title: The association between obstructive sleep apnea and metabolic syndrome: a systematic review and meta-analysis.
abstract: BACKGROUND
  Obstructive sleep apnea (OSA) is characterized by repeated episodes of obstruction of the upper airway. Numerous studies have indicated a relationship between OSA and metabolic syndrome (MS), but the results remain debatable. We aimed to perform a systematic review and meta-analysis to evaluate the association between OSA and MS.
  
  
  METHODS
  We searched electronic databases (PubMed, EMBASE, and ISO Web of Knowledge) up to September 2014 with English-language restriction. Cross-sectional, case-control, and cohort studies in which the presence of OSA was assessed by objective measurements, the exposure of interest was OSA, and the outcome of interest was the presence (or incidence) of MS were included. The adjusted odds ratios (ORs) (or relative risk) and 95 % confidence intervals (CIs) were extracted and pooled. Sensitivity analyses were conducted, and heterogeneity and publication bias were assessed.
  
  
  RESULTS
  Overall, 15 cross-sectional (2456 patients with OSA and 1705 subjects without OSA), five case-control (1156 OSA patients and 404 controls), and no cohort studies were included. The pooled ORs of MS in individuals with OSA for cross-sectional and case-control studies were 2.87 (95 % CI: 2.41-3.42) and 2.56 (95 % CI: 1.98-3.31), respectively. There was clinically unimportant (I (2) = 20 %) and moderate (I (2) = 35 %) between-study heterogeneity of the analysis. The pooled crude ORs of MS in individuals with mild and moderate-to-severe OSA was 2.39 (95 % CI: 1.65-3.46) and 3.45 (95 % CI: 2.33-5.12), respectively, and there was substantial heterogeneity in the meta-analyses (I (2) = 53 % and I (2) = 63 %, respectively). However, no evidence of publication bias was detected.
  
  
  CONCLUSIONS
  OSA is shown to be associated with MS, although causality between these two factors has not been demonstrated yet. Future cohort and randomized controlled studies are needed.
authors.affiliation: Department of Endocrinology, Xijing Hospital, Fourth Military Medical University, 169 Changle Road West, Xi'an, 710032, China. yoji_xu@hotmail.com.
authors.forename: Shaoyong
authors.initials: S
authors.lastname: Xu
authors.affiliation: Department of Health Statistics, School of Preventive Medicine, Fourth Military Medical University, 169 Changle Road West, Xi'an, 710032, China. wanyi@fmmu.edu.cn.
authors.forename: Yi
authors.initials: Y
authors.lastname: Wan
authors.affiliation: Department of Cardre Ward, Lanzhou General Hospital of Lanzhou Military Region, People's Liberation Army, 333 Binhe Road South, Lanzhou, 730050, China. xuming078@gmail.com.
authors.forename: Ming
authors.initials: M
authors.lastname: Xu
authors.affiliation: Department of Endocrinology, Xijing Hospital, Fourth Military Medical University, 169 Changle Road West, Xi'an, 710032, China. mingjie1207@163.com.
authors.forename: Jie
authors.initials: J
authors.lastname: Ming
authors.affiliation: Department of Endocrinology, Xijing Hospital, Fourth Military Medical University, 169 Changle Road West, Xi'an, 710032, China. drxingying@gmail.com.
authors.forename: Ying
authors.initials: Y
authors.lastname: Xing
authors.affiliation: Department of Otolaryngology Head and Neck Surgery, Lanzhou General Hospital of Lanzhou Military Region, People's Liberation Army, 333 Binhe Road South, Lanzhou, 730050, China. anfei0201@gmail.com.
authors.forename: Fei
authors.initials: F
authors.lastname: An
authors.affiliation: Department of Endocrinology, Xijing Hospital, Fourth Military Medical University, 169 Changle Road West, Xi'an, 710032, China. qiuheji@hotmail.com.
authors.forename: Qiuhe
authors.initials: Q
authors.lastname: Ji
mesh_terms: D015897:Comorbidity; D006801:Humans; D015994:Incidence; D024821:Metabolic Syndrome / Q000453:epidemiology*; D016017:Odds Ratio; D012720:Severity of Illness Index; D020181:Sleep Apnea, Obstructive / Q000453:epidemiology*