VO2 Max and Biological Age: What Apple Watch Cardio Fitness Really Means

Apple Watch Cardio Fitness estimates VO2 max. How it relates to biological age, what one score misses, and how Nuvo uses it among 21 biomarkers.

Written by the Nuvo Longevity editorial team at Firefly Labs LLC.

Cardio Fitness on Apple Watch is an estimate of VO2 max — the most oxygen your body can use during hard exercise, reported in milliliters per kilogram per minute (mL/kg/min). A higher number is a strong marker of cardiorespiratory fitness and, in large cohort studies, of lower long-term mortality risk. It is not a biological age. Biological age is a composite of many signals. Nuvo is an iPhone and Apple Watch app that calculates a daily biological age from 21 biomarkers: HealthKit data, including Cardio Fitness from Apple Watch, plus optional lab results. VO2 max is one input in that score, not a substitute for it.

This page is for people who opened Cardio Fitness in the Health app and want to know what the level means for how they are aging, and what to do next. It does not replace a broader Apple Watch habits guide. For resting heart rate, HRV, sleep, and recovery alongside fitness, see how Apple Watch data maps to biological age. For the difference between the birthday on your license and a biology-based age, see biological age vs chronological age.

What Apple Watch Cardio Fitness actually measures

Apple’s support article defines cardio fitness as a measurement of VO2 max: the maximum amount of oxygen your body can consume during exercise, and a predictor of long-term health. Apple Watch Series 3 or later estimates it from heart rate and motion during an Outdoor Walk, Outdoor Run, or Hiking workout. Apple’s current support article says the watch supports a VO2 max range of 14–65 mL/kg/min, with classifications for users 20 and older. An initial estimate can take at least 24 hours of wear plus several of those outdoor workouts, and passive measurements, before a number appears. See Track your cardio fitness levels.

The Health app then places that estimate in one of four levels for your age and sex: Low, Below Average, Above Average, or High. There is no “Average” bucket. If the level is consistently low, Apple can notify you. Apple’s May 2021 healthcare white paper, Using Apple Watch to Estimate Cardio Fitness with VO2 max, explains the low-notification threshold: for ages 20–59 it is the lowest quintile by sex and age decade in the FRIEND registry (Fitness Registry and the Importance of Exercise National Database); for ages 60 and older, Apple uses absolute cutoffs of 18 mL/kg/min for men and 15 mL/kg/min for women.

The same paper is clear that these are submaximal estimates, not a mask-and-treadmill cardiopulmonary exercise test (CPET). You do not have to hit peak heart rate. The estimate can be off if age, sex, or weight in the Health app is wrong; if you are pregnant; if you carry a heavy pack, push a stroller, or walk on sand; or if heart rate is raised by heat, caffeine, dehydration, or altitude. People who take heart-rate-limiting medicines such as beta blockers should record that in Health Details. In Apple’s validation cohort, failing to flag those medicines produced large overestimates.

Why exercise physiology treats VO2 max as a longevity marker

Cardiorespiratory fitness has been tied to survival for decades. Blair and colleagues reported in JAMA (1989) that physical fitness predicted all-cause mortality in healthy men and women. In 2016 the American Heart Association published a scientific statement arguing that cardiorespiratory fitness should be assessed routinely, as a clinical vital sign (Ross et al., Circulation).

The quantitative reviews are the ones worth remembering when a watch shows “Below Average”:

