In the largest exercise-testing cohort to date — 562,234 adults followed for a mean 10.2 years — each additional MET of peak cardiorespiratory fitness (CRF) was independently associated with a 16% lower hazard of all-cause death (HR 0.84). The least-fit quintile (below 5.0 METs) carried a 10-year mortality risk of 32.9%, compared with 13.4% among fit individuals — a 2.5-fold difference. Adding CRF to a conventional risk model (age, sex, hypertension, diabetes, dyslipidemia, smoking) improved continuous net reclassification by 0.37 and raised the C-statistic from 0.662 to 0.682, statistically and clinically meaningful gains.
The scale of this cohort makes it among the most statistically authoritative analyses ever conducted on fitness and mortality. While the Veterans Affairs population skews heavily male and older, the magnitude of the CRF effect dwarfs many conventional risk factors clinicians already act on routinely. Observational design means residual confounding cannot be ruled out — fitter people differ in dozens of unmeasured ways — but the dose-response relationship and biological plausibility are robust, supported by decades of mechanistic and interventional data. Crucially, CRF is modifiable: structured aerobic exercise reliably raises peak METs. The practical implication is direct: a treadmill test result belongs alongside blood pressure and cholesterol in routine risk conversations. For adults hovering near the 5-MET threshold, even modest fitness gains could shift mortality trajectory meaningfully.