Overweight and obesity have increased in pandemic dimensions (1–5). To make the situation more complexing, regional differences exist in both obesity prevalence and trends between countries and within countries (1–4). Data (2) show that higher-than-optimal BMI caused an estimated 5 million deaths from noncommunicable diseases such as cardiovascular diseases, diabetes, cancers, neurological disorders, chronic respiratory diseases, and digestive disorders in 2019. To address the rising pandemic of obesity, there are widespread calls for regular monitoring of the trends in adult overweight and obesity prevalence in all populations (3–5). The prevalence of overweight and obesity in China have been on the rise in recent decades (6–8) and are projected to increase further (9), primarily driven by an increasing adoption of a western lifestyle and decreased physical activity (10, 11). Literature consistently shows that the prevalence of overweight and obesity varied considerably among provinces and regions in China (6–8). More than 14% of China’s population live in Southwestern China (12), where about 53 million living in two megacities - Chengdu and Chongqing (13, 14). Southwestern China has economically developed fast over the past few decades, resulting in dramatic changes of lifestyle and probably growing prevalence of overweight and obesity. However, data on the prevalence and related factors of adult overweight and obesity in this area were lacking. In the current article, we estimate the prevalence of adult overweight and obesity and explore their potential influencing factors in Southwestern China.

We excluded 638 participants from the 14,016 eligible residents according to the protocol and then 2,282 participants due to incomplete information, resulting in a total of 11,096 participants included in the final analysis. The mean age of the participants was 55.09 ± 10.96 years, 91.0% were married, 64.5% were women, 23.7% were self-reported having high school education or higher, 58.9% were physically active; 22.3% were smokers, 75.1% were non-smokers and 2.6% were ex-smokers; 17.2% were alcohol drinkers, 81.3% were non-drinkers and 1.5% were ex-drinkers. The mean values of systolic and diastolic blood pressure were 130.92 ± 21.53 mmHg and 78.50 ± 11.38 mmHg, respectively. The mean FPG and 2-h PG levels were 5.67 ± 2.05 mmol/L and 7.92 ± 3.83 mmol/L, respectively.

Compared with men (Table 1), women appeared to have higher rates of non-smokers (95.3% vs. 38.4%, p < 0.001), alcoholic non-drinkers (95.5% vs. 55.4%, p < 0.001) and being physically active (60.0% vs. 57.0%, p = 0.002), and have a lower rate of being married (88.8% vs. 95.0%, p < 0.001), having lower education (19.0% vs. 32.2%, p < 0.001). Women had significantly lower mean systolic and mean diastolic blood pressure values (129.90 ± 21.96 mmHg vs. 132.77 ± 20.08 mmHg, p < 0.001 and 77.45 ± 11.37 mmHg vs. 80.40 ± 11.14 mmHg, p < 0.001, respectively) and a significantly higher mean 2-h PG level (8.01 ± 3.83 mmol/L vs. 7.75 ± 3.82 mmol/L, p = 0.001). No significant difference for the FPG level was observed between male and female (5.66 ± 2.14 mmol/L vs. 5.69 ± 1.89 mmol/L, p = 0.513).

In the overall participants, the mean BMI was 23.90 ± 3.53 kg/m² (Table 2). One-way ANOVA analysis showed a significant difference across the age groups (p < 0.05). Additionally, women (24.04 ± 3.60 kg/m²) had a higher BMI than men (23.64 ± 3.39 kg/m²) (p < 0.05). As for overweight and obesity (Table 3), the prevalence were 29.7 and 4.4%, respectively. The prevalence of overweight increased significantly with advancing age (P for trend <0.05). Comparing with men, women have a significantly higher prevalence of overweight (30.5% for women and 28.1% for men, p < 0.05) and obesity (5.1% for women and 3.2% for men, p < 0.05). Similar results were observed based on the BMI cutoffs recommended by the Working Group on Obesity in China (Table 4).

Univariable logistic regression analysis suggested that women, non-smokers, ex-smokers, non-drinkers, being hypertensive and diabetic were associated with a higher prevalence of obesity and that being physically active was associated with a lower obesity prevalence (Table 5). Multivariable logistic regression analysis further supported that women, non-smokers, ex-smokers, being hypertensive and diabetic were related to higher obesity prevalence, and that physically active adults and those aged 65–79 years were less likely to have obesity.

