The present study, in which the most recent data from CHNS, including the 2015 survey, were analyzed, showed that mean WC, and the prevalence of abdominal obesity have increased dramatically among Chinese adults with normal weight, irrespective of which criterion was used, since 1993. The increases occurred in both genders, all age groups, rural and urban residents, and all educational attainment groups. Moreover, men, younger participants, and rural residents showed relatively greater increases. Our results are of particular concern as abdominal obesity, which reasonably represents visceral adiposity, is closely associated with obesity-related conditions and mortality in those with a normal BMI8.
Although few studies have analyzed trends in mean WC and prevalence of abdominal obesity among people with acceptable BMI, our results are generally in line with those of previous studies showing that a considerable proportion of individuals suffered from abdominal obesity among people with normal BMI 22. Further, emerging evidence showed that WC increased much faster than BMI during the same periods, and hence the relative increase in abdominal obesity at the same periods is much larger than that of general obesity23,24. Our previous study showed a dramatic upward trend in the prevalence of abdominal obesity among people with normal BMI from 1993 to 200914. The present study showed that the increasing trend from 2009 to 2015 appeared to continue rather than slow or level off. Our results together with previous reports suggest that body composition has changed over time. Evidence showed that approximately 20% higher risk of mortality occurred in individuals with normal BMI who were abdominally obese compared with their counterparts with normal BMI who were not abdominally obese25 and that the association between WC and mortality was strongest in those with a normal BMI8. Thus, depicting the changing trend of abdominal obesity among people with acceptable BMI may provide additional information for more accurately assessing the prevalence of obesity-related disorders. Considering the more deleterious effect of visceral fat on metabolic disorders than subcutaneous fat26–29, the increase in WC is likely to be due to a relatively greater increase in visceral adipose tissue than that of subcutaneous fat. Therefore, it is urgent to take interventional strategies to reverse abdominal obesity trends and reduce the likely medical costs of the increase in abdominal obesity in normal-weight people.
The continuing rapid increase in the prevalence of abdominal obesity among individuals with normal BMI in China from 1993 to 2015 is attributed to several factors. Unhealthy lifestyles and behavioral changes are probably major drivers. Increased availability, accessibility, and affordability of energy-dense foods and a more sedentary lifestyle that have followed urbanization and increasingly mechanized transportation and labor are responsible for excess energy intake and reduced energy expenditure, respectively, and thus induce fat accumulation in the body. For example, China has been experiencing westernization of their diet. As a result, the consumption of plant-based foods such as cereals and starchy roots has dramatically declined; in contrast, the intake of foods rich in sugar, fat, and refined carbohydrates as well as animal-based food such as red meat, and processed meat has dramatically increased 30. Active transportation, such as walking or cycling, which was associated with a decrease in obesity and weight gain, covered up to 80% of daily travel in China until the 1990s, but this situation declined dramatically thereafter31,32. TV ownership increased dramatically during the recent two decades, with 38 sets per 1,000 persons in 1985 and 112 to 135 sets per 100 households in 201133. Future unfavorable trends in dietary pattern and physical activity level will exacerbate the increase in the prevalence of abdominal obesity.
Our findings that the rates of such an increase in abdominal obesity prevalence varied by sex, age, and rural/urban regions are also a characteristic noted in other studies1,23,34. Although the root causes that induce the difference is not clear, disparities between subgroups in genetic, sociocultural, socioeconomic, and behavioral factors, such as disparities in calorie intake, knowledge and means to adopt healthy lifestyles as well as weight management programs, and mechanized transport and work, have been considered as potential drivers30. Despite the inequalities in the abdominal obesity prevalence by sex, age, and rural/urban regions, our study showed that the abdominal obesity prevalence increased over time in all subgroups, indicating a leading role of the obesogenic environment in China in the recent two decades. Further, our finding that the continuous increase in the prevalence of abdominal obesity was much faster in younger people is concerning because it predicts the prevalence of abdominal obesity prevalence among normal-weight people should keep increasing in the next few years.
In the present study, a significant dissociation between general obesity defined by BMI and abdominal obesity defined by WC was noted, with approximately 25% with the presence of both general obesity and abdominal obesity occurring together. Further, approximately two-thirds of individuals with obesity would be missed if WC is not taken into account for the identification of obesity. Hence, the absence of a WC measurement might result in substantial misclassification of individuals who are actually in a risk category based on WC as being in a low-risk category based on an acceptable range of BMI. Indeed, our previous study together with other reports evidenced that more than 20% of the normal weight population encountered an increased WC and a cluster of cardiovascular risk factors, including insulin resistance, atherogenic lipid profiles, hypertension, and non-acholic fatty liver disease 9. Since increased WC and the mentioned cardiovascular risk factors exhibited increased incidences of diabetes, cardiovascular diseases, and all-cause mortality35–38, adding WC measurement into the BMI assessment would contribute to assessing whether an individual was actually in a risk state and thus could provide an opportunity for proper intervention.
Our study has several strengths. First, it maintains a large sample size and includes individuals from diverse and representative regions in China, which allows for exploring the prevalence of obesity over a range of demographic groups. Second, all study measurements are made by trained staff following a standard protocol. A vigorous quality assurance program and the same sampling and strict methodology are used to ensure the quality of the data collection over the entire study period, allowing direct comparisons of results over time. Third, we assessed the secular trends in abdominal obesity in people with normal BMI, using different cut-off points for BMI and identified consistent associations. However, the limitations of the present study require careful consideration. Firstly, the sample is partial nationally representative, therefore the generalizability of the results to regions not studied may be limited. Secondly, other social and environmental variables such as dietary habits, sleep duration, and physical activity, which would have an impact on obesity, were not considered. Third, measurements of body fatness were not available.