Twelve studies (eight prospective and four cross-sectional observational) met the inclusion criteria, but the cohorts overlapped in two studies, so 11 were analysed (eight prospective, for outcomes) with 67,127 participants (range 550 to 25,626). In the prospective studies, follow-up ranged from three to 30 years. Study quality was judged to be good, with scores ranging from 5 to 8 out of 9.
Overweight metabolically healthy: Without metabolic syndrome, overweight people had a similar risk of death or cardiovascular events to people of normal weight (RR 1.10 95% CI 0.90 to 1.24; seven studies; Ι²=0). In studies with at least 10 years of follow-up, overweight people were at a higher risk, but the results were not statistically significant (RR 1.21 95% CI 0.91 to 1.61; three studies; Ι²=70%).
Obese metabolically healthy: Without metabolic syndrome, obese people had a similar risk of events to people of normal weight (RR 1.19 95% CI 0.98 to 1.38; eight studies; Ι²=15%). In studies with at least 10 years of follow-up, overweight people were at a higher risk (RR 1.24 95% CI 1.02 to 1.55; four studies; Ι²=34%).
Metabolically unhealthy: For people of normal weight, those with metabolic syndrome, compared with those without it, were at a higher risk of events (RR 3.14 95% CI 2.36 to 3.93; eight studies; Ι²=97%). For overweight people, those with the syndrome were at a higher risk than people of normal weight without it (RR 2.70 95% CI 2.08 to 3.30; seven studies; Ι²=96%). For obese people, those with the syndrome were at a higher risk than people of normal weight without it (RR 2.65 95% CI 2.18 to 3.12; eight studies; Ι²=95%). Various sensitivity analyses were reported to address the heterogeneity in these results. These analyses reduced heterogeneity substantially only for overweight people.
There was no evidence of publication bias.