Objectives: The purpose of this research was to assess the association between the metabolic syndrome (MetSyn) and cardiovascular events and mortality by meta-analyses of longitudinal studies. Background: Controversy exists regarding the cardiovascular risk associated with MetSyn. Methods: We searched electronic reference databases through March 2005, studies that referenced Reaven's seminal article, abstracts presented at meetings in 2003 to 2004, and queried experts. Two reviewers independently assessed eligibility. Longitudinal studies reporting associations between MetSyn and cardiovascular events or mortality were eligible. Two reviewers independently used a standardized form to collect data from published reports. Authors were contacted. Study quality was assessed by the control of selection, detection, and attrition biases. Results: We found 37 eligible studies that included 43 cohorts (inception 1971 to 1997) and 172,573 individuals. Random effects meta-analyses showed MetSyn had a relative risk (RR) of cardiovascular events and death of 1.78 (95% confidence interval [CI] 1.58 to 2.00). The association was stronger in women (RR 2.63 vs. 1.98, p = 0.09), in studies enrolling lower risk (<10%) individuals (RR 1.96 vs. 1.43, p = 0.04), and in studies using factor analysis or the World Health Organization definition (RR 2.68 and 2.06 vs. 1.67 for National Cholesterol Education Program definition and 1.35 for other definitions; p = 0.005). The association remained after adjusting for traditional cardiovascular risk factors (RR 1.54, 95% CI 1.32 to 1.79). Conclusions: The best available evidence suggests that people with MetSyn are at increased risk of cardiovascular events. These results can help clinicians counsel patients to consider lifestyle interventions, and should fuel research of other preventive interventions.
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