In the present study, we determined the effects of objectively measured light-intensity lifestyle activity and sedentary time on the prevalence of MetS in Japanese adults. The major finding of our study is that both light-intensity lifestyle activity and sedentary time are significantly associated with MetS, independent of MVPA. Most of the earlier studies investigating the relationship between physical activity and MetS focused on self-reported MVPA in leisure time [6–9]. To prevent lifestyle-related diseases, the EPAR2006 recommends that adults engage in 23 METs-h/week of MVPA . However, for most people it is difficult to achieve this level of MVPA in leisure time. In 2009, Shibata et al. reported that only ~30% of Japanese adults engaged in the recommended level of MVPA . On the other hand, several recent studies have revealed that lifestyle activity in daily life is associated with health benefits in adults [15–17]. For example, Camhi et al.  reported that the accumulation of light-intensity lifestyle activity measured using an accelerometer is independently associated with lower odds for certain cardiometabolic risk factors. Therefore, the results of this study and previous studies suggest that public health messages and guidelines should be updated to promote light-intensity lifestyle activity and decrease sedentary time, alongside increasing MVPA.
Our results are consistent with those of another study in which higher light-intensity activity was associated with decreased 2-h plasma glucose in Australian adults without diagnosed diabetes . Another study has noted that light-intensity lifestyle activity measured using an accelerometer is associated with cardiometabolic risk factors . The results of the present and other studies suggest that accumulating light-intensity lifestyle activity could be a convenient approach to increase physical activity for the prevention of MetS. For example, increasing lifestyle activities, including light-intensity activities (e.g., slowly walking around the home, store or office) [18, 19], may be an option to increase overall physical activity to help prevent the MetS. Although lifestyle activities may not be vigorous in terms of aerobic effort, they do account for a considerable proportion of the total daily activity, and represent important sources of energy expenditure [20, 21]. These activities may also contribute to the amount of daily activity required to improve health. Although MVPA is an important component of the public health message for preventing MetS, interventions aimed at replacing sedentary behavior with light-intensity activities may be more successful, particularly since ~70% of Japanese people do not conduct adequate amounts of physical activity to realize any health benefits . However, it is important to consider that a greater amount of light-intensity lifestyle activity may be required to prevent MetS, as compared with MVPA .
In this study, we also determined the impact of sedentary time on the prevalence of MetS. The prevalence of MetS was approximately 2.27-times higher among in the highest tertile of sedentary time versus the lowest tertile. These results are consistent with those of earlier studies that examined the associations among sedentary time, obesity, MetS and mortality [22–25, 37]. Several factors could explain this association between sedentary time and MetS. First, sedentary time may independently predict adverse health conditions. Ford et al.  and Bertrais et al.  also reported significant associations of television viewing time and duration of computer use with MetS, independent of MVPA. Furthermore, several studies have shown that objectively measured sedentary time is associated with MetS risk factors [26, 27]. Thus, dual approaches may be necessary to increase someone’s physical activity or reduce sedentary time. Second, the impact of sedentary time on the risk of MetS may be the reverse of that of light-intensity activity. However, a common trait of these studies, as described above, did not estimate light-intensity activity, which may confound the association between sedentary time and health outcome. Consistent with previous studies, we found only weak significant association between sedentary time and MVPA (R2 = 0.244), but sedentary time was more strongly associated with light-intensity lifestyle activity (R2 = 0.78). The proportion of participants in the highest tertile of lifestyle activity increased from 2.5% to 15.2%, and 79.2% showed a change in category from the highest to middle or lowest tertiles of sedentary time (P < 0.05, data not shown). Moreover, when we statistically controlled for differences in objectively measured light-intensity lifestyle activity, the association between sedentary time and MetS lost its significance in the logistic regression model (P > 0.1, data not shown). Accordingly, the results of our study suggest that the effects of sedentary time on the risk of MetS are at least partly due to differences in light-intensity lifestyle activity.
Strengths and limitations
The important strength of our study is that physical activity and sedentary time were objectively measured using triaxial accelerometers. However, accelerometers are not sensitive to all activities such as biking, standing and upper-body movement. The Sedentary Behaviour Research Network (SBRN) defines sedentary behavior as any waking behavior characterized by an energy expenditure of ≤ 1.5 METs while in a sitting or reclining posture . However, in this study, we defined as sedentary time and light-intensity activity based on intensity levels (METs) as mean values of 1-min epochs. Therefore, it is possible we misclassified some instances of light-intensity activity, sedentary time and non-wear time.
A major limitation of this study is that we cannot infer causality or specify the direction of the effect because of its cross-sectional design. The prevalence of MetS in our subjects (men: 34.6%; women: 16.4%) was higher than that in a general population of Japanese individuals aged 30–69 years (men: 26.3%; women: 8.3%) included in the National Health and Nutrition Examination Survey (2009) . Furthermore, the proportion of individuals who achieved the recommended MVPA level was higher in our study (52.2%) than in the general Japanese population (30.1%) . Therefore, our current findings may not be generalizable to the Japanese population. In this study, the mean sedentary time was approximately 2–3 h/day lower in our participants than that reported in study performed in the USA and Australia [22, 27, 39]. This may also represent a limitation of our study, although this may also be due to differences in accelerometer wear time, cut-off values for sedentary activity, and the methods used to enrol the participants.
Although we controlled for several covariates, there may be residual confounding by other variables, such as sociocultural factors, family history of metabolic diseases, alcohol intake and physical fitness, which may partly explain our findings. Nevertheless, we took into account factors directly associated with the prevalence of MetS including MVPA, calorie intake and smoking [40, 41]. Despite these limitations, our findings are important from a public health perspective, particularly in terms of designing interventions aimed at promoting physical activity by focusing on lifestyle activity among middle-aged Japanese individuals.