• Kodama et al., JAMA (2009). A meta-analysis of 33 studies (102,980 people and 6,910 deaths for the all-cause analysis). Each 1-MET higher maximal aerobic capacity — about 3.5 mL/kg/min, roughly 1 km/h faster jogging — was associated with a pooled risk ratio of 0.87 for all-cause mortality (about 13% lower) and 0.85 for coronary heart disease or cardiovascular events (about 15% lower).
• Mandsager et al., JAMA Network Open (2018). 122,007 adults who had exercise treadmill tests at Cleveland Clinic, with fitness reported as estimated METs rather than directly measured VO2 max. Risk-adjusted all-cause mortality was lowest in the elite group. Elite versus low fitness had an adjusted hazard ratio of 0.20 (95% CI 0.16–0.24). Low versus elite was 5.04. Below average versus above average was 1.41, in the same range as the adjusted hazard ratios the paper reported for smoking (1.41) and diabetes (1.40), and higher than coronary artery disease (1.29). The authors found no upper limit of benefit in that cohort.
• Laukkanen et al., Mayo Clinic Proceedings (2016). This one measured VO2 max directly with respiratory gas exchange, twice, about 11 years apart, in 579 men in the Kuopio Ischaemic Heart Disease Risk Factor Study. After adjustment for baseline fitness and conventional risk factors, each 1 mL/kg/min higher long-term change in VO2 max was associated with a hazard ratio of 0.91 (95% CI 0.87–0.95) for later all-cause death. The useful point is change over years, not a single reading.

None of these papers say a wearable estimate is your biological age. They say cardiorespiratory fitness — measured or carefully estimated — tracks mortality risk in a dose-response way. Apple Watch Cardio Fitness is a convenient estimate of that fitness, with error bars.

How VO2 max connects to biological age — and where the link stops

Biological age is an estimate of physiological aging, not the count of birthdays. Epigenetic clocks built from DNA methylation are one research version of that idea. Fitness shows up in that literature, with limits.

McGreevy and colleagues built DNAmFitAge, a blood methylation clock that includes a methylation surrogate of VO2 max along with gait speed, grip strength, FEV1, and GrimAge (Aging, 2023). Those methylation fitness markers correlated only modestly with the measured traits in validation sets (average r about 0.16–0.48). People with a younger, fitter DNAmFitAge had lower mortality and coronary risk in the cohorts they studied. That is evidence that fitness is part of biological aging biology. It is not evidence that a watch VO2 max can stand in for a methylation test.

Kawamura and colleagues, in WASEDA’S Health Study (Aging Cell, 2024), measured oxygen uptake directly in 144 Japanese men aged 65–72. Both VO2 at the ventilatory threshold and peak VO2 were negatively associated with GrimAge acceleration after adjustment for age, smoking, and drinking. In their multiple regression, calf circumference, triglycerides, carbohydrate intake, and smoking contributed more to GrimAge acceleration than cardiorespiratory fitness did. Fitness lined up with a younger epigenetic age. It did not explain biological age by itself.

That is the distinction this page is for. A high Cardio Fitness level is a good longevity signal. A low one is a reason to train, and a reason to look at the rest of the picture: glucose exposure, inflammation, lipids, sleep, and recovery. A single outdoor-run estimate will not show a borderline HbA1c or a high CRP.

VO2 max alone versus a full biological age

Use Cardio Fitness as the fitness vital sign. Use a multi-biomarker biological age when you want one daily number that also includes labs and the other wearable signals. They answer different questions.

A recovery readiness score is a third, separate idea: how prepared you are today, mostly from HRV, resting heart rate, sleep, and recent load. It is not VO2 max. If that morning number is what you are looking at, see what a recovery readiness score means.

How close is the watch number to a lab VO2 max?

Apple’s 2021 white paper compared the watch estimate with submaximal projections from CPET. On the held-out validation set (221 people), the mean error was 1.4 ± 4.7 mL/kg/min. Average error was under 1 MET (1 MET is about 3.5 mL/kg/min). The standard deviation means a given person can still be several mL/kg/min away from the reference projection. Test-retest reliability on that validation set was an intraclass correlation of 0.86. About 78% of outdoor pedestrian workouts longer than 5.75 minutes produced an estimate.

Read the level and the multi-week trend. Do not treat a 1–2 mL/kg/min swing after one humid run as a change in biological age. The 2021 algorithm description also covered a range of 14–60 mL/kg/min; Apple’s current support article lists 14–65 mL/kg/min. Either way, the number is an estimate with known limits, which is why it belongs inside a broader score rather than standing as the score.

