Physical Activity, Cardiorespiratory Fitness, and Population-Attributable Risk

To determine population-attributable risk (PAR) and exposure impact number (EIN) for mortality associated with impaired cardiorespiratory fitness (CRF), physical inactivity, and other risk markers among veteran subjects.

The sample included 5890 male subjects (mean age 58±15) who underwent a maximal exercise test for clinical reasons between January 1, 1992, and December 31, 2014. All-cause mortality was the end point. Cox multivariable hazard models were performed to determine clinical, demographic, and exercise-test determinants of mortality. Population-attributable risks and EIN for the lowest quartile of CRF and for inactive behavior were analyzed, accounting for competing events.

There were 2728 deaths during a mean ± standard deviation follow-up period of 9.9±5.8 years. Having low CRF (<5.0 metabolic equivalents [METs]) was associated with an approximate 3-fold higher risk of mortality and a PAR of 12.9%. Each higher MET achieved on the treadmill was associated with a 15% reduction in mortality (hazard ratio [HR]=0.85; 95% confidence interval [CI], 0.83 to 0.88; P<.001). Nearly half the sample was inactive, and these subjects had a 23% higher mortality risk and a PAR of 8.8%. The least fit quartile (<5.0 METs) had relative risks of ≈6.0 compared with the most-fit group (HR=5.99; 95% CI, 4.9 to 7.3). The least-active tertile had ≈2-fold higher risks of mortality vs the most active subjects (HR=1.9; 95% CI, 0.91 to 4.1). The lowest EIN was observed for low fitness (3.8; 95% CI, 3.4 to 4.3, P<.001), followed by diabetes, smoking, hypertension, and physical inactivity (all P<.001 except for diabetes, P=.008).

Both higher CRF and physical activity provide protection against all-cause mortality in subjects referred for exercise testing for clinical reasons. Encouraging physical activity with the aim of increasing CRF would have a significant impact on reducing mortality.