Is VO2 max the same as cardio fitness?

Yes. Cardio fitness, or cardiorespiratory fitness, is the general term, and VO2 max is the standard way to measure it. Apple calls its estimate Cardio Fitness, Fitbit and Google use Cardio Fitness Score, and Oura uses Cardio Capacity.

How accurate is VO2 max on a smartwatch?

Watch estimates are usually close on average but can be several points off for an individual, and estimates based on resting data tend to read high. Use your trend on one device rather than comparing single readings across brands.

Why did my VO2 max suddenly drop?

Short-term drops are usually caused by heat, hills, poor sleep, illness, dehydration or a change in body weight, all of which affect your heart rate at a given pace. A drop that lasts several weeks is more likely to reflect a real change.

How long does it take to improve VO2 max?

Most people see measurable gains within six to eight weeks of consistent training, with the biggest gains for those starting from a low base.

Can blood tests measure VO2 max?

No. VO2 max can only be measured directly with an exercise test or estimated by a wearable. Blood tests can show factors that limit it, such as low haemoglobin, low iron or an underactive thyroid.

Does losing weight increase VO2 max?

It raises the score your wearable shows, because VO2 max is divided by body weight. Losing fat without changing your training lifts the number without any change in how much oxygen your heart and muscles can handle. Training is what raises that underlying capacity.

What Your VO2 Max Says About Your Health, and What Your Blood Tests Can Explain

What is VO2 max, what is a good score for your age, and how accurate is your watch's estimate? Learn how cardio fitness connects with blood sugar, cholesterol, iron and thyroid results on your health report.

Genie Health Editorial Team · 19 July 2026 · 14 min read

What is VO2 max?

VO2 max is the maximum amount of oxygen your body can take in and use during hard exercise. It is the standard measure of cardiorespiratory fitness, and one of the strongest single predictors of long-term health.

Apple Watch, Fitbit, Google Pixel Watch, Garmin, Oura and WHOOP all estimate VO2 max, under names such as Cardio Fitness and Cardio Capacity. This article explains what VO2 max measures, what a good score looks like for your age, and how it connects with the markers on your health report.

VO2 max is measured in millilitres of oxygen per kilogram of body weight per minute (mL/kg/min). A VO2 max of 40 means that at full effort, your body can use 40 millilitres of oxygen for every kilogram you weigh, every minute. Because the number is divided by body weight, two people with the same heart and lungs can have different scores if one weighs more.

Every step of oxygen's journey through the body affects your score. It works like a supply chain with four links:

The lungs take oxygen from the air into the blood.
The heart pumps oxygen-rich blood around the body. The amount it pushes out with each beat, called stroke volume, is the biggest single factor in VO2 max, and it is what aerobic training enlarges most.
The blood carries the oxygen. Almost all of it travels bound to haemoglobin, the iron-containing protein in red blood cells.
The muscles pull oxygen out of the blood and use it to make energy. This depends on how many small blood vessels (capillaries) supply the muscle, and how many mitochondria, the structures that burn fuel using oxygen, the muscle cells contain.

VO2 max is often thought of as purely a heart measure, but the muscles matter as much as the heart. A strong heart cannot make up for blood that carries too little oxygen, or for muscles that cannot use it.

How your wearable estimates VO2 max

Wearables estimate VO2 max from how hard your heart works at a given effort, because a fitter heart beats more slowly at the same pace. Each device needs its own kind of data before it can show a score:

Apple Watch (Cardio Fitness) estimates VO2 max during outdoor walks, runs and hikes recorded in the Workout app, on fairly flat ground and at an effort that raises your heart rate well above resting. It needs at least a day of wear and several of these workouts before the first reading, and it takes account of your age, sex, weight, height and any heart rate medications you have entered.
Fitbit and Google Pixel Watch (Cardio Fitness Score) start with a range based on your resting heart rate, age, sex and weight. To narrow it to a single number, track an outdoor run of at least 10 minutes with GPS, ideally on flat ground.
Garmin needs an outdoor run of at least 10 minutes with GPS and heart rate, and some models also require you to reach about 70% of your maximum heart rate. It compares your heart rate with your pace, or with power from a power meter for cycling, and the estimate settles after a few runs.
Oura (Cardio Capacity) gives a starting estimate from your age, sex, height and weight, then refines it with a guided six-minute outdoor walking test that uses your phone's location to track your pace.
WHOOP needs at least 14 nights of recovery data within 21 days before it shows a VO2 max estimate. It combines heart rate, movement and recovery data, updates weekly, and lets you enter a lab-tested VO2 max to calibrate it.

