We sought to conduct a systematic review and meta-analysis of the cardiovascular risk associated with the metabolic syndrome as defined by the 2001 National Cholesterol Education Program (NCEP) and 2004 revised National Cholesterol Education Program (rNCEP) definitions.

Numerous studies have investigated the cardiovascular risk associated with the NCEP and rNCEP definitions of the metabolic syndrome. There is debate regarding the prognostic significance of the metabolic syndrome for cardiovascular outcomes.

We searched the Cochrane Library, EMBASE, and Medline databases through June 2009 for prospective observational studies investigating the cardiovascular effects of the metabolic syndrome. Two reviewers extracted data, which were aggregated using random-effects models.

We identified 87 studies, which included 951,083 patients (NCEP: 63 studies, 497,651 patients; rNCEP: 33 studies, 453,432 patients). There was little variation between the cardiovascular risk associated with NCEP and rNCEP definitions. When both definitions were pooled, the metabolic syndrome was associated with an increased risk of cardiovascular disease (CVD) (relative risk [RR]: 2.35; 95% confidence interval [CI]: 2.02 to 2.73), CVD mortality (RR: 2.40; 95% CI: 1.87 to 3.08), all-cause mortality (RR: 1.58; 95% CI: 1.39 to 1.78), myocardial infarction (RR: 1.99; 95% CI: 1.61 to 2.46), and stroke (RR: 2.27; 95% CI: 1.80 to 2.85). Patients with the metabolic syndrome, but without diabetes, maintained a high cardiovascular risk.

The metabolic syndrome is associated with a 2-fold increase in cardiovascular outcomes and a 1.5-fold increase in all-cause mortality. Studies are needed to investigate whether or not the prognostic significance of the metabolic syndrome exceeds the risk associated with the sum of its individual components. Furthermore, studies are needed to elucidate the mechanisms by which the metabolic syndrome increases cardiovascular risk.