Background
Physical activity reduces mortality and the incidence of non-communicable diseases [
1,
2]. However, a large proportion of the population in Japan and in many other countries remains physically inactive [
3]. Promotion of physical activity is therefore a major public health concern globally. In Japan, the number of walking steps per day has decreased over time from around 1998–2000 [
4], and a health promotion policy is needed to increase physical activity again. To increase participation in physical activity, it is necessary to understand the factors that influence this, and to design relevant policies and effective interventions based on that understanding.
Socioeconomic position (SEP), including for example, household income and educational level, is a determining factor of health status, and it is particularly important to promote a healthy lifestyle in low SEP populations [
5]. In previous studies, lower SEP was associated with poorer health status and unhealthy behaviour among Japanese adults [
6‐
8].
Studies that examined the association between SEP and total physical activity have, however, been inconsistent [
9‐
13]. In a systematic review of European studies of the association between SEP and physical activity across the domains of work, travel and recreation, lower SEP was associated with greater occupational physical activity, but with less recreational physical activity [
14]. It is also important to examine the association between SEP and domains of physical activity (DPA) such as work, travel, and recreation, to identify lifestyle factors that can be targeted to increase physical activity [
15].
There is no comprehensive study examining the association between SEP and DPA among Japanese subjects. Japanese culture and lifestyles are different from those in Europe, so examining the relevant associations between socioeconomic position and physical activity in each domain of life in Japan is important for improving health promotion activity [
16]. The association between SEP and health behaviours varies according to gender [
7]; there are, however, few studies that have examined this [
14]. The aim of this study was therefore to examine the association between SEP and DPA in Japanese men and women.
Discussion
This study is the first to examine the gender-linked association between SEP and physical activity in Japanese adults. In accordance with the previous systematic review of European studies [
14], this study found various associations between SEP and different DPA in Japanese adults. There were also differences in these associations between the sexes. It is therefore important to take into account SEP, DPA, and gender when formulating proposals for the promotion of physical activity.
In work-related physical activity, the men with higher household income or educational status were less active than men with lower SEP. However, the relationship between SEP and work-related physical activity was not significant in women. Previous European studies had shown a consistent association between SEP and work-related physical activity in both sexes [
14]. This difference in this study may be related to a difference in the assessment of work-related physical activity. Assessment of work-related physical activity in this study included housework, while previous studies had excluded this [
22,
23]. Since 70.2 % of women in this study reported that they worked part-time (data not shown), most work-related physical activity for women in this study was likely to be housework. This could account for the lack of association between SEP and work-related physical activity in women.
In travel-related physical activity, the men with higher household income or educational status were more active than men with lower SEP. Also, the women with higher educational status were more active than women with lower educational status. However, full time employed women were less travel-related physically active than non-full time employed women The European review found no consistent association between SEP and travel-related physical activity in either sex [
14]. In a Japanese study, Ishii et al. [
24] reported that higher SEP was associated with more active commuting modes (e.g., walking, bicycle, public transport). In Japan, previous studies reported a significant association between the neighbourhood environment and travel-related physical activity [
16,
24]. The significant direct association between SEP and travel-related physical activity found in this study might therefore be affected by environmental factors. However, the reasons for the differences between men and women in this study are unknown, and further study is therefore required.
In recreational-related physical activity, both men and women who showed high household income were more active than lower household income workers. There was also a direct association between SEP and recreational physical activity in both sexes in the European review [
14], and earlier Japanese studies [
7,
25]. Leslie et al. reported that high SEP residents had easier access to parks than lower SEP residents and used them more [
26]. Psychological factors such as self-efficacy and self-control, and social factors, such as social interaction, mediate the relationship between SEP and recreational physical activity [
27‐
29]. This study did not indicate why there was no significant association between educational level and recreational physical activity, though there was a trend towards an association in both sexes.
In total physical activity, both men and women who showed high household income were more active than lower household income workers. Previous Japanese studies have reported inconsistent associations between SEP and total physical activity [
9‐
13]. Although high household income was associated with less work-related physical activity, this appears to be more than compensated for by greater travel and recreational physical activity.
This study had two major limitations. First, physical activity was not measured using an objective instrument. Objective measurements (using, for example, a pedometer) are generally recommended, but cannot provide data about physical activity in each domain. A self-report questionnaire was therefore used in this study to examine the association between SEP and the different domains of physical activity. It is therefore possible that the validity and reliability of GPAQ v2 is lower than an objective measure. However, GPAQ has been standardized and is used worldwide [
19,
30].
Secondly, this study may have a sampling bias. Participants voluntarily registered with the Internet research company [
31], and many Internet survey registrants expect an incentive from the research company. It is therefore possible that the study subjects were not representative of the local general population. This study may also have shown selection bias, since the response rate was lower in women, people who were younger and those with a lower income.
Conclusions
Although this study has some limitations, our results suggest that both men and women with higher household income are more active in recreational-related and total physical activity than the both sexes with lower income. However, the relationships between SEP and work or travel-related physical activities are different in gender. To increase physical activity in Japanese adults with lower SEP, it will therefore be important to focus on increasing travel and recreational physical activity. In a follow-up study, the mechanism of the association between SEP and DPA will be examined.
Acknowledgements
This study was supported by a Grant-in-Aid for Challenging Exploratory Research (No. 25560357) from the Japan Society for the Promotion of Science, and Global COE Program “Sport Sciences for the Promotion of Active Life” from the Japanese Ministry of Education, Culture, Sports, Science, and Technology.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MM, KH, and TA participated in the design of this study. MM performed the data mining and the statistical analysis, and contributed to the writing and editing of the manuscript. KH contributed to the statistical analysis and editing of the manuscript. TA assisted in the writing and editing of the manuscript. All authors read and approved the final manuscript.