Our data showed that the associations of regular snoring with unfavorable levels of inflammatory markers and adipokines were mainly explained by obese status. However, regular snoring was significantly associated with an elevated MetS risk, when potential risk factors like lifestyle factors, adiposity, inflammatory markers and adipokines were extensively controlled in apparently healthy Chinese men and women. The findings of our study provide further insights into potential mechanism(s) involved in the association between snoring status and metabolic disorders.
In consistent with the findings from a study in which snoring frequency was found to be independently associated with triglycerides, HDL and adiponectin in American diabetic women 
, we also observed that frequent snoring was significantly associated with an adverse profile of cardiometabolic biomarkers including insulin, HOMA-IR, triglycerides, HDL, adipokines (HMW-adiponectin, leptin) and inflammatory markers (CRP, IL-6, IL-18 and LBP) in an apparently healthy Chinese population. However, unlike that study, most of the associations, particularly adiponkines and inflammatory markers in our study seem to be explained by accumulated adipose status indicated by BMI or waist circumference. Indeed, controversial results have been reported in several studies when considering the influence of obesity on the associations 
. For instance, Ulukavak and coworkers 
observed that serum leptin was associated with apnea-hypopnea index (AHI) independent of BMI in obese Turkish SDB patients; whereas, Ip et al 
indicated that body fat parameters rather than AHI were predictors for leptin levels in German SDB patients. The discrepancies among studies might be due to differences in ethnics, study design and/or severity of obesity. Since adipokines and/or proinflammatory cytokines are largely derived from adipose tissue 
, it is also unclear that to what degree the ‘metabolically obese’ phenomenon and different profile of inflammatory markers in Chinese population could influence the above associations 
. Certainly, more studies are deserved in this aspect.
In the present study, BMI is showed to be the most influential factor involved in the associations of regular snoring with the MetS and its features, indicated by controlling for BMI largely attenuated the snoring-MetS association and even abolished significant associations with elevated blood pressure and low HDL cholesterol (). It appears that accumulating adiposity in our population served as a critical mechanistic linking between snoring and metabolic disorders. Supporting evidence also came from some of cohort and intervention studies 
. For example, based upon the data from the Wisconsin Sleep Cohort Study, Peppard et al 
discovered that weight gain was associated with an increased development and severity of SDB; whereas weight loss resulted in improvement of SDB. Notably, a strong positive association between snoring frequency and central obesity was persistent, even extensively adjusting for risk factors such as BMI, lifestyle, depressive symptoms, sleeping quality and duration, as well as multiple inflammatory cytokines and adipokines. As active endocrine organ, adipose tissue secretes a number of adipokines and promotes expression of inflammatory markers which are proposed to mediate the adverse effects of obesity on the development of metabolic diseases like type 2 diabetes and CVD 
. In addition, fat deposition in the upper airway lumen and muscle could reduce tracheal traction and lung volume, resulting in and worsening the obstruction of upper airway 
, which might provide potential mechanism(s) between obesity and the pathogenesis of SDB/snoring. Taken together, findings of our study further emphasized the dominant role of obesity on the relationship between snoring and the risk of MetS when potential confounders were extensively controlled.
Our study provided more supporting evidence that frequent snoring was a strong and independent risk factor for MetS among Chinese, regardless the fact that controlling for BMI remarkably reduced the association (). The independent role of snoring status was also suggested by the observations that no interactions were detected between snoring with overweight/obesity, inflammatory markers or adipokines in the joint classification analyses (). Therefore, our findings implicates that frequent snoring might attribute to metabolic abnormalities via
mechanism(s) beyond obesity and related inflammation and adipokine dysregulation. Although specific mechanism(s) is not fully understood, existing literatures suggested that SDB induced hypoxia and hypercapnia might stimulate sympathetic nervous activity 
and generate more circulating catecholamine and cortisol 
, which consequently increase insulin resistance 
. Furthermore, hypoxia is also believed as an atherogenic factor which might increase the risk for future cardio-metabolic disturbance 
. Previously, habitual snoring was reported to add prognostic value for type 2 diabetes and CVD independent of obesity by some of prospective studies 
. Meanwhile, the findings from our study also supported that frequent snoring could provide additional information over putative risk biomarkers such as inflammatory markers and adipokines for MetS in apparently healthy Chinese. Collectively, self-reported snoring might be useful as a low-cost and no-invasive indicator in screening persons with higher cardio-metabolic risk, especially in developing countries.
To the best of our knowledge, this is the first study to thoroughly examine how obese status, multiple inflammatory markers and adipokines are related to snoring frequency and its association with MetS risk in the Chinese population. Our study has provided further insights in understanding the mechanism(s) linking snoring to cardio-metabolic disorders. Admittedly, there are limitations for the study. Firstly, the information of snoring habits in our study was obtained by questionnaires, a commonly used subjective method, which might introduce recall bias. However, previous studies have suggested the self-reported snoring to be a reliable measurement in epidemiology studies 
. Secondly, we did not collect information about chronic obstructive pulmonary disease (COPD), which may overlap with SDB in pathophysiology and cardio-metabolic outcomes 
. However, existing data showed that the prevalence of COPD in urban Shanghai was 3.9% among subjects aged 40 years and older 
. Moreover, the COPD and obstructive sleep apnea syndrome only coexist in 1% adult males 
. Therefore, the snoring-MetS association was unlikely to be altered substantially by taking COPD into account. Moreover, the cross-sectional design could not allow establishing the causal relation. The case-control nature of the sample might limit our findings to be generalized in general populations. Certainly, future studies are needed to confirm our results prospectively in different populations.
Our study indicates that snoring frequency is significantly associated with MetS, independent of obesity, inflammatory factors and adipokines in apparently healthy Chinese. Whether snoring status could serve as an economic and no-invasive indicator for high risk individuals need to be determined prospectively.