In this large, prospective cohort study of women who were free from clinical depression or severe depressive symptoms at baseline, increased time spent in daily PA was associated with a reduced risk of clinical depression, whereas increased television watching was associated with a trend toward an increased risk. The decreased risk of depression was the strongest when assessing the joint influence of high PA level (≥90 minutes/day) and low amount of time spent watching television (0–5 hours/week). Additionally, walking at an average or brisk/very brisk pace, but not slow walking, was associated with a reduced risk of depression. Although previous longitudinal studies have addressed the link between PA and depression, most of these studies examined the relation between PA and prevalent depressive symptoms. The unique contribution of our study is that it actually examined risk of incident depression with an additional advantage of using long-term follow-up, multiple assessments of PA and depression, and a large sample size.
Our results are consistent with those of previous longitudinal studies that reported inverse associations between PA and depressive symptoms in men and women (11
). Among the 6 studies that analyzed women separately (7
), 5 reported a protective association between PA and depression symptoms (7
), whereas 1 study found no association (24
). In our cohort, we noted a relative risk of 0.80 for clinical depression when we compared the highest level of PA (≥90 minutes/day) with the lowest level (<10 minutes/day). Similarly, in the Copenhagen City Heart Study, which comprised 5,937 women with a 26-year follow-up period, Mikkelsen et al. (20
) noted a higher risk of clinical depression (multivariate relative risk = 1.80, 95% CI: 1.29, 2.51) when comparing women with a low level of PA (<2 hours/week) with those with a high level of PA (≥4 hours/week of light PA or ≥2–4 hours/week of vigorous PA). In another cohort of 9,207 middle-aged Australian women in whom those with high habitual PA levels (>300 minutes of moderate-intensity PA/week) were compared with those with very low levels (<60 minutes of moderate-intensity PA/week), Brown et al. (13
) noted an inverse association with depressive symptoms (multivariate odds ratio (OR) = 0.62 for an MHI-5 score ≤52 and OR = 0.62 for a Center for Epidemiologic Studies Depression Scale score ≥10). An effect of similar magnitude to our results was noted in the US Black Women’s Health Study (n
= 35,224) (15
). Compared with those who engaged in no vigorous activity (running, aerobics, basketball, or swimming), women with ≥7 hours/week of vigorous activity had a multivariate odds ratio for depressive symptoms (20-item Center for Epidemiologic Studies Depression Scale score ≥16) of 0.75 (95% CI: 0.65, 0.87). Despite some variations in PA type and duration, study design, and analysis across studies in women, a consistent inverse dose-response relation between PA duration and clinical depression risk was evident in all of these studies. Although the US Black Women (15
) and Danish (20
) cohorts, as well as our own, excluded women with physician-diagnosed depression at baseline, other studies did not (7
). A subtle difference from our results was observed in the US Black Women’s cohort (15
), in which the inverse dose-response relation ceased beyond 3–4 hours/week of vigorous PA (equivalent to 26–34 minutes/day).
In the present analyses, walking at an average or brisk/very brisk pace, but not slow walking, was associated with a significantly reduced risk of clinical depression, indicating that PA has to reach a certain intensity level for clinical depression risk reduction. However, this association was attenuated and no longer significant in analyses in which we used the stricter definition of depression, and it is thus of uncertain interpretation. Among the 4 cohorts in which the effect of walking was analyzed prospectively, 3 studies found an inverse association with depressive symptoms (9
) and 1 study did not find any association (15
); however, none of the published articles provided information on walking pace.
We also noted that the risk of depression increased with increasing television watching (Ptrend
= 0.01). A marginally significant (P
= 0.05) increased depression risk of about 13% was noted among women who spent 21 hours/week or more watching television compared with those who did so for 0–1 hour/week. Similarly, in the Seguimiento University of Navarra (SUN) cohort (25
), comprising 10,381 men and women, participants with the highest sedentary index score were found to be at increased risk (OR = 1.31, 95% CI: 1.01, 1.68) of mental disorders (self-reported physician’s diagnosis of depression, bipolar disorder, anxiety, or stress or use of antidepressant medication or tranquilizers) when compared with those with the lowest score. However, when depression (self-reported physician-diagnosed or use of antidepressants) was analyzed separately, the association with the sedentary index score lost significance (for the highest category vs. the lowest category, OR = 1.35, 95% CI: 0.94, 1.94).