What to do after you see the level

If you do not have a Cardio Fitness value yet, wear the watch and log a few outdoor walks, runs, or hikes in the Workout app. Indoor cycling and strength sessions can be excellent training and still never update this particular metric, because Apple’s estimator is built on outdoor pedestrian effort with usable GPS and heart rate.

If the level is Low, that is the tier Apple built a notification around, and it is the tier where the treadmill-fitness literature shows the steepest mortality gap. Moving out of low fitness matters more than polishing an already high number. The practical next step is regular aerobic work you can repeat: brisk outdoor walking is enough to start populating the metric and to raise fitness if you have been sedentary. People who already train can add harder intervals. Apple’s white paper summarizes training studies in which high-intensity interval programs, typically 6–12 weeks, produced on the order of 5–10% higher VO2 max, and notes that a few weeks of inactivity can drop VO2 max by as much or more. Genetics still explain a large share of the differences between people, so compare yourself with your own trend.

If you take a beta blocker or similar medicine, confirm Health Details before you decide the estimate is “too good.” Apple reported that unflagged heart-rate-limiting medication was a major source of overestimation.

Then put the fitness number next to the signals it cannot see. A practical pairing is Cardio Fitness plus a recent lab panel — especially HbA1c and CRP — and the other watch trends covered in the Apple Watch biological age guide. If you are choosing an app to hold that combined picture, the 2026 biological age app comparison is the wider field. This page stays on VO2 max and biological age only.

This is general education, not a training prescription or a diagnosis. If you have heart disease, unexplained chest pain, or a new exercise limitation, talk with a clinician before you raise intensity.

How Nuvo uses Cardio Fitness

Nuvo: Longevity Coach (App Store id 6762535065) is an iPhone and Apple Watch app. It calculates a daily biological age from 21 biomarkers drawn from HealthKit and from optional labs. Cardio Fitness is one of those wearable inputs. The score updates daily, so a month of better aerobic work can show up in the trend without waiting for the next blood draw. Adding labs is optional; when you do, results such as HbA1c, fasting glucose, HDL, CRP, and creatinine sit in the same score as the watch data.

Coaching is on-device (Nuvo Intelligence). There is no account. Apple’s privacy nutrition label for Nuvo is Data Not Collected. Health data stays on the phone. Price is $4.99/month or $24.99/year after a 7-day trial. The annual plan is $2.08/month billed annually. Download app. For how on-device coaching is set up, see health app privacy and on-device AI.

Nuvo will not turn Cardio Fitness into a clinical VO2 max, and a high watch number will not cancel a lab result you have not entered. The point of the 21-biomarker score is to stop treating one impressive fitness tile as the whole of biological age.

Frequently Asked Questions

What is Apple Watch Cardio Fitness?
Cardio Fitness is Apple’s estimate of VO2 max in mL/kg/min. Apple Watch Series 3 or later derives it mainly from heart rate and motion during outdoor walks, runs, and hikes. The Health app classifies it as Low, Below Average, Above Average, or High for your age and sex. Apple documents the feature in its support article “Track your cardio fitness levels” and in the 2021 white paper “Using Apple Watch to Estimate Cardio Fitness with VO2 max.”

Is Cardio Fitness the same thing as a lab VO2 max?
No. A laboratory VO2 max comes from cardiopulmonary exercise testing with measured oxygen in inhaled and exhaled air. Apple Watch uses a submaximal prediction. In Apple’s held-out validation set of 221 people, mean error versus a submaximal CPET projection was 1.4 ± 4.7 mL/kg/min. Useful for tracking fitness. Not a replacement for a clinical test.