The most accurate way to measure VO2 max is a cardiopulmonary exercise test (CPET) in a lab or clinic, where you exercise to exhaustion on a treadmill or bike while wearing a mask that analyses the air you breathe. Wearables estimate the same thing indirectly.

A review of wearable validation studies found that estimates based on exercise data were accurate on average, while those relying on resting data tended to overestimate VO2 max. Individual errors were wide in both cases.

A sudden drop in your score rarely means your fitness has disappeared. Because the estimate depends on heart rate, anything that raises your heart rate at the same pace lowers it: heat and humidity, hills, dehydration, poor sleep, illness or caffeine. Weight gain lowers it too, and an out-of-date weight in your app skews it in either direction. Your trend over several weeks on the same device is far more reliable than any single reading.

What VO2 max tells you about your health

A higher VO2 max for your age is linked to a longer life and a lower risk of heart disease and type 2 diabetes. Cardiorespiratory fitness predicts the risk of death more strongly than most of the risk factors measured at a health screening, which is why the American Heart Association has called for it to be treated as a clinical vital sign.

In a study of more than 750,000 US veterans, fitness predicted the risk of death better than smoking, high blood pressure, diabetes or high cholesterol. Each small step up in fitness (3.5 mL/kg/min of VO2 max) was linked to a 14% lower risk of death. The biggest benefit comes from moving out of the least-fit group, so if your VO2 max is low, even a small improvement counts.

Fitness matters this much because it reflects how well the whole body works together, from the heart and blood vessels to how the muscles handle glucose and fat. That is also why it connects with so many markers on a health report. Your wearable shows whether your fitness is rising or falling, but not why your score is low or has stopped improving despite regular training. Blood sugar, triglycerides and HDL cholesterol tend to move with fitness, while haemoglobin, iron and thyroid results can explain a score that will not move.

What is a good VO2 max for your age?

A good VO2 max is one at or above the average for your age and sex, and higher is better at every age. Values peak in your twenties and then fall by roughly 10% per decade in people who are not regularly active. Women's values are typically lower than men's, mainly because men have larger hearts, more muscle mass and higher haemoglobin levels.

The tables give a snapshot of lab-measured VO2 max in healthy adults from the US FRIEND registry. The 50th percentile is the median: half of people tested score above it and half below. As a rough guide, a score below the 25th percentile is low for your age, between the 25th and 75th is around average, and above the 75th is high. Full charts by age and sex cover more percentiles and age groups.

Men (mL/kg/min)

Age	25th percentile	50th percentile (median)	75th percentile
20 to 29	37.3	45.4	52.6
30 to 39	31.3	38.6	46.5
40 to 49	28.4	34.8	41.8
50 to 59	23.9	29.4	35.5
60 to 69	19.7	24.4	29.9

Women (mL/kg/min)

Age	25th percentile	50th percentile (median)	75th percentile
20 to 29	28.6	35.6	42.2
30 to 39	23.1	28.3	34.5
40 to 49	21.3	25.9	30.9
50 to 59	19.5	23.1	27.3
60 to 69	16.4	19.4	23.1

Most wearables also place your score in a category, such as below average, above average or high, compared with people of your age and sex. Those categories use each company's own reference data, so they will not match this table exactly.

Genetics shapes both where your VO2 max starts and how much it responds to training, so people following the same programme improve by different amounts. Age does not stop it improving, though. Older adults who start structured aerobic training still raise their VO2 max, often by 10% or more.

