The hypothesized associations between overall physical activity dose and odds of depressive symptoms were not found in this study, as there were no significant associations between total walking, moderate, vigorous or overall physical activity (i.e. across all domains) and odds of depressive symptoms. These findings imply that it may not be the physiological effects of physical activity that are important in reducing the odds of depressive symptoms, since, if this were the case, a stronger association of odds of depressive symptoms with total dose (across all domains) of physical activity would have been expected.
The only domain of physical activity found to be inversely associated with odds of depressive symptoms was leisure-time activity. When controlling for age, marital status and physical health, those who reported more than 31/2 hours of total leisure time physical activity per week (summed across intensities) had lower odds of depressive symptoms when compared to those who undertook less than this. In fact, no significant associations were found between undertaking less than 3 1/2 hours of total weekly leisure-time physical activity and odds of depressive symptoms, suggesting that a high duration of leisure-time physical activity may confer greater mental health benefits. This finding supports the dose of physical activity recommended in the US, UK and Australian national physical activity guidelines which suggest a minimum duration of physical activity equivalent to 2 1/2 to 3 1/2 hours per week (at least 30 minutes on most, if not all days of the week [17
]. This is consistent with findings from several observational [8
] and intervention studies [52
] which found the public health dose of exercise to be strongly associated with a lower risk of depression. The Australian national physical activity guidelines are based largely on evidence concerning benefits for physical health. These findings suggest they could be extended to mental health. Although other studies have also found a shorter duration of physical activity to be associated with lower risk of depression [14
], these studies used measures of physical activity and risk of depression that differed from the current study. For example, unlike the current study, the method used by Hassmen et al [19
] was unable to determine different intensities or domains of physical activity.
When examining different intensities of leisure-time physical activity, the results showed reduced odds of depressive symptoms amongst women in the highest tertiles of moderate- and vigorous-intensity leisure-time physical activity, but those performing vigorous-intensity physical activity showed an even greater reduction in odds. These findings are supported by previous studies that found a strong association between undertaking higher durations of moderate-intensity [9
] or vigorous-intensity [51
] physical activities and odds of depression. Findings from the current study suggest that the greater the dose (intensity and duration) of leisure-time physical activity, the lower the odds of depressive symptoms. However, there was also an inverse trend approaching significance between depressive symptoms and walking, suggesting that vigorous-intensity physical activity may not be necessary to achieve the reduced odds of depressive symptoms associated with physical activity. Targeting physical activity of specific intensities perhaps guided by personal preference and enjoyment may be a point of consideration when prescribing or recommending a dose of physical activity to confer mental health benefits, as this may result in an increased likelihood of maintenance of the training program and consequently maintenance of reduced depressive symptoms [55
]. However, given there were strong inverse associations of depressive symptoms with vigorous activity, women should be encouraged to include at least some activity of this intensity in their weekly physical activity routine if possible.
Several researchers have hypothesised that physical activity reduces risk of depression through physiological pathways – for instance, exercise may activate endorphin secretion, which reduces pain and produces a euphoric sensation [1
]. However, the finding that leisure-time physical activity was the only domain associated with odds of depressive symptoms suggests some alternative possible explanations, such as a sense of enjoyment or a perceived control or choice when undertaking activities in leisure-time that may be lacking when physical activity is undertaken in domains other than leisure-time (i.e. work-related, domestic, and transport-related). Alternatively, the finding that different domains of physical activity were differentially associated with odds of depression could be attributed to recall difficulties, as leisure-time physical activity may be more accurately recalled than physical activity undertaken in other domains [56
]. This finding is in contrast to previous observational studies [24
] that did find weak correlations between physical activity undertaken in other domains (domestic and transport) and likelihood of depression. However, it was difficult to interpret the results from those studies in terms of optimal domain of physical activity for reducing likelihood of depression, since in these studies, domain was simply equated with intensity (eg. domestic physical activity = low-intensity physical activity), yet physical activities undertaken from the domestic domain are not always of a low intensity, and not all low-intensity physical activity is necessarily domestic in nature [57
]. Few studies have examined the association between multiple domains of physical activity and likelihood of depression. However, concurrent with findings in this study, one previous observational study that compared leisure-time physical activity with domestic and work-related physical activity, found only leisure-time physical activity to be inversely associated with depression [26
The findings of the current study are generally consistent with the social interaction hypothesis, which posits that the improvements in mental health following exercise are at least partly related to the mutual support and social relationships that are provided when participating in physical activity [27
]. In the current study, lower social support for physical activity (eg. being discouraged by others) was associated with higher odds of depressive symptoms, whilst greater social support (eg. being often physically active with a family member) was associated with lower odds of depressive symptoms. These findings, however, are not consistent with one previous study of men and women that suggested that physical activity was associated with lower odds of depressive symptoms regardless of the social format [22
]. The sample in that study differed from the current study (i.e. included both men and women), which may explain the contrasting results; women may value and benefit more from the social aspects of physical activity more than men [58
]. In the present study, no association was seen between being physically active with a friend/colleague, having no-one to exercise with or having a dog to walk regularly and odds of depressive symptoms. It may be that the support for being physically active from family is more important for mental health among women than support from other sources.
Several limitations of this study should be acknowledged. Firstly, the cross-sectional nature of the study did not allow us to determine the causality or direction of relationships. For example, those without depressive symptoms may be more capable and motivated to participate in physical activity. We were also unable to determine whether the association between support for physical activity and depressive symptoms is due to the social aspect of the physical activity, or due to having a generally more supportive social network. Further longitudinal and intervention studies are needed to confirm the associations observed here, and also to investigate the mechanisms by which physical activity might protect against risk of depression. Such hypothesized mechanisms include 'distraction', whereby improvements in mental well-being following exercise are due to diverting negative thoughts and unpleasant stimuli during the activity [1
]; or 'mastery', whereby improvements following physical activity are due to achieving goals and a feeling of success [1
]. This study relied on self-report measures which possess the potential for error in judgement, recall difficulties and the possibility of socially desirable responses. Nevertheless, measures of depressive symptoms, physical activity and social influences were valid and widely accepted [35
]. Finally, the response rate for the physical activity survey was only 43.5%, whilst the response rate from those who received the diet questionnaire first was 21.2%. This is a potential limitation of our study as participants who responded may be more interested in physical activity and possibly be more active than non-respondents. However, comparison of physical activity levels of the women in this study with those of Australian women reported elsewhere [46
] showed similar levels of activity across the two samples.
Strengths of this study include the large, population-based sample of participants who were generally representative of Australian women in terms of both their physical activity levels and depressive symptoms [46
]. Further, 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, and few have examined the social context of physical activity and its association with odds of depressive symptoms. Therefore, this study provides a novel perspective on the optimal dose, domain and social context of physical activity that may confer mental health benefits in women.