In this cross-sectional study, we found that total physical activity mainly composed of occupational activity, was strongly inversely associated with fasting insulin and insulin resistance. More importantly, we further assessed the independent association between different types of physical activity and HOMA for the first time in Chinese population. We identified a negative association between occupational physical activity and insulin resistance, which was found to be robust since results remained consistent throughout the analyses independent of potential confounders. In addition, transportation and domestic physical activities were also observed to be negatively associated with insulin resistance despite of relative weak association. There was a joint effect of low physical activity and short sleep in relation to insulin resistance.
In our study, non-exercise physical activity, especially occupation activity, contributed vast majority of total physical activity. Such physical activity pattern are similar in developing countries [10
], but different from that in developed countries [37
]. Therefore, it is necessary to analyze the independent association between individual components of physical activity and insulin resistance.
A number of studies have consistently demonstrated the inverse association between physical activity and insulin resistance [7
]. Physical activity was independently associated with HOMA in both sexes among adolescents from Europe [38
] and in US [7
]. Physical activity could reduce insulin resistance and improve insulin sensitivity in adults [40
]. Similar results were found in Iranian adults, together with the result that work-time physical activity had a higher contribution to such an association [10
]. Our results not only confirm these previous findings, but also extend the four types of physical activity both separately and combined in relation to insulin resistance. A dose-response association was also detected in different types of physical activity.
Domestic physical activity and transportation activity was also shown beneficial effect for health [8
]. Hu et al.
] found that moderate and high physical activities including commuting physical activity independently and significantly reduced risk of Type 2 diabetes among the middle-aged general population. Again, he found that daily commuting to and from work reduced the risk for total and CVD mortality among patients with type 2 diabetes [9
]. Esteghamati [10
] reported a significant negative relationship between commuting activity and insulin resistance. Domestic physical activity was also observed having gender-specific effects on health indicators in Europe [42
]. Negative associations between transportation/domestic physical activities and insulin resistance, despite of relative weak magnitude, were also observed in current data, which was consistent with what they found to a large degree.
Unexpectedly, a positive association was found between leisure physical activity and HOMA both in crude model and the model after adjustment for age and gender in our study. It can possibly be explained by that persons performing more leisure physical activity would also do less occupational physical activity. As we can see, in the higher quartiles of HOMA, where the leisure time physical activity was high, the occupational physical activity was lower. The mean occupational activity among those with leisure physical activities was significantly lower than those without leisure physical activities (50.1 vs. 132.1 MET-hrs/week, p
< 0.001). Therefore, it is not the high leisure physical activity that is associated with the high HOMA but rather the high leisure time activity might be reflective of lower occupational activity which itself was associated with a high HOMA. Furthermore, people who were diagnosed with chronic diseases could have purposefully increased leisure time activity or exercise which could not be detected in our study due to cross-sectional nature. It has been shown that leisure physical activity is an effective method to reduce the risk of insulin resistance and impaired glucose tolerance [43
], type 2 diabetes [8
], obesity [42
], the risk of metabolic syndrome, and all-cause mortality [44
Physical activity is now recognized as a key component to lifestyle modification strategies. In US, the Centers for Disease Control and Prevention and the American College of Sports Medicine recommend that every US adult should accumulate 30 min or more of moderate-intensity physical activity on most, preferably all, days of the week [45
]. More recently, the Federal government has issued the "2008 Physical Activity Guidelines for Americans" [46
], which is more flexible for everyone to follow. The China Diet Guide (2007) issued by China Ministry of Health also recommends that an adult should have body activity equivalent to or more than accumulative 6000 steps every day. It is better to have moderate activity for 30 min if a person is in good shape [47
To the extent that non-exercise physical activity (occupational, domestic and transportation) constituted a major source of physical activity, it is important to consider the synergistic effect by combining them together when developing relevant intervention strategies [48
]. Particularly, it appears that occupational physical activity has a great potential to decrease insulin resistance and therefore to reduce the risk of adverse health outcomes, especially in developing countries like China.
Short sleep duration was related to impaired glucose tolerance, and type 2 diabetes [16
]. It may be partially indicative of psychological stress, while the latter is also regarded as quasi-indicator of insulin resistance. We found a similar association in this study. Interestingly, a combination of low physical activity and short sleep duration had the highest odds of insulin resistance among all combinations, which had not been reported before.
The present study has several mentionable strengths. Firstly, the use of concept of metabolic equivalent (MET)-hours-per-week generated more precise estimation which takes both intensity and time spent on different types of activity into consideration. Second, different types of physical activity were assessed separately and independently. The associations found are more reliable since they were controlled for numerous confounders including diet, demographic factors, physiochemical parameters, and so on. Third, the questionnaires used in the study were developed with a standard method [29
] and a Chinese language version was used.
The main limitation of our study is its cross-sectional nature, as was previously stated. Recall bias may exist in our study due to the use of questionnaires. Additionally, the "gold standard" for evaluating insulin resistance is the euglycemic hyperinsulinemic clamp method [49
]. Due to its invasive nature, cost, we prefer to use HOMA instead in such a large epidemiological study. However, HOMA has been globally used as a reliable surrogate method in measuring insulin resistance [50