If you could only measure one thing about your health — one number that predicts your risk of dying from any cause at least as well as smoking status or diabetes — a strong candidate would be your cardiorespiratory fitness. Specifically, your VO₂max.
This isn't speculation. In 2018, a study of over 122,000 patients published in JAMA Network Open showed that the mortality risk associated with low cardiorespiratory fitness was comparable to or greater than that of smoking, diabetes, or coronary artery disease. (In that study, fitness was estimated from treadmill performance rather than measured directly by gas exchange.) And unlike most risk factors, fitness is something you can improve at any age.
What VO₂max actually measures
VO₂max is the maximum amount of oxygen your body can use during intense exercise. It's measured in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). The test is done on a treadmill or stationary bike with gradually increasing intensity while you breathe through a mask that measures gas exchange.
It's not a comfortable test — you push to your limit, which takes about 10 to 15 minutes. But it produces the single most precise measurement of your cardiovascular and metabolic fitness available.
Why it predicts mortality so powerfully
VO₂max is not just a measure of how fit you are in the gym. It reflects the integrated function of your heart (how much blood it can pump), your lungs (how well they exchange oxygen), your blood vessels (how efficiently they deliver oxygen), and your muscles (how well they use it).
When any of these systems starts to decline — often years before it shows up on standard tests — VO₂max drops. It's like a single number that summarizes the health of your entire oxygen delivery chain.
The numbers from the research are striking. Each 1 MET increase in fitness (roughly 3.5 mL/kg/min of VO₂max) is associated with a 13% reduction in all-cause mortality. In the 2018 study, people in the least fit quarter had about five times the mortality risk of the fittest (elite) group, and risk fell with every step up in fitness.
In the 2018 study, no upper limit of benefit was observed. Being fit is good. Being very fit was better. No plateau was seen.
What your result means
Your VO₂max is compared to age-and-sex-matched percentiles. A 50-year-old man with a VO₂max of 40 mL/kg/min is doing well. The same number in a 25-year-old woman would be roughly average to slightly above. Context matters — a physician or sports medicine specialist can explain where you stand and what it means for your risk profile.
Importantly, VO₂max is modifiable. Unlike your Lipoprotein(a) level, which is genetically fixed, you can improve your VO₂max at any age with the right training. Training studies show meaningful improvements within a few months of structured exercise, including in older adults. This makes it not just a risk marker but a target for intervention.
Who should consider testing
VO₂max is measured by spiroergometry (cardiopulmonary exercise testing), typically offered by sports medicine practices, cardiology departments and some performance labs. It is often paired with grip strength testing and body composition analysis (BIA) to get a more complete fitness picture.
Standard check-ups rarely include spiroergometry. It requires specialized equipment and trained personnel. But given that fitness is one of the strongest mortality predictors that can be measured, it is a reasonable test to ask about — particularly if you are planning to start or intensify training, have cardiovascular risk factors, or simply want a baseline to track over time. Whether it makes sense for you, and whether a medically supervised test is needed, is worth discussing with your own physician.
The actionable part
A useful test result comes with more than a VO₂max percentile: it should lead to a specific exercise prescription — frequency, intensity, type, and duration — calibrated to your current fitness level and risk profile. Not "exercise more," but "here's exactly what to do and why."
References
- Mandsager K, et al. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Netw Open. 2018;1(6):e183605. doi:10.1001/jamanetworkopen.2018.3605.
- Ross R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. A scientific statement from the American Heart Association. Circulation. 2016;134(24):e653-e699.
- Kodama S, et al. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: a meta-analysis. JAMA. 2009;301(19):2024-2035.
This article is general information and not individual medical advice. Decisions about tests and treatment belong in a conversation with your own physician.