The study
Cleveland Clinic researchers looked at 122,007 patients who took a treadmill exercise test, then followed who lived and who died (Mandsager et al., JAMA Network Open, 2018).
What they found
Compared with the fittest group, the least fit had about five times the risk of death (hazard ratio 5.04).
For comparison, smoking carried a hazard ratio of about 1.41 and diabetes about 1.40 in the same analysis.
There was no ceiling: the fittest group did best of all.
The honest catch
It’s observational, so fitness may not be the only cause. But fitness is one of the most trainable numbers you have, at almost any age.
Decoded / the concept
Hazard ratio
5.04× means: at any moment in the study, the least fit had about five times the risk of dying.
Not five times shorter lives. A ratio of risk, measured across the whole study.
What you can do
- Learn your VO2 max: a treadmill test or a good wearable estimate is a start.
- Two hard cardio sessions a week, plus easy movement most days.
- Check with your doctor before you start hard training.
Common questions
Is low fitness worse than smoking?
In 122,007 adults who took a treadmill test, the least fit had about five times the risk of death of the fittest (hazard ratio 5.04), a bigger gap than smoking (about 1.41) in the same analysis. It’s a strong link, not proof of cause.
Can you improve cardiorespiratory fitness later in life?
Yes. Fitness is trainable at almost any age. Check with your doctor before starting hard training.
Published 2026-09-29 · Last reviewed 2026-09-29
Educational content, not medical advice. Talk to your clinician before changing anything about your health.