To our knowledge, this is the first large population-based face to face survey to investigate the prevalence of overweight and obesity in Chengdu and Chongqing in Southwestern China. The current study shows that 34.1% of the adults in this area had overweight or obesity. However, the obesity prevalence among the Southwestern China adults in our study (4.4%) is lower than the national prevalence of obesity in China (5.2%) (7), much lower than the adult obesity prevalence in the United States (41.9%) (17), that in Asian Americans (16.1%) (17), and that in the European Region (23%) (18), there are estimated to be several million adults with a BMI of ≥30 kg/m² due to the large population in this area. Furthermore, a recent research (9) suggests that the prevalence of overweight and obesity in Chinese adults are projected to increase further. Unsurprisingly, obesity and overweight represent major health challenges for local health-care system.

The prevalence of overweight and obesity in Southwestern China in our study were comparable to those in regions of South China (19) but much lower than those in North China (20–23). The striking North–South difference of the obesity prevalence in China has been documente with a BMI of ≥30 kg/m² d before (6–8). The regional disparities of obesity prevalence within countries have also been reported in Germany (24) and in the United States (25). The regional differences between North and South China are at least partly related to or resulted from differences in climate, diet and living habits between the two areas (26, 27).

Obesity is a complex condition with many causal contributors including genetic ones and many environmental factors (28). Unsurprisingly not only the prevalence but also the associated risk factors of obesity vary across different regions of China. Our study shows that BMI, overweight prevalence and obesity prevalence in women were significantly higher than in men, in line with the findings of some researches in China (21). Multivariate logistic regression analysis suggested that women were related to a higher obesity prevalence. However, large heterogeneity in obesity prevalence in women and men existed in the published literature. A higher obesity prevalence in women than in men was also found at the global level (1). However, a lower obesity prevalence among women than among men was reported in some provinces of China (22, 23). In a research conducted in the overall China (8), men had a lower prevalence of obesity than did women in 2004, but this pattern had reversed by 2018. In the United States, there was no significant differences of adult obesity prevalence between men and women although the prevalence of severe obesity in adults was higher in women than men (17). The disparities suggest women may not be an independent risk factor of obesity, and understanding the drivers of these disparities might help to provide guidance for the most promising intervention strategies.

The current study shows the prevalence of obesity increased progressively with increasing age until 65 years, then dropped after 65 years, similar to previous studies (6), while the prevalence of overweight grew with increasing age across all age groups. The definite cause of the obesity prevalence drop in the older adult seems elusive. An assumption in a previous study (6) was that it was due to a decrease in total energy intake, muscle loss with age, chronic disease consumption in the older adult (29, 30). However, this assumption failed to explain the increased prevalence of overweight in the adults aged of 65–79 years. Another theory is that it could be due to survival disadvantage associated with obesity in the older population (28), which could explain the paradoxical changes of prevalence of overweight and obesity in the age group 65–79 since in older adults, as a meta-analysis showed (31), being overweight was not found to be associated with an increased mortality risk while having a BMI >33 kg/m² was.

Our study shows ex-smokers as well as non-smokers were more likely to have obesity, consistent with prior study (19). Smoking cessation at every age was associated with longer survival (32) and undoubtedly should be encouraged for all smokers even in the presence of a potential risk of weight gain. However, professional advice for weight control is reasonable for a greater health benefit even after 3 years of smoking cessation.

There are several limitations in this study. Firstly, the participants in our study were enrolled from Chengdu and Chongqing which made it inappropriate to extrapolate its conclusion to other regions of China. Secondly, the survey was conducted in urban areas of Southwestern China. As a result, it should be cautious to extrapolate the results to rural areas although recent evidence showed narrowing urban–rural difference among men and even higher mean BMI and obesity prevalence in women in rural settings compared with women in urban settings (8). Thirdly, the causal relationships between overweight/obesity and their associated factors cannot be established due to the cross-sectional design of the current study.

In summary, our study provides important information on the prevalence of overweight and obesity among adults in Southwestern China and important reference significant for preventing, screening, and controlling overweight and obesity in this vast population.