How VO2 max relates to blood sugar, cholesterol and blood pressure

VO2 max and the metabolic markers on a health report tend to move together. People with a higher VO2 max usually have lower blood sugar and triglycerides, higher HDL cholesterol and lower blood pressure. The two share the same underlying causes: how much you exercise, how much body fat you carry, and how well your muscles use glucose and fat.

The training that raises VO2 max improves these markers too. A large analysis of 160 exercise trials found that as fitness rose, triglycerides, HDL cholesterol, insulin resistance and inflammation markers all improved.

That makes your fitness trend a useful companion to these results. A VO2 max that has been rising or falling for months shows whether the habits behind your blood sugar and cholesterol are changing, between blood tests that may be a year apart.

Blood sugar: fasting glucose and HbA1c

The clearest link is with blood sugar. Muscle is the body's largest store of glucose, and fitter muscle has more mitochondria and more of the transporters that pull glucose out of the blood. Some of this happens without insulin, during and after exercise. Over time, the body also becomes more responsive to insulin, so it needs less to keep glucose steady.

People with a lower VO2 max tend to have higher fasting glucose and HbA1c. Timing matters when you compare the two: VO2 max can rise within six to eight weeks of consistent training, while HbA1c reflects your average blood sugar over the previous two to three months. A three-month climb in your fitness trend lines up with the window your next HbA1c covers.

Triglycerides and HDL cholesterol

Fitter muscles burn more fat for fuel. Aerobic training increases lipoprotein lipase, an enzyme in muscle that breaks down triglycerides in the blood so they can be used for energy. As a result, triglycerides tend to fall and HDL cholesterol tends to rise as VO2 max improves. These are the most consistent lipid changes linked to fitness.

LDL cholesterol and ApoB are a different matter. They are not directly linked to VO2 max, and they depend more on diet, body weight and genetics. Training can improve them modestly as part of wider changes, but a rising fitness score is no guarantee that your LDL will fall.

Blood pressure and hs-CRP

As stroke volume increases, the heart pumps more blood per beat, so it can beat more slowly at rest. Regular aerobic training also keeps blood vessels more flexible, which helps lower blood pressure. A falling resting heart rate on your wearable often moves alongside a rising VO2 max.

Higher fitness is also linked to lower hs-CRP, a blood marker of low-grade inflammation. Part of this comes from losing fat around the waist, which produces inflammatory signals, but the link has held in some studies even after accounting for body weight.

The link may also run the other way. Chronic low-grade inflammation is linked to stiffer blood vessels that widen less during exercise, which limits blood flow to working muscles.

Other markers on a health report, such as ALT (a liver enzyme), tend to improve when training reduces liver fat. They are not direct VO2 max markers, though, and they are more closely tied to weight and diet.

A health report on its own can show that your HbA1c or triglycerides have crept up, but not what changed in your daily life. A fitness trend that has slipped over the same months adds a likely part of the answer. Genie brings the two together. It connects with Oura, WHOOP, Fitbit, Apple Health and Google Health and tracks the reports you upload over time, so its Sparks insights can link your VO2 max trend with your report results. A Connected Signals card might show that your VO2 max has fallen over six months while your HbA1c and triglycerides have risen across your last two reports. When your fitness climbs, Sparks can show your blood sugar and lipids improving with it.

Why your VO2 max is low or not improving: what your blood tests can show

When VO2 max stays low or stops improving despite consistent training, the cause can sit in one of the links of the oxygen supply chain. Some of those links show up directly on a health report. A blood test cannot measure your VO2 max, but it can show whether your blood is carrying enough oxygen, or whether your thyroid is slowing your heart and muscles.

Link in the chain	What can limit it	Markers to look at
Blood carrying oxygen	Low haemoglobin (anaemia) or low iron stores	Haemoglobin, haematocrit, ferritin
Heart and muscle energy	An underactive thyroid	TSH, with free T4

Haemoglobin and iron

The most direct link between a blood test and VO2 max is haemoglobin. It carries almost all the oxygen in your blood, so when it is low, less oxygen reaches your muscles with each heartbeat, however strong your heart is. Anaemia lowers VO2 max and can make exercise feel much harder than it should, with your heart rate rising quickly at an easy pace. Your wearable will usually read that higher heart rate as lower fitness.

