In this video, Dr. Jason Tso, clinical director of the Sports Cardiology Program at Stanford Health Care, explains the concept of VO2 max and its impact on cardiovascular health and overall health. He goes into detail on how exercise strengthens the heart, improves energy and endurance, lowers the risk of heart disease, diabetes, and stroke, and supports healthy aging.

1. What Is VO2 Max and Why Does It Matter?

Dr. Tso opens the talk with an introduction from James Liu of the Stanford Health Library, noting the strong interest in this session, which more than 500 people registered for. Dr. Tso reveals that today's topic is exercise, VO2, and cardiovascular (CV) health, and lays out three goals for the talk:

• Defining cardiopulmonary fitness: he will explain it in relation to VO2.
• Understanding cardiopulmonary exercise testing (CPET) and VO2 max.
• Explaining the relationship between fitness or VO2 and overall health, especially cardiovascular health.

After asking the audience whether they have a medical or physiology background, Dr. Tso continues the talk by answering the very first question that comes up: "What is VO2?" VO2 has been around for 50 years, but in recent years it has gained huge popularity among "longevity influencers" and the "general fitness community."

Put simply, VO2 means the ability to consume oxygen. Even at this very moment, all of us are breathing and using oxygen to carry out our daily functions. VO2 max refers to the maximum of this oxygen consumption capacity, and he explains that reaching this maximum requires maximal exercise. In other words, VO2 max is an objective number representing the maximum amount of oxygen the body can take in while performing maximal exercise.

"VO2 max is the maximum amount of oxygen each individual can use. And to reach this maximal oxygen capacity, you need maximal exercise."

VO2 max is a multi-organ measurement in which not only the heart, lungs, and muscles but the entire body—including the brain and fat cells—uses oxygen. The unit of measurement is milliliters (of oxygen) / minute (time) / kilogram (body weight), which expresses oxygen use relative to body weight and allows comparison across people of different body types. The actual measurement is done by measuring the volume of air going in and out through a mask.

2. VO2 Max and the Role of the Body's Organs

To explain how VO2 max works, Dr. Tso shows a complex diagram (the Wasserman diagram) and walks through, step by step, the process by which oxygen molecules enter the body and generate energy. 😮

1. Lungs: Oxygen that enters through the mouth and nose goes into the lungs upon inhalation.
2. Heart: Oxygenated blood from the lungs enters the right side of the heart and is pumped out to the whole body from the left ventricle.
3. Muscles: Blood is delivered to the muscles through the arterial system, and the mitochondria inside muscle cells use oxygen to generate all of the body's energy (ATP).

He explains that a problem at any stage in this process can affect the ability to use oxygen.

2.1. The Relationship Between Muscle and VO2 Max

Dr. Tso introduces a study explaining how muscle affects VO2. By analyzing thigh muscle samples, the study showed that the greater the muscle mass, the higher the capacity to consume oxygen.

• Heart failure patients (red): They have muscle but can use almost no oxygen.
• General population (white): They use far more oxygen than heart failure patients.
• Endurance athletes (blue): They can use far more oxygen than either of the other two groups.

These results show that a muscle's oxygen-use capacity has a linear relationship with an individual's overall oxygen-use capacity, and they emphasize that the greater the muscle mass, the greater the oxygen capacity and exercise ability. However, Dr. Tso made the important point that "exercise capacity is limited by the heart, not by the muscle."

"You are limited by your heart, not by your muscles. But your VO2 is reflected in your muscle mass and muscle capacity."

2.2. The Role of the Lungs and Heart

The lungs serve to take in oxygen and deliver it to the blood. He explains that, in this process, the main factor limiting oxygen delivery is not the volume of air in the lungs themselves but the volume of blood (blood flow) circulating around the lungs.

The most important organ is the heart. Dr. Tso said that the main factor limiting exercise capacity is cardiac output. Cardiac output refers to the amount of blood the heart can pump over a given period of time.

• Diastole: The phase when the heart fills with blood. During this phase, the maximum volume of blood fills the ventricles.
• Systole: The phase when the heart pumps blood. During this phase, blood is supplied throughout the body.

Stroke volume is the amount of blood pumped per single heartbeat, and it is calculated as cardiac output = stroke volume x heart rate. The higher this cardiac output, the better the exercise capacity.

In particular, chronic exercise—especially endurance training—changes the shape of the heart, making it larger, and as a result it can pump more blood. Maximum heart rate declines slightly with age, but if stroke volume increases through training, cardiac output rises and oxygen consumption capacity improves as well. In other words, as the heart grows larger through training, it becomes able to deliver more oxygen throughout the body.

"When you train, your heart gets bigger so it can deliver more oxygen throughout your body."

He says the exercises that induce the most dramatic cardiac changes are long-term endurance sports that require many hours and much effort over years, such as cycling, rowing, swimming, and cross-country skiing.

3. Changes in VO2 Max and the Importance of Exercise

3.1. VO2 Max by Age, Sex, and Fitness Level

Dr. Tso explains in detail how VO2 max varies by age, sex, and fitness level.

• Age: Unfortunately, VO2 max declines with age. Typically it peaks in the mid-30s to early 40s, after which it declines by about 10% per decade.
  • Athletes: Active people start at a higher VO2 max and decline relatively slowly.
  • Inactive people: They start at a low VO2 max and fall to even lower levels, so as they age they face a greater risk of losing independence in daily life.

"VO2 max declines with age. This is an unfortunate reality of life. In our mid-30s to early 40s, our true potential plateaus. And after reaching the peak, declining by about 10% per decade is typical."

