Depression is a major cause of physical and psychosocial illness and all-cause mortality 1. Depression affects more than 340 million people worldwide 2, with women at particular risk 3. Conventional management of depression has typically involved treatment from clinical professionals such as general practitioners, psychiatrists, psychologists, and has included counselling and anti-depressant medication 4. However, in recent years research has focused on the role of physical activity as a potential component in the prevention and/or management of depressive symptoms or clinical depression 1.

Several reviews examining observational and intervention studies have assessed the relationship between physical activity and depression/depressive symptoms 567. These reviews generally draw a similar conclusion: that physical activity is positively associated with reduced likelihood of depression or depressive symptoms. However, little is known about the specific components of physical activity that are important, particularly the dose, domain or social context of physical activity that might confer mental health benefits.

Whilst numerous studies have explored the associations between the dose – particularly the frequency and/or duration of physical activity – and likelihood of depression, findings from these studies were varied. A number of observational 8910 and intervention studies 1112 concluded that higher levels of physical activity (eg. greater than 60 minutes of physical activity per week) were associated with lower odds of depression. In contrast, other studies 13141516 have found that even low levels of physical activity (eg. exercising as little as 30 minutes per week) was related to improved mental health. Another study 11 has concluded specifically that the commonly recommended 'public health dose' of physical activity (at least 30 minutes of moderate intensity physical activity on most, if not all days of the week) 1718 was an effective treatment for reducing depressive symptoms.

Conflicting results were also found in studies investigating the association between physical activity intensity and depression. Numerous longitudinal 9, cross-sectional 19 and intervention studies 2021 have found that participating in physical activity at a moderate-intensity was significantly associated with lower odds of depression whilst few studies have examined the relative strengths of associations between physical activity of moderate or high intensity and risk of depression 2223.

Most studies supporting an inverse relationship between physical activity and risk of depression have focussed on leisure-time physical activity. Very little research has investigated the association of depressive symptoms with other domains of activity, such as work-related, transport-related or domestic physical activity. Only three studies have been identified which specifically compared leisure-time physical activity with physical activity in other domains 242526, with all three suggesting leisure-time physical activity was more strongly associated with lower odds of depression than physical activity undertaken in any other domain (domestic, transport-related, work). To our knowledge, no previous research has compared leisure-time, domestic, transport-related and work-related physical activity with odds of depressive symptoms in the same study.

It has been hypothesized that the associations between physical activity and depressive symptoms may be due to the social support and interaction that comes with physical activity undertaken in a group or social setting 27. Little research, however, has examined this hypothesis. Only one intervention study specifically compared physical activity undertaken alone with that undertaken with others 22. It found that physical activity had a moderate effect on alleviating depressive symptoms regardless of the social context in which it was undertaken.

The aim of this study was to examine the associations between various components of physical activity (dose, domain and social context) and odds of depressive symptoms in women using data from a large population-based cohort of women. Due to the cross-sectional nature of this study, only participants' current levels of physical activity and depressive symptoms were examined. Since a dose-response relationship has been previously reported 11, it was hypothesized that higher doses of physical activity (similar to that of the public health recommendations) would be more strongly inversely associated with odds of depressive symptoms than would lower doses. It was also hypothesized that leisure-time physical activity would be more strongly inversely associated with odds of depressive symptoms than other domains of physical activity (eg. work-related, transport-related and domestic), since activities performed during leisure-time are generally more enjoyable and undertaken by personal choice. A third hypothesis was that social physical activities and social support for physical activity would be more strongly associated with lower odds of depressive symptoms than physical activity performed alone or without social support.

The analyses in this study were based on cross-sectional survey data collected formerly from 1,554 women aged between 18 and 65. Full details of the methods are described elsewhere 2829.

Table 1 presents the socio-demographic characteristics and presence of depressive symptoms among participants. The mean age of participants was 42 years. The majority of the women were born in Australia (75%) and were married or in a defacto relationship (65%). A total of 615 (41%) participants reported their highest qualification as completing year 12 (or equivalent) or an apprenticeship or certificate/diploma, with 566 (37%) having completed a university or higher degree. Over one-quarter of the women were employed in a professional occupation (27%) and the majority of women surveyed did not have children living at home (60%). These demographic characteristics are comparable to those reported in other population-based studies conducted in Melbourne and nationally 45464748. A total of 421 (30%) of participants were classified as currently experiencing depressive symptoms (according to the GHQ-30). This is not dissimilar to previous findings which reported that almost 24% of Australian women had been diagnosed with depression 49.

Table 2 shows the proportion of women experiencing depressive symptoms according to sociodemographic and physical activity variables from chi-square analyses. The only domain of physical activity to be inversely associated with the presence of depressive symptoms was leisure-time (p = 0.009). Women who reported walking or moderate-intensity physical activity in leisure time were less likely to be experiencing depressive symptoms (p = 0.027 and p = 0.048 respectively). For example, 22% of those who reported more than 1.5 hours of moderate-intensity leisure-time physical activity (i.e. the highest tertile) were currently experiencing depressive symptoms, compared with 31% of those reporting no moderate-intensity leisure-time physical activity.

Being discouraged from physical activity and having no one to exercise with were associated with an increased presence of depressive symptoms (p = 0.002 and p = 0.037 respectively), whilst being a member of a sporting/recreational club, having someone to walk with in the neighbourhood and being active with a family member were associated with a lower presence of depressive symptoms (p = 0.044, p = 0.037, and p = 0.011 respectively).

Table 3 shows the crude and adjusted OR and 95% CI from logistic regression models predicting odds of depressive symptoms according to physical activity variables. Results from the unadjusted logistic regression models predicting odds of depressive symptoms according to duration of physical activity (summed across intensities) within each domain, showed that compared to those in the lowest tertile of leisure-time physical activity (i.e. <0.67 hours/week leisure-time physical activity), those women in the highest tertile (>3.5 hours/week) had lower odds of depressive symptoms (p = 0.002). Adjusting for covariates had little effect on this association. No other physical activity domain was associated with odds of depressive symptoms in logistic regression models.