Interactive effects of physical fitness and body mass index on risk of stroke: A national cohort study.

Background: High body mass index (BMI) and low physical fitness are risk factors for stroke, but their interactive effects are unknown. Elucidation of interactions between these modifiable risk factors can help inform preventive interventions in susceptible subgroups.

Methods: National cohort study of all 1,547,294 military conscripts in Sweden during 1969-1997 (97-98% of all 18-year-old males). Standardized aerobic capacity, muscular strength, and body mass index measurements were examined in relation to stroke identified from inpatient and outpatient diagnoses through 2012 (maximum age 62 years).

Results: Sixteen thousand nine hundred seventy-nine men were diagnosed with stroke in 39.7 million person-years of follow-up. High body mass index, low aerobic fitness, and (less strongly) low muscular fitness were associated with higher risk of any stroke, ischemic stroke, and intracerebral hemorrhage, independently of family history and sociodemographic factors. High body mass index (overweight/obese vs. normal) and low aerobic capacity (lowest vs. highest tertile) had similar effect magnitudes, and their combination was associated with highest stroke risk (incidence rate ratio, 2.36; 95% CI, 2.14-2.60; P < 0.001). Aerobic capacity and muscular strength had a positive additive and multiplicative interaction (P < 0.001), indicating that low aerobic capacity accounted for more strokes among men with low compared with high muscular strength.

Conclusions: High body mass index and low aerobic capacity in late adolescence are associated with increased risk of stroke in adulthood. Low aerobic capacity and low muscular strength also have a synergistic effect on stroke risk. These findings suggest that preventive interventions should include weight control and aerobic fitness early in life, and muscular fitness especially among those with low aerobic capacity.