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Introduction: Numerous studies have reported a strong inverse association between BMI and physical activity in western populations. Recently, the association between BMI and physical activity has been considered bidirectional. This study aimed to examine the associations of body mass index (BMI) with physical activity and sedentary behavior and to explore whether those associations were modified by socio-demographic characteristics., Materials and Methods: We conducted a multistage random sampling survey in three districts of Hangzhou, China, in 2012. The International Physical Activity Questionnaire long form was used to collect data regarding physical activity and sedentary behavior. A multilevel mixed-effects regression model was used to assess the associations of BMI with physical activity and sedentary behavior., Results: A total of 1362 eligible people (624 men and 738 women, ages 23-59 years) completed the survey. People who are young or middle-aged and have the highest education level are the most inactive. Significant differences in the associations between physical activity and BMI across socio-demographic groups were identified (sex*BMI, P=0.018; age*BMI, P<0.001; education level*BMI, P=0.030). Women or individuals older than 50 had a higher level of physical activity with increasing BMI. There was no statistically significant association between BMI and sedentary behavior (P=0.450)., Conclusions: The associations between BMI and physical activity were modified by sex, age, and education level in Hangzhou, China.

With increasing family income, the prevalence of overweight has risen and become a serious threat to individual health and a major public health challenge in China. This study attempts to shed light on the mechanism of income impact on the adult health outcomes of BMI and overweight through five potential channels: nutritional intakes, dietary diversity, dietary knowledge, food preference, and dining out. Using the panel data from the China Health and Nutrition Survey (CHNS), we investigate the causal relationship between income and health by considering the changes in the minimum wage as a valid instrument to address the endogeneity problem of income in health estimation. The results indicate that rising income increases the adults' BMI and the propensity to be overweight; approximately 15.58% and 16.20% of income impact on BMI and overweight could be explained by the five channels considered, respectively. Among the five channels, dietary diversity plays the most significant role in explaining the income impact. We observe significant heterogeneity in income-BMI gradients across various income quantiles and subsamples. Specifically, income-BMI gradients tend to increase along with income percentiles, and income has a significantly positive impact on BMI and overweight for the male sample but it shows no significant impact for the female sample. • Five important mechanisms of income impact on BMI and overweight are highlighted. • The endogeneity of income is addressed by using minimum wage as an instrument. • Rising income increases the adults' BMI and the propensity to be overweight. • Dietary diversity plays the most significant role in explaining the income impact. [ABSTRACT FROM AUTHOR]