Background Age and body mass index (BMI) are positively associated with the development of the metabolic syndrome (MetS). Cardiorespiratory fitness (CRF) can attenuate BMI-related increases in prevalence of MetS, but the nature of this association across different age strata has not been fully investigated.

Aim To identify the association between CRF and MetS prevalence across age strata (20–69 years) and determine whether associations are independent of BMI.

Methods CRF was estimated from incremental treadmill exercise in British men attending preventative health screening. Participants were placed in four age strata (20–39, 40–49, 50–59 and 60–69 years) and classified as fit or unfit using age-related cut-offs. The presence of MetS was defined using the National Cholesterol Education Program Adult Treatment Panel III criteria.

Results 9666 asymptomatic men (48.7±8.4 years) were enrolled. The prevalence of MetS was 25.5% in all men and ranged from 17.1% in those aged 20–39 years to 30.6% in those aged 60–69 years. Fit men's likelihood of meeting the criteria for MetS was half (OR=0.51, 95% CI 0.46 to 0.57) that of unfit men. The likelihood of MetS was 32–53% lower across age strata in fit, compared with unfit men. Adjustment for BMI attenuated the association, though it remained significant in men aged 20–49 years.

Conclusions The cardiometabolic benefits of CRF are independent of BMI particularly in men <50 years. Public health messages should emphasise the important role of CRF alongside weight management for enhancing cardiometabolic health.