Iron is needed to make haemoglobin, so a low ferritin, the marker of your iron stores, can come before anaemia. Low iron without anaemia is less clearly linked to a lower VO2 max, but it can still affect energy and endurance. Low iron is especially common in women who menstruate, people who eat little or no meat, and endurance athletes. Vitamin B12 and folate are also needed to make red blood cells, and low levels can cause anaemia too.

A very high haemoglobin or haematocrit is not a sign of better fitness. It can reflect dehydration or other causes, and it is worth discussing with a doctor.

Thyroid: TSH and free T4

Thyroid hormones set the pace of energy use across the body, including how strongly the heart contracts and how actively the muscles' mitochondria work. An underactive thyroid reduces both, and studies have found lower oxygen uptake during exercise in people with hypothyroidism. Alongside a stalled VO2 max, it often brings tiredness, feeling cold and weight gain.

TSH is the usual first test. A high TSH suggests an underactive thyroid, and free T4 helps confirm it.

Raised hs-CRP and poor blood sugar control can also go with a lower VO2 max, as the previous section explains, but they limit oxygen delivery less directly than low haemoglobin or an underactive thyroid.

A wearable can show that your VO2 max has plateaued for three months despite regular training, but it cannot see your blood. Your health report can show a low ferritin or a raised TSH, but it cannot tell you that your training has stopped paying off. With both in Genie, you can Ask Genie questions such as "Why has my VO2 max stopped improving?" Genie looks at your fitness trend alongside your activity, resting heart rate and report results, such as haemoglobin, ferritin and thyroid markers, and explains which might be relevant. It can also help you prepare questions for your doctor, who can decide whether further tests are needed.

How to improve your VO2 max

VO2 max improves when you regularly exercise hard enough to push your heart and breathing close to their limits. Sustained aerobic training over several weeks to months raises it at almost any age and starting point.

Brisk walking works if you are starting from a low base. For someone who is unfit, a brisk or uphill walk raises the heart rate enough to improve VO2 max. Once you are fitter, the same walk becomes too easy to push it further, which is why daily walking alone does not guarantee a high score.
Steady, moderate exercise builds the base. Longer sessions of jogging, cycling or swimming at a pace where you can still talk (often called Zone 2) increase the capillaries and mitochondria in your muscles, improving how well they extract and use oxygen.
Intervals raise the ceiling. Short bouts at 85% to 95% of your maximum heart rate push stroke volume to its limit. In one well-known study, the "4x4" format (four minutes hard, three minutes easy, repeated four times) raised VO2 max by about 7% in eight weeks, more than steady training using the same total energy.
Consistency matters most. Most programmes combine mostly moderate sessions with one or two harder ones a week. VO2 max falls within weeks if training stops.

If you have a heart condition, high blood pressure or symptoms such as chest pain or breathlessness during exercise, speak to your doctor before starting high-intensity training.

Why VO2 max is worth reading alongside your health report

VO2 max reflects how well your lungs, heart, blood and muscles work together. Your wearable tracks it over months, and your health report shows what is happening in your blood. Read together, they can explain why a score is stuck and show whether your training is improving your blood sugar and cholesterol.

Connect your wearable and upload your health reports to Genie to see your VO2 max in the context of the rest of your health.

