This study intended to explore association between individual SES and obesity in a Chinese adult population. Based on descriptive statistics, we found two prominent features of obesity distribution in study population. One is that, by either WGOC or WHO cut-offs, the age-and-sex standardized prevalence of general obesity in study population was considerably lower than the average level of western countries (which normally ranges from 10% to 25%), and such phenomenon has been repeatedly observed by previous Chinese studies [21–23]. The second is that, a considerable discrepancy has been found between the prevalence of general obesity and abdominal obesity in study population, say 7.6% (95%CI: 7.2%-8.0%) compared with 11.9% (95%CI: 11.4%-12.4%). We think several explanations can be made on this difference. First of all, it has been well proved that BMI can not provide information on the distribution of body fat , so theoretically, a certain level of discordance could exist between two indexes. Secondly, many previous studies have suggested that compared with Caucasians, Asians have a more central distribution of body fat under a given BMI [25, 26], which can magnify the numerical difference between two obesity rates. Thirdly, based on literature review results, from the year 1993 to 2009, although all kinds of obesity were observed significant increases among Chinese mainlanders, the rise of abdominal obesity was the most staggering , such unbalanced growths can further widen prevalence gap between general obesity and abdominal obesity in Chinese populations.
In their exhaustive reviews on association between SES and obesity, Sobal  and Mclaren  concluded that, in developed societies, SES was inversely related to obesity in women, whereas in men, such association was more likely to be non-significant. In developing countries, a consistent positive correlation between SES and obesity was expected in both genders. After controlling for possible influencing factors, we found that in study population, there were significant disparities in SES-obesity association between two genders when measured by different SES indicators. In women, both general and abdominal obesity were inversely associated with education, but exhibited no significant correlation with income. In men, on the contrary, a positive association was identified only between income and obesity. Such findings indicated that the prosperity of local economy in Zhejiang province has already accelerated the transition of SES-obesity relationship from “developing country pattern” into “developed country pattern”.
It has been concluded that along with social economy development, at first the positive SES-obesity association will be attenuated, then, the negative association will gradually take a hold . During the past few years, in several developing countries such as Thailand and the Philippines, a similar atypical SES-obesity relationship has also been found [11, 28], which revealed that SES indicators, especially income level, were more often positively related to obesity in men, while in women, null associations were usually observed. Such results suggested that the transition of SES-obesity association in women was comparatively faster than that in men. We think Bourdieu’s “habitus theory”  plays a vital role in this gender-related difference. To women, a thinner figure is socially valued, and such notion can ultimately transform into formidable impetus in keeping fit figure. Some previous studies have found that, even in obesity-promoting environments, such impetus can still successfully offset the collective effect of other proponents, eventually preventing women from obesity . Thus it is not surprising to find an expedited transition of SES-obesity relationship in women. But to men, a larger body size is usually valued as a sign of physical dominance and prowess, therefore social culture is comparatively tolerant to male obesity. Under such circumstance, it is much less imperative for men to pursuit thinness.
In model fitting process, after the inclusion of multiple lifestyle factors, we only discerned a minute attenuation of SES-obesity association among men, which indicated the limited and gender-specific effect of lifestyle factors as the possible intermediate variables in SES-obesity association. Such result was quite different from a previous Sweden study , which found a considerable proportion (18%-29%) of SES-obesity association can be explained by lifestyle factors, although the lifestyle factors in two studies were largely comparable. Besides, we also explored the independent associations between lifestyle factors and obesity. We found that in men, marriage was positively related to obesity whereas cigarette smoking was inversely related to obesity. Such results were similar to many previous studies. Moreover, among physically active male respondents, the odds of general obesity or abdominal obesity were about one quarter lower to physically inactive ones, but in women, no significant association was found between physical activity level and obesity. Such result may indicate that being physically active is a more effective way for men to prevent obesity than women. As to dietary habits, we did not find concordant significant associations between total fat intake, total meat intake and obesity. We think such result may not reflect the real situation because the daily intake amount of fat and meat among Chinese is generally low, so in our study, the quantity difference between dichotomized “high intake” and “low intake” subgroups may be too small to detect the real effect. However, to fruits and vegetables, we did observe an expected positive relationship between intake amount and obesity in both genders. We think the main reason that fruits and vegetables intake can be a plausible surrogate for total energy intake among Chinese is that China is an agricultural country, the prices of fruits and vegetables are comparatively low, so they are generously consumed in every meal with staples (commonly rice or wheaten food), in this instance, fruits and vegetables intake amount can be positively related to staples intake amount. And in China, staples are the predominant sources of daily energy intake.
Several limitations of this study should be noticed. Firstly, it was a cross-sectional study, which means we can not draw any causal-effect conclusion based on study results. Secondly, although the response rate was high (91.2%), it turned out that some traits of participants were still slightly different from general population of Zhejiang, such trivial discrepancies could still bring bias to study results. Thirdly, despite fruits and vegetables intake can be an ideal surrogate for daily energy intake, it is very likely that the results may exhibit a certain degree of discordance if we were able to use daily energy intake in the first place. At last, considering the huge discrepancies in demographic characteristics, lifestyle pattern and economic strength between different regions of China, our study results might not be applicable to other Chinese subpopulations as well as the whole Chinese population.
Despite all limitations, the major findings of our study have important secular significance. China is a developing country with highly unbalanced regional development. In economically prosperous areas, like Zhejiang, based on our findings, SES-obesity association has already resembled to which in developed societies. But to those impoverished inland provinces, such relationship can be totally different. Thus it is of great importance to implement SES-obesity association studies in other parts of China, in order to accurately target high risk groups of obesity, and successfully implement intervention measures. Furthermore, many previous studies have revealed that macro environment (neighborhood and area) was closely related to the well-being of residents [31, 32], and some scholars have already explored its relationship with obesity in specific subpopulations (mostly in children) [33, 34], but nearly all of these studies were implemented in developed countries, so it is also of great necessity to carry out relevant studies in Chinese populations in the near future.