Leisure-Time, Occupational, and Commuting Physical Activity and Risk of Type 2 Diabetes in Japanese Workers: A Cohort Study

Toru Honda; Keisuke Kuwahara; Tohru Nakagawa; Shuichiro Yamamoto; Takeshi Hayashi; Tetsuya Mizoue

Disclosures

BMC Public Health. 2015;15(1004) 

In This Article

Discussion

In this large study of Japanese workers, engagement in vigorous-intensity exercise alone or vigorous-intensity exercise combined with moderate-intensity exercise was associated with a reduction in the risk of type 2 diabetes not only for doses above the recommended dose of physical activity but also for doses below the recommended level. In contrast, moderate-intensity exercise alone was not associated with a risk of type 2 diabetes even above the recommended dose of physical activity. This is the first study to compare the diabetes risk between individuals who engaged in vigorous-intensity exercise alone and individuals who engaged in moderate-intensity exercise alone. This is also among a few studies examining the relationship between occupational and commuting physical activity with diabetes.

A recent meta-analysis of cohort studies[9] suggested that risk reduction of diabetes is greater for vigorous-intensity activity compared with moderate-intensity activity. Our results of moderate-intensity and vigorous-intensity exercise dose (Additional file 1: Table S1 http://www.biomedcentral.com/1471-2458/15/1004/additional) agree with the results of meta-analysis. However, this type of grouping used in the previous studies[9] has a limitation because individuals who engaged in moderate-intensity exercise might also perform vigorous-intensity exercise. To minimize the possibility of mixed results, we assessed the risk among individuals who engaged in moderate-intensity exercise alone or those who engaged in vigorous-intensity exercise alone (Table 3). Then, diabetes risk associated with vigorous-intensity exercise alone tended to decrease whereas that associated with moderate-intensity exercise alone did not. Our results indicate that engagement in vigorous-intensity but not moderate-intensity exercise can reduce the risk of type 2 diabetes in Asian individuals.

Although it has been suggested that any increase in the dose of physical activity from a sedentary level can reduce the risk of type 2 diabetes,[7] only two US cohort studies addressed this issue. In the two studies, Japanese American men[13] and Black women[16] who engaged in <60 min of vigorous-intensity exercise per week had a slightly lower risk of diabetes (6 % and 10 %, respectively) compared with individuals who did not engage in any vigorous-intensity exercise. Again, these findings are limited due to the potential mixing of activities with different intensities. In the present study, engagement in vigorous-intensity exercise alone and both moderate- and vigorous-intensity exercise below the recommended dose (7.5 MET-hours per week) had an approximately 30 % lower risk of diabetes compared with no exercise. Although the definition of vigorous exercise and the magnitude of association differ among studies, our results together with previous findings[13,16] support the view that vigorous-intensity exercise can reduce the risk of type 2 diabetes even when individuals engage in vigorous-intensity exercise below the recommended dose. Further studies are needed to identify the minimum amount of vigorous-intensity exercise for type 2 diabetes prevention.

In the present study among Japanese workers, moderate-intensity exercise alone was not associated with a lower risk for type 2 diabetes at even high doses. This finding disagrees with a meta-analysis of cohort studies,[9] which were mainly performed in Caucasians, that demonstrated a significant inverse association between moderate physical activity and the risk of type 2 diabetes. However, as mentioned above, the approach for classification used in the previous studies has a limitation. This may partly explain the differences between ours and previous finding. In addition, cohort studies among Japanese[19] and Japanese American individuals[13] did not detect inverse associations for moderate-intensity activity. Given the limited evidence in Asians on the association between moderate-intensity exercise and the risk of type 2 diabetes[13,19] and a finding of decreased HbA 1c level after moderate-intensity exercise among normal weight and obese Japanese women,[30] which may be inconsistent from ours in that moderate-intensity exercise may prevent type 2 diabetes, further investigations are needed in this population.

Of the 18 items of leisure-time exercise, jogging, aerobics, tennis, soccer, and swimming were associated with a large risk reduction. In addition to these activities, we recently reported a significant risk reduction associated with strength training in detail from the same population.[31] Caution should be exercised in the interpretation of these findings because small proportion (<10 %) of workers engaged in each of activity, leading to unstable point estimates with wide confidence intervals.

We found that occupational physical activity was not associated with diabetes. This null finding is supported by three studies among Taiwanese,[32] Chinese,[20] and Japanese American.[13] In contrast, a Finnish study[33] reported a significant risk reduction associated with high occupational physical activity. In addition, a Japanese study[34] showed an inverse association of occupational physical activity with impaired glucose status, although the reduction was not statistically significant. We do not have a plausible explanation for inconsistent finding, however, differences in work load or socioeconomic status across the studies may partly explain.

The mechanisms of differential effects of physical activity according to the intensity level on the risk of type 2 diabetes remain unclear, but there are some possible explanations. Intervention studies among young women[35] and elderly adults[36] showed that high-intensity exercise reduced more visceral fat than moderate-intensity exercise. An intervention study among obese adults showed that the reduction in visceral fat through lifestyle modification was followed by increases in the levels of adiponectin,[37] which is a protein that improves insulin sensitivity.[38] In another study among obese adults,[39] a single session of high-intensity exercise showed a greater reduction in the postprandial glucose level than moderate-intensity exercise. Vigorous-intensity exercise may be effective for maintaining normal glucose metabolism for East Asians, who have a higher postprandial glucose level compared to Caucasians.[40–42]

The present study has some strengths, including a large dataset of workers from a large company in Japan and the annual follow-up assessments with data on blood glucose and HbA 1c . In most previous studies, incident cases were identified based on self-reports,[11,16,20,23,43] which resulted in the under-detection of incident cases. There are several limitations in our study. First, the physical activity questionnaire used in this study was not validated. Nonetheless, the activity questionnaire in the present study is similar to the questionnaire for leisure-time physical activity used in the Shanghai Women's Health Study[20] in which exercisers were asked to report details for up to three types of exercises/sports (i.e., type, hours/week, and years of participation in each activity).[20] A validation study for the activity questionnaire in China[44] reported that energy expenditure of exercise estimated by the questionnaire was highly correlated with the energy expenditure of the physical activity log (r = 0.74). Second, we only used the physical activity information collected at baseline. The subjects may have changed their physical activity during the follow-up period. Nevertheless, such changes in physical activity would be non-differential and lead to an underestimation of the inverse association with diabetes risk. Third, unmeasured confounders (i.e., sitting time and diet) and residual confounders might have influenced the association. Fourth, the participants in the present study are young and middle-aged workers from a large company in Japan. Therefore, caution should be taken when generalizing our findings for a population with a different background.

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