Does a higher VO2 max mean a younger biological age?
Higher cardiorespiratory fitness is associated with lower mortality and, in epigenetic studies, with slower age acceleration. It does not equal biological age. DNAmFitAge (McGreevy et al., Aging, 2023) builds fitness, including a VO2 max surrogate, into a methylation clock. WASEDA’S Health Study (Kawamura et al., Aging Cell, 2024) found higher measured peak VO2 associated with lower GrimAge acceleration in older men, while smoking and other lifestyle factors still explained more of that clock. Biological age needs more than one biomarker.

Why can my Cardio Fitness look fine while biological age does not?
VO2 max does not measure HbA1c, CRP, lipids, or kidney markers. Mandsager et al. (JAMA Network Open, 2018) showed fitness is a powerful mortality signal, and also that smoking, diabetes, and coronary disease carry their own risk. A strong outdoor-run estimate can coexist with a metabolic or inflammatory problem the watch cannot see. That is why Nuvo combines HealthKit data with optional labs in a 21-biomarker daily score.

What should I do if Apple Watch says my Cardio Fitness is Low?
Confirm the estimate has had enough outdoor walk, run, or hike data, and that age, sex, weight, and any heart-rate-limiting medicines are entered correctly in Health. If the level is truly low, consistent aerobic training is the intervention with the strongest fitness evidence. In Mandsager et al., the adjusted mortality gap between low and higher fitness was large. Raise intensity only if it is safe for you. Use the trend over weeks, not one workout.

How does Nuvo use VO2 max?
Nuvo reads HealthKit on iPhone, including Apple Watch Cardio Fitness, as one of 21 biomarkers in a daily biological age. Optional lab results can be added to the same score. Coaching runs on-device. No account is required. Apple’s privacy label is Data Not Collected. Nuvo is $4.99/month or $24.99/year ($2.08/month billed annually) after a 7-day trial. Download app on the App Store (id 6762535065).

Is a recovery score the same as VO2 max?
No. Recovery or readiness scores emphasize today’s autonomic state, sleep, and recent training load. VO2 max describes cardiorespiratory capacity. Both can inform training. Only a multi-biomarker model is trying to estimate biological age. See the recovery readiness score explainer for the daily readiness number, and keep Cardio Fitness as the fitness vital sign.

Sources

• Apple Support. Track your cardio fitness levels.
• Apple. Using Apple Watch to Estimate Cardio Fitness with VO2 max. May 2021.
• Kaminsky LA, Arena R, Myers J. Reference standards for cardiorespiratory fitness measured with cardiopulmonary exercise testing: data from the Fitness Registry and the Importance of Exercise National Database. Mayo Clinic Proceedings. 2015;90(11):1515–1523. doi:10.1016/j.mayocp.2015.07.026.
• Blair SN, Kohl HW, Paffenbarger RS, et al. Physical fitness and all-cause mortality: a prospective study of healthy men and women. JAMA. 1989;262(17):2395–2401. doi:10.1001/jama.1989.03430170057028.
• Ross R, Blair SN, Arena R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. Circulation. 2016;134(24):e653–e699. doi:10.1161/CIR.0000000000000461.
• Kodama S, Saito K, Tanaka 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. doi:10.1001/jama.2009.681.
• Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Network Open. 2018;1(6):e183605. doi:10.1001/jamanetworkopen.2018.3605.
• Laukkanen JA, Zaccardi F, Khan H, Kurl S, Jae SY, Rauramaa R. Long-term change in cardiorespiratory fitness and all-cause mortality: a population-based follow-up study. Mayo Clinic Proceedings. 2016;91(9):1183–1188. doi:10.1016/j.mayocp.2016.05.014.
• McGreevy KM, Radak Z, Torma F, et al. DNAmFitAge: biological age indicator incorporating physical fitness. Aging. 2023. doi:10.18632/aging.204538.
• Kawamura T, Radak Z, Tabata H, et al. Associations between cardiorespiratory fitness and lifestyle-related factors with DNA methylation-based ageing clocks in older men: WASEDA’S Health Study. Aging Cell. 2024;23(1):e13960. doi:10.1111/acel.13960.