older adults with chronic obstructive pulmonary disease, a 10-week exercise training program significantly reduced depressive symptoms. However, other studies have failed to show an antidepressant effect of exercise training in patients with pulmonary disease.40 Brosse and colleagues7 concluded that many of the studies conducted with patients with pulmonary disease suffered from design flaws, making interpretation of the results difficult.
Dementia and Cognitive Functioning
Findings regarding the effects of exercise training on cognitive functioning have been inconsistent. In a meta-analysis of 30 RCTs of participants aged 65 and older with some degree of cognitive impairment or a preexisting diagnosis of dementia, it was found that participants who underwent exercise training (either cardiovascular or strength training) showed significant improvement in performance on cognitive tasks in comparison with participants in control conditions.41 Another meta-analysis of exercise training in older adults42 concluded that exercise was associated with improved cognitive functioning, especially in the domain of executive control processes (eg, working memory, ability to plan, ability to multitask). However, other interventional studies have failed to show improvements in cognitive functioning,43–45 so that the effect of exercise training on cognitive functioning remains uncertain.
Since the publication of the Colcombe and Kramer42 meta-analysis, results of a recent well-controlled, prospective study adds to our knowledge of the relationship between exercise and cognitive functioning. Larson and colleagues46 followed a sample of 1740 men and women who were at least 65 years of age and without cognitive impairment to examine the temporal association between self-reported physical activity and development of dementia. At the end of the follow-up period (average = 6.2 years), 158 participants had developed dementia (including Alzheimer's, vascular, and other types of dementia). Results showed that participants who reported exercising 3 or more times per week were significantly more likely to remain dementia free relative to those who exercised fewer than 3 days per week.
In summary, although results are mixed, there are data to suggest that exercise training may improve cognitive functioning, especially among individuals with some degree of cognitive impairment. Regular participation in exercise may also delay the onset of dementia.
Several hypotheses that involve biochemical, physiological, and psychosocial mechanisms have been offered to account for the effect of exercise on mental health. Despite continuing research that demonstrates the positive effects of exercise on depressed mood, no single theory adequately explains how exercise leads to a reduction in depressive symptoms. A plethora of physiological and psychological theories have been proposed to explain the interaction between mood state and exercise. While many theories focus on physiological factors such as an increase in core body temperature (thermogenic model)20 and differential neocortical activation following exercise,47 other models focus on biochemical mechanisms such as increased secretion of amine metabolites, as well as serotonin synthesis and metabolism (monoamine hypothesis).48–50 The central premise of these biochemical theories is that exercise acts on the same pathway that antidepressant medications target in the treatment of clinical depression.51
Additional biochemical theories focus on specific areas of the brain (ie, hypothalamic-pituitary-adrenal [HPA] axis),52 or endogenous chemicals (eg, endorphins, enkephalins) or systems (eg, endocannabinoid), to explain the antidepressant properties of exercise. Theories such as the HPA axis model propose that stress hormones are released in response to physical (eg, exercise) and psychological stress experiences. Accordingly, habitual levels of exercise are thought to decrease the amounts of stress hormones secreted from the HPA axis,52 resulting in lower levels of depression and stress reactivity.53
In addition to the aforementioned physiological models, several psychological theories have been proposed to explain the effect that exercise has on depressed mood.53–56 Psychological theories of exercise focus on different constructs such as intrinsic motivation,57 self-determination,58 and self-efficacy.59 In addition, several social-cognitive and expectancy-value theories (eg, theory of reasoned action; theory of planned behavior)60,61 address issues of intention, perception of control, and actual engagement in an activity (exercise). In general terms, these models focus on a variety of mechanisms that might explain the link between exercise and reduced depressed mood (eg, improved accomplishments and confidence, positive distraction, improved self-esteem, environmental reinforcement, increasing positive coping skills available for use during stressful situations).
In an attempt to bridge the gap regarding exercise and mood in these various psychological models, Salmon53 has proposed an integrative model called the stress adaptation model. According to this model, an increase in life structure occurs as a result of including exercise in one's daily routine. In addition, individuals who exercise on a daily basis may experience a simultaneous decrease in stress reactivity.53 Support for this theory can be drawn from previous research that found reductions in the perceived experience of stress in daily living in individuals who exercise on a regular basis.56 According to the stress adaptation model, regular exercise protects individuals from the effects of stress, and ultimately prolongs euthymic periods or prevents depressive relapse. In this model, exercise is also conceptualized as a stressor, albeit one with potential positive benefits. In addition, Salmon53 proposes that the controllability of the exercise stressor may provide a substantial benefit. This controllability may result in more efficient adaptation to other stressors, decrease the likelihood of relapse as a result of routine regulation, lead to the development of more effective coping responses, and decrease reactivity to stressful life events.53
Adherence to exercise over time is generally low, and may be particularly poor in medically ill populations in rehabilitation settings. For example, approximately 50% of cardiac patients who begin an exercise program will drop out within 6 months.62
Risk of nonadherence to exercise may be higher for individuals with depression. For example, Glazer and colleagues63 found that, for patients enrolled in cardiac rehabilitation, depression status at program entry was predictive of the number of exercise sessions attended as well as the dropout rate from the program. A significant relationship between depression and exercise adherence may not be surprising, as individuals with depression may find it more difficult to stay engaged in an exercise program secondary to common symptoms of depression such as fatigue and a loss of interest in people and activities.
Thus, exercise specialists are likely to encounter individuals who have difficulty adhering to an exercise regimen. Researchers have found that assessing an individual's level of motivation to exercise is an important first step in intervening to enhance adherence.64 Motivation appears to be a critical aspect of maintaining exercise over time; there is evidence that brief counseling tailored to an individual's level of motivation results in greater likelihood of meeting exercise participation goals.65 When problems with adherence do arise, they should be viewed as learning opportunities. That is, lapses in exercise participation can be used to identify an individual's unique barriers to adherence. Such barriers can then be explored in an effort to develop solutions.
Future investigations should examine psychological variables associated with low adherence to exercise regimens. For example, it has been found that participant characteristics, such as anxiety and life satisfaction, predict drop out from an exercise program in exercisers with depression.66
The purpose of this review was to examine the effectiveness of exercise training in the treatment of psychiatric symptoms and disorders. In summary, it generally appears that exercise is beneficial for individuals with symptoms of depression and anxiety. More data are necessary to allow for conclusions for the effect of exercise in treating BD. Exercise training also appears to be a viable treatment option for medically ill patients with comorbid depression, and may serve to delay the onset of dementia.
Interpretation of results of the literature reviewed is made difficult because of significant design flaws present in many of the studies. There is a need for well-designed trials investigating the beneficial effect of exercise training on psychiatric functioning. These trials should include appropriate control groups and adequate periods of participant follow-up.
Future research should also focus on defining the optimal dose of exercise training to achieve a favorable effect, as well as differences in effectiveness between aerobic exercise and other forms of training, such as resistance training, in individuals with psychiatric symptoms. Finally, further exploration of the mechanisms by which exercise training improves psychiatric symptoms is needed.
This study was supported, in part, by grants HL080664 and MH049679 from the National Institutes of Health.
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