Results from the present analysis demonstrate that for men and women with high blood pressure and mild to moderate levels of depression, depressive symptoms may be reduced to a greater extent by an exercise program, either alone or combined with a weight management intervention, compared with participants receiving no treatment. Furthermore, among participants in the active treatment groups, reduced levels of depression were associated with increased aerobic capacity such that participants with greater improvements in VO2 peak showed greater reductions in depression.
Our results are consistent with previous studies, which indicated that exercise can reduce depressive symptoms in healthy individuals [15
] as well as patients with conditions such as heart disease [18
] and arthritis [44
]. Because both hypertension [28
] and elevated depressive symptoms [45
] are independent risk factors for CHD, an exercise intervention that reduces blood pressure and depression may be especially beneficial among hypertensive patients, a group at elevated risk for the development of CHD.
We observed that the antidepressant effect of exercise was moderated by pretreatment levels of depression. Participants with initially elevated levels of depressive symptoms benefited from exercise, whereas exercise resulted in minimal changes in depression for participants who were initially non-depressed. It is possible that there was a “floor effect” such that participants with low depressive symptoms had no room for improvement, hence only those participants with elevated depressive scores could improve. Alternatively, it is possible that, among individuals with elevated levels of depressive symptoms at baseline, those whose depressive symptoms were more rapidly reduced were able to engage in the exercise portion of the intervention more effectively.
Exercise has repeatedly been shown to reduce depressive symptoms in a variety of depressed populations[7
] and to be equally effective[11
] compared to anti-depressant therapy. In one of the first clinical trials to evaluate the use of aerobic exercise as a treatment for major depressive disorder (MDD), Blumenthal and colleagues [11
] found that a 16-week aerobic exercise intervention was equally effective to treatment with Sertraline in alleviating depressive symptoms. Moreover, in a follow-up analysis from this original trial, Babyak and colleagues [12
] found individuals who participated in the exercise portion of the study were less likely to have a recurrent depressive episode 10 months following study completion and that reduced risk was associated with higher levels of physical activity in this subset of patients. Similarly, in a recent meta-analysis of 11 treatment outcome studies utilizing physical activity in the treatment of depression, Stathopoulou and colleagues [7
] found a clear advantage of exercise compared to control conditions.
Mechanisms by which exercise may reduce depressive symptoms by affecting changes in central monoamines, increasing hypothalamic-pituitary-adrenal (HPA) axis regulation, altering endorphin levels, increasing positive self-evaluations, and by enhancing cardiopulmonary functioning. Improved aerobic capacity assessed by VO2
peak has been associated with improvement in depression in several previous studies among both cardiac patients [18
] and healthy patients [48
]. Recent studies have shown that decreased aerobic capacity predicts increased levels of depressive symptoms as early as two weeks following exercise withdrawal among healthy individuals [50
]. Among individuals with depression, Dunn and colleagues (CIT) recently demonstrated that a dose response relationship may exist between increased levels of aerobic fitness and decreased levels of depression. Our results provide further evidence to support this relationship by showing that training-induced improvement in aerobic capacity was associated with a reduction in symptoms of depression. These findings suggest that improved fitness may have contributed to the antidepressant effects of the exercise interventions. In contrast to previous findings [51
], weight reduction was not significantly associated with improvement in depression.
The findings of this study had several notable limitations. First, because participants exhibited relatively low levels of depression at baseline our findings may not be generalizable to individuals with more severe depressive symptoms. Second, because depressive symptoms were assessed currently at two time points, the direction of the relationship was between changes in depressive symptoms and changes in aerobic fitness is unclear: improved aerobic capacity could result in reduced depression, or reduced depression may contribute to improved aerobic capacity. Finally, the WLC group in our study did not receive as much attention from the study staff as the active treatment groups, which may have affected their mood over the course of the 6-month treatment program. Future studies investigating this relationship would be improved with the addition of an attentional control group.
Results of this study may have important clinical implications. Previous studies have shown that depression is associated with adverse medical outcomes [52
]. Because depressive symptoms have been associated with a greater incidence of adverse events in the presence of hypertension [33
], we feel that our findings may have important implications for the management of depression among hypertensive individuals. Furthermore, given that depression has been associated with a greater risk for both the development of CHD [54
] and increased mortality following cardiac events [55
], interventions that alleviate depressive symptoms in this at-risk population may have important public health implications. Future research should examine the benefits of exercise on clinical outcomes among depressed hypertensive patients, including assessment of other important biomarkers of risk.