In this study, a positive relationship was found between SES and BMI both among middle and old men and women, same with some literatures in developing countries. Mocanu V pointed that a positive relationship of SES and BMI was existed in China and Russia. However, in developed countries, there was an inverse relationship between SES and BMI. Kilson Moon et al. found a negative relationship in German adults and a negative relationship between SES and BMI was found in Europe and American[59, 60]. The possible explanations for the positive relationship between SES and BMI in this research maybe the lifestyle and nutritional determinants attaching with rising SES, such as decreased physical activities, sedentary lifestyle, altered eating patterns, and increased fat content of the diet. What’s more, individuals with higher SES may have the widespread ownership of televisions, computers and cars, and the supermarket system of high calorie food, which caused the higher BMI[62, 63].
The mediation role of lifestyles was only existed in women, but not in men. Women with high SES had a lower level of physical activity, which was related to a higher BMI. The inverse relationship between SES and physical activity may be caused by occupation of the participants. 68.7% of the women’ occupation was agriculture which leaded to a high level of physical activity and higher level of physical activity always linked with more energy consumption and a lower BMI.
Women with high SES had a longer sleeping time, which was related to a higher BMI. In this study, age, education and income were the SES indicators and the younger age, higher education, higher income linked the higher score of SES. Bliwise found that sleep duration often declines across the lifespan and Rachel P pointed that short sleep duration had been associated with lower education and income, so these may lead people with higher SES associated with the longer sleep time. Kristen L. Knutson found that long sleep was examined in relation to obesity. Westerlund et al. also found that shorter sleep durations during nights were associated with an increased consumption of high calorie foods and then lead the lower BMI. What’s more, most studies reporting positive associations rely on self-reported measures of sleep, indicating that long sleepers may be reporting more time in bed but not more time spent asleep, and in our study the sleep time was self-reported. Moreover, some research found that the association between less sleep and weight gain may be more consistent among children and weaken with age. However, many studies had the converse conclusion that less sleeping time contributed to the weight gain[50, 70, 71]. In their studies, sleep reduction or deprivation may affect the vary of the hormones (like leptin and ghrelin) and then disturb the metabolism and energy balance of our body, and lead to the weight gain.
Women with high SES smoked less, which was related to a higher BMI. Many researches had the same conclusion with our study. Cornelie Nienaber-Rousseau et al. found women in Europe and South Africa smoking less associated with higher BMI and Kilson Moon et al. found that smokers had a lower BMI than non-smokers. Mechanisms for a possible relationship between smoking and a lower BMI may include as following: firstly, maybe the reduction of appetite lead the metabolic efficiency increased or the caloric absorption decreased [27, 74]. Secondly, smokers were more likely to choose smoke instead of consuming desserts. Thirdly, people with higher BMI may give up smoking due to their poor health then increased the BMI among non-smokers. While Alicia García-álvarez et al. found that current smoking especially high intensity have been associated with increased risk of weight gain. A possible mechanism they gave for a greater BMI among high intensity smokers was the higher fasting plasma cortisol concentrations seen in smokers as compared to non-smokers, which was strongly associated with visceral adipose tissue (VAT) and in turn strongly associated with BMI, and the higher cortisol concentrations could be a consequence of the stimulation of sympathetic nervous system activity that is induced by smoking.
Among women, the association between SES and BMI was partly mediated by lifestyles. Women with higher SES were more likely to have healthy habits. Comparing to the lower SES group, women with higher SES were more likely concentrated on the health related information and easier to adopt new lifestyle related behaviors since they were more likely to afford the high cost of newer, exotic habits and had better access to high-quality healthcare and health recommendations. Thus over time, people with higher SES were more likely to develop healthier behaviors like reduction of tobacco smoking and the sufficient sleep and avoid harmful ones comparing to people with lower SES[77, 78]. Besides of lifestyles, sex itself (sex hormones affect the amount and distribution of body fat) was also a factor influencing body composition, its oxidation and mobilization, which can cause more storage of fat in women than that in men. What’s more, gave birth to a child was associated with increased long-term central obesity and higher BMI for women,but this never happened in men.
While among men, SES was not significantly related with lifestyles, and only the smoking was positively related with BMI, which meant the smokers were less likely to have excess BMI. Comparing to women, the rate of smoking in men was significantly high, up to 54.6% and the rate of smoking cessation was 23.1%, which may make the influence of SES not significant on smoking. What’s more, the variability of sleep time among men was small, and then may lead the difference between sleep time and SES among men not significant. Moreover, comparing to men, women of all ages were displaying more weight-related concern, regarding eating habits, body weight, and physical appearance which may make the distribution of lifestyles between SES and BMI among men was not significant. Furthermore, other lifestyles like the intake of energy didn’t include may lead the mediation of lifestyles among men not significant.