FAQs

Is VO2 max the same as cardio fitness?: Yes. Cardio fitness, or cardiorespiratory fitness, is the general term, and VO2 max is the standard way to measure it. Apple calls its estimate Cardio Fitness, Fitbit and Google use Cardio Fitness Score, and Oura uses Cardio Capacity.
How accurate is VO2 max on a smartwatch?: Watch estimates are usually close on average but can be several points off for an individual, and estimates based on resting data tend to read high. Use your trend on one device rather than comparing single readings across brands.
Why did my VO2 max suddenly drop?: Short-term drops are usually caused by heat, hills, poor sleep, illness, dehydration or a change in body weight, all of which affect your heart rate at a given pace. A drop that lasts several weeks is more likely to reflect a real change.
How long does it take to improve VO2 max?: Most people see measurable gains within six to eight weeks of consistent training, with the biggest gains for those starting from a low base.
Can blood tests measure VO2 max?: No. VO2 max can only be measured directly with an exercise test or estimated by a wearable. Blood tests can show factors that limit it, such as low haemoglobin, low iron or an underactive thyroid.
Does losing weight increase VO2 max?: It raises the score your wearable shows, because VO2 max is divided by body weight. Losing fat without changing your training lifts the number without any change in how much oxygen your heart and muscles can handle. Training is what raises that underlying capacity.

References & Sources

Kokkinos P, et al. "Cardiorespiratory Fitness and Mortality Risk Across the Spectra of Age, Race, and Sex." Journal of the American College of Cardiology, 2022. https://www.jacc.org/doi/10.1016/j.jacc.2022.05.031
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. https://www.ahajournals.org/doi/10.1161/cir.0000000000000461
Mandsager K, et al. "Association of Cardiorespiratory Fitness With Long-term Mortality Among Adults Undergoing Exercise Treadmill Testing." JAMA Network Open, 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC6324439/
Kaminsky LA, et al. "Updated Reference Standards for Cardiorespiratory Fitness Measured with Cardiopulmonary Exercise Testing: Data from the Fitness Registry and the Importance of Exercise National Database (FRIEND)." Mayo Clinic Proceedings, 2022. https://www.sciencedirect.com/science/article/pii/S0025619621006455
Molina-Garcia P, et al. "Validity of Estimating the Maximal Oxygen Consumption by Consumer Wearables: A Systematic Review with Meta-analysis and Expert Statement of the INTERLIVE Network." Sports Medicine, 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC9213394/
Balady GJ, et al. "Clinician's Guide to Cardiopulmonary Exercise Testing in Adults: A Scientific Statement From the American Heart Association." Circulation, 2010. https://www.ahajournals.org/doi/10.1161/cir.0b013e3181e52e69
Lin X, et al. "Effects of Exercise Training on Cardiorespiratory Fitness and Biomarkers of Cardiometabolic Health: A Systematic Review and Meta-Analysis of Randomized Controlled Trials." Journal of the American Heart Association, 2015. https://www.ahajournals.org/doi/10.1161/jaha.115.002014
Helgerud J, et al. "Aerobic high-intensity intervals improve VO2max more than moderate training." Medicine & Science in Sports & Exercise, 2007. https://pubmed.ncbi.nlm.nih.gov/17414804/
Bouchard C, et al. "Genomic predictors of the maximal O2 uptake response to standardized exercise training programs." Journal of Applied Physiology, 2011. https://pubmed.ncbi.nlm.nih.gov/21183627/
"Anemia in sports: a narrative review." https://pmc.ncbi.nlm.nih.gov/articles/PMC8472039/
"Pulmonary consequences of hypothyroidism." https://pmc.ncbi.nlm.nih.gov/articles/PMC5541969/
"Association between cardiorespiratory fitness and glycaemic markers." https://pmc.ncbi.nlm.nih.gov/articles/PMC4407752/
Apple. "Track your cardio fitness levels" (Apple Support). https://support.apple.com/en-gb/HT211856
Google. "Cardio Fitness Score" (Fitbit Help). https://support.google.com/fitbit/answer/14237924
Oura. "Cardio Capacity." https://ouraring.com/blog/cardio-capacity/
WHOOP. "Estimate your VO2 max with WHOOP." https://www.whoop.com/thelocker/estimate-your-vo-max-with-whoop-/
Garmin. "Performance Measurements" (Garmin owner's manual). https://www8.garmin.com/manuals/webhelp/GUID-676967A0-1B23-4384-9BC9-76F3D643F1C8/EN-US/GUID-F5BF67CE-C94E-4842-AE96-A7A05C85B732.html

Note: This article is for education and awareness only. Genie does not provide medical advice. Please consult a healthcare professional for anything specific to your health.