This study aimed to investigate the prevalence and characteristics of metabolic syndrome with different MetS definitions at the national level in China. The results showed that the overall prevalence of MetS in the Chinese populations aged ≥ 35 years according to the definition of IDF, the revised ATP III, and JCDCG was 26.4%, 32.3%, and 21.5%, respectively. The MetS was less prevalent in men than women according to IDF definition (22.2% vs 30.3%) and the revised ATP III (29.2% vs 35.4%) definition, but the opposite was true according to JCDCG definition (24.4%vs 18.5%). Our result also showed that JCDCG definition was not in good agreement with IDF and the revised ATP III among women. In addition, our study indicated that area, age, education, smoking, alcohol use, and family history of CVD were related to MetS, but the effects and correlation intensity of these factors varied with gender and definition.
The prevalence of MetS in our population lies well within the data previously obtained in China.[13, 22, 23] In one of the nationwide studies for people over 45 years old, the prevalence of MetS was 34.8%, 39.7%, and 25.6%, according to IDF, the revised ATP III, JCDCG criteria, respectively. [22] The participants in that study were older than that in our study. In a survey conducted in participants aged 18 years and over, the prevalence of MetS defined by the revised ATP III was 24.2%, much lower than the 32.3% that we obtained when using the same definition .[13]However, the MetS prevalence of the age group of 45-54, 55-64, and ≥ 65 years of age in that study was 32.12%, 36.97%, 37.81%, respectively. In our study, the MetS prevalence for the age group of 45-54, 55-64, 65-64, and ≥ 75 years of age was 33.4%, 39.4%, 37.9%, and 38.1%, respectively (Table 2). The numbers are very close.
It has been seen that the prevalence of MetS was closely related to age and gender.[24] In our study, the prevalence of MetS in the total population peaked at 55-64, which is close to Wu’s study, which peaked at the age of 60-69.[25] In addition to age, gender cannot be ignored. In our study, the prevalence of MetS in women over 45 years old remained at a high level (table 2), and the odds ratio of women over 45 years old reached around 3 (table 4). Menopause may explain this phenomenon, for menopause generally occurs around the age of 50.[26] The loss of heart and kidney protection of female hormones with age may lead to the sharp increase in hypertension and cardiovascular disease in postmenopausal women.[27] The prevalence of MetS in our study reached its highest in men aged 45-54 and then decreased and 65 years and above became a protective factor. This marked reversal of gender difference in older adults may be partly attributable to the men prone to metabolic disease who had died before the age of 75 or refused to participate in this study.[27, 28] The characteristics of MetS vary by gender, suggesting reasonable comparisons should be made by gender.
In addition to age and gender, our study showed there were some other factors associated with MetS. This study revealed that the individuals living in urban areas had a higher risk of MetS, which is the same as some other studies.[25, 29] The reason for this phenomenon may be that, in China, compared with rural areas, in economically developed urban areas with rapid industrialization, animal food and fast food with high fat and purine content increased significantly, while grain consumption was the opposite[30]. Our results also revealed gender differences in the association between education and MetS, with a positive association in women and negative in men. This was consistent with a study conducted by the Korea National Health and Nutrition Examination Surveys.[31] One possible explanation was that women with higher education might have a favorable opportunity to get more nutrition knowledge and prefer healthy food consumption patterns. [32] And men with higher education are more likely to consume high-calorie foods and alcohol, while avoiding physically demanding tasks.[31] It is worth noting that a family history of CVD was an independent risk factor for MetS in our study, which indicated that more attention should be paid to individuals with a CVD family history.[33] There was a significant negative correlation between smoking and MetS defined with the revised ATP III and JCDCG definition in men. Although the association between smoking and MetS was not significant in women, its OR value was smaller than that in men, which may be caused by the small number of women smoking and insufficient test power. This phenomenon was contrary to the general conclusion that smokers had higher insulin resistance and a higher risk of fatal coronary artery disease than non-smokers.[34] One possible explanation is that some smokers weigh less than non-smokers due to the effects of nicotine on metabolism.[35] Interestingly, we found an arguable result that alcohol use was a protective factor for women and a risk factor for men, which was also reported in Sampson’s study.[36] Men have a higher drinking rate and tend to drink more alcohol. Heavy drinking, especially > 30 g/day in men, is often accompanied by an increase in energy intake and changes in the concentration of steroid hormones that may cause central fat storage, which will aggravate higher blood pressure, elevated plasma glucose, and central obesity.[37] Women drink less often and consume lower quantities of alcohol. Some studies have shown that drinking a small amount of alcohol may have cardiovascular protective effects.[38] However, the protective effect of drinking a small amount of alcohol is still controversial and needs to be further studied.[39]
Our study compared three Mets definitions of IDF, the revised ATP III, and JCDCG. The prevalence of Mets defined by JCDCG criteria was the lowest mainly due to the strictest central obesity standard (WC 85 cm for women). The consistency test showed the consistency between any two definitions of men and the revised ATP III definition and IDF definition of women was relatively good, while the consistency between JCDCG and IDF definition (kappa =0.58) and the revised ATP III (kappa =0.58) was relatively poor in women. Based on the marked gender disparity, we may speculate that JCDCG criteria may need more in-depth study, especially for women. Some studies had suggested WC cut-off values of 86 cm in men and 78 cm in women.[40] Existing studies have shown that the revised ATP III definition was the best predictor of cardiovascular disease.[22, 41] To solve this problem, more longitudinal studies may be needed. On the other hand, the difference in definition leads to the difference of relevant risk factors. Although the impact and correlation intensity of these factors vary by gender and definition, the difference is not very large. Our results showed that the risk factors under the three definitions were almost the same, which may be that although these definitions are different, they still overlap greatly. A previous study reported that the overlap rate between IDF and JCDCG was 56.1%,[42] In our study the rate is 64.9% in men and 46.8% in women, respectively.
Our study has some advantages. Firstly, the study population of the CHS study was randomly selected from the whole country by stratified and multistage sampling, and the sample is nationally representative. Our results show that MetS has become a problem that cannot be ignored in China. Secondly, strict quality control ensured the high quality of data and reliability of the findings. The uniform research protocol, strict training and examination, and the centralized detection of blood glucose and lipids in the central laboratory ensure the accuracy and comparability of the data.
The limitations of this study need to be recognized. Firstly, we only compared the revised ATP III, IDF, and JCDCG definitions due to the lack of some indicators, such as the data of insulin resistance. Secondly, we explored some related factors of MetS, but we cannot claim causality because of the cross-sectional design. Thirdly, in this study, we explored the related factors of MetS, some variables which may affect MetS were not included in our study, such as physical activity and dietary patterns. In addition, due to funding and other reasons, the investigation lasted for a long time, and some related factors may have changed.