The main hypothetical reason for the positive association we noted between television watching and depression is that television watching typically displaces PA. In our cohort, women who spent more time watching television tended to exercise less. A joint influence of television watching and PA on depression risk was noted (). In the SUN cohort (25
), the only other longitudinal study that evaluated prospectively the joint association between sedentary behavior and PA and mental disorders, the combination of PA (above the median) and sedentary index (below the median) was associated with 25% decreased odds of mental disorders (OR = 0.75, 95% CI: 0.60, 0.93) when compared with the reference group (PA below the median and sedentary index above the median). Therefore, the combinations of PA and sedentary behavior, such as television watching, could help to identify subjects at higher risk.
Several mechanisms have been proposed to explain the impact that increased PA has on depressive mood, such as increased sense of self-esteem, diversion from negative thoughts, perception of control and mastery (40
), increased circulating beta-endorphin (42
) and monoamine (43
) levels, alterations in hypothalamic-pituitary-adrenal axis (45
) and brain plasticity, and neurogenesis enhancement (46
). Although PA has beneficial effects on cardiovascular health, in our analyses, the inverse relation between PA and depression seemed independent of other beneficial effects on cardiovascular health. In their recommendation for older adults, the American College of Sports Medicine and the American Heart Association indicated that all adults aged 65 years or older need moderate-intensity aerobic PA for a minimum of 30 minutes on 5 days each week or vigorous-intensity aerobic activity for a minimum of 20 minutes on 3 days each week (47
). About 42.8% of our participants in 2004 did not met these American College of Sports Medicine/American Heart Association recommendations, whereas this rate was 47.9% for all US women and 63.7% for those older than 65 years of age (in 2005) (48
). A sedentary lifestyle has become pervasive in US society, as reflected by the 2009 Nielsen’s Three Screen Report (49
), which indicated that the amounts of time spent watching television and Internet and mobile video were escalating among Americans. On average, Americans watch television at home approximately 153 hours/month (approximately 5 hours/day) (49
). Given the relation observed between television watching and depression risk, public health campaigns should promote replacement of these sedentary behaviors with PA. Substantial health benefits can be gained by even convenient activities such as walking at an average pace or higher. When conducting future research in this area, investigators should consider taking repeated measurements of both sedentary behavior (television watching and computer use) and PA.
The major strengths of the present study included its large sample size, prospective design, and repeated measures of PA and other covariates. The use of our validated PA questionnaire in 5 assessments over a period of 8 years was another unique strength of our study. This study also had limitations, and the results should be interpreted with caution. Some outcome misclassification bias was inevitable because of a combination of errors in reporting depression or antidepressant intake, low depression recognition by physicians (50
), undertreatment of depression (51
), and prescription of antidepressants for indications other than depression (e.g., neuropathic pain (52
), premenstrual syndrome (53
), and hot flushes (54
)). Bias could have resulted if this misclassification was related to PA or television watching. For example, the inverse association between PA and depression could have been underestimated if women with lower socioeconomic status were both less active and, if depressed, less likely to be diagnosed with depression. The robustness of the results after adjustment for socioeconomic status, however, suggests that bias from this source is likely to be modest. Exposure misclassification could also be likely, particularly for television watching, which was assessed only in 1992 and might not accurately represent the total television-watching time during follow-up. Because women with subclinical depression might be more prone to develop clinical depression, a spurious inverse association between PA and the risk of depression could also be observed if women with subclinical depression reduced their PA level. Moreover, because of lack of information on depression history, we were not able to discern whether our incident cases were first onsets, as late-onset depression mainly occurs among people who have already been diagnosed with other disorders (55
). Further, our results might not be generalizable to younger women or men, who were not included in this study. Our incidence is not directly comparable to that observed in unselected populations because to minimize reverse causation, we excluded women with severe depressive symptoms at baseline, thus artificially selecting a healthier population and eliminating a group of women at higher risk of depression. Last but not least, the observational nature of the present study cannot prove a causal relation between television watching or PA and depression. For example, genes involved in regulation of brain monoamides (dopaminergic monoamides, norepinephrine, and serotonin) have been suggested as likely candidates to affect both voluntary leisure-time regular PA and depressive mood (26
). In conclusion, our results in this large cohort of older women indicate that regular PA and lower television-watching time may contribute to a reduced risk of depression.