• Sex: Men generally have higher VO2 max numbers than women. Young men peak in their 50s, while young women peak in their 40s.
• Type of sport: Endurance-based sports such as cross-country skiing, running, cycling, speed skating, rowing, and swimming require the highest VO2 max, and their athletes show high numbers. By contrast, sports that require less sustained high-intensity activity, such as baseball or basketball, have relatively lower VO2 max numbers.

3.2. How to Improve VO2 Max and Detraining

One of the fastest ways to raise VO2 max is to reduce excess body fat. Because fat does not use as much oxygen as muscle, losing weight mathematically increases the VO2 max number.

• Effects of exercise:

  • Sedentary people: A healthy person who does not exercise can expect roughly a 20% improvement in VO2 max. Someone who is severely obese and has low fitness may see an even greater improvement.
  • People who already exercised: Those who previously exercised can also gain some benefit, but they may not see as large an effect as a sedentary person.

• Time: The effects of exercise begin to appear after about 2 to 3 months, and you need to exercise consistently for about 6 to 12 weeks to see consistent change.

• Type of exercise:

  • High-intensity interval training (HIIT): In typical 8-week short-term studies, it has been shown to be more effective than moderate-intensity continuous exercise.
  • Strength training: Especially for older people with low muscle mass, building muscle mass through strength training can increase VO2 max.

"Every form of exercise increases VO2. Different approaches show slight differences, but everything helps."

However, with detraining—that is, stopping exercise—those effects disappear quickly. Even just 10 days without exercise causes VO2 max to decline steeply, because cardiac output and stroke volume drop and the heart's size shrinks. The fact that it takes 3 months to gain VO2 max through exercise but only 10 days to lose it is a somewhat disappointing reality. 😥

4. VO2 Max, Longevity, and the Controversy Over Excessive Exercise

Dr. Tso emphasized that VO2 max, or exercise capacity, is closely related to longevity. Citing the results of large-scale studies from the U.S. Department of Veterans Affairs (VA) and the Cleveland Clinic, he showed that the better one's exercise capacity, the significantly lower the risk of death.

• VA study: In a study of hundreds of thousands of people, regardless of age, those with better fitness were more likely to live longer, and the lower the fitness, the higher the mortality rate.
• Cleveland Clinic study: In a treadmill test of 120,000 people, the longer someone could last on the treadmill—that is, the better their fitness—the higher their survival rate 10 years later.

"The higher your fitness, the more likely you are to live longer."

However, Dr. Tso also mentioned the controversy over excessive exercise. According to a theory called the "reverse J-shaped curve," there is an argument that appropriate exercise greatly reduces health risk, but excessive exercise beyond a certain point may actually be harmful to health.

• Potential risks of excessive exercise: There is an argument that very active people (exercising more than 10–25 hours per week) may face an increased risk of the heart becoming too large, abnormal heart rhythms (atrial fibrillation, AFib), and certain types of coronary artery disease.
• Dr. Tso's view: He takes a skeptical stance toward this argument, explaining that most studies have failed to demonstrate a clear link between excessive exercise and increased mortality. In particular, he pointed out that in the case of the Copenhagen Heart Study, the data's margin of error was so large that it was hard to draw conclusions.

"I think the claim that excessive exercise increases mortality still lacks data."

If anything, studies of elite athletes show the opposite.

• Tour de France athlete study: Former Tour de France athletes lived longer than the general French population and had lower mortality.
• U.S. Olympic athlete study: Former U.S. Olympic athletes were significantly less likely than the general population to develop various diseases (digestive, pulmonary, endocrine, etc.).

In conclusion, Dr. Tso summarized that "people who exercise a lot may have some heart-related risks, but overall they live longer," delivering the message that for most people, "more exercise is better."

5. Determinants of VO2 Max and Measuring Exercise Intensity

5.1. Determinants of VO2 Max

VO2 max is determined by several factors.

• Age: The younger you are, the higher the VO2 max.
• Sex: Men generally have higher VO2 max than women.
• Body size: The greater the muscle mass, the higher the VO2 max.
• Training effect: VO2 max can be improved through exercise training.
• Genetic factors: Who can improve the most and who cannot is still an area of research, and genetic factors are highly likely to play a role as well.

5.2. The VO2 Max Test (CPET) Process

The CPET (Cardiopulmonary Exercise Test) used to actually measure VO2 max proceeds as follows.

• Medical-grade equipment: An electrocardiogram (EKG) and pulmonary function testing are performed together.
• Individualized protocol: The treadmill or bicycle exercise load is adjusted for each individual so that they can exercise as hard as possible within about 12 minutes.
• Wearing a mask: A mask is worn to accurately measure the volume of air (oxygen) being breathed.
• Results: This yields the VO2 max value and exercise data.

5.3. Measuring Exercise Intensity with Smart Devices and Its Limits

Recently, it has become possible to measure exercise intensity and VO2 max through smart devices such as the Apple Watch or Garmin.

• Maximum predicted heart rate: The formula 220 minus age is used, but accuracy is low because individual variation is large. (a range of 220 minus age ± 22)
• Heart Rate Recovery: This indicates how quickly the heart rate drops when you stop exercising. The better your fitness, the faster the heart rate drops, and it can be a good indicator of health.
  • A decrease of at least 15 BPM (beats per minute) within one minute after stopping exercise is a healthy sign.
  • For athletes this can drop by much more, and it decreases with age.
  • Dr. Tso explained: "How quickly your heart rate drops after intense exercise is also an indicator of how quickly your body can recover from stress. People with stronger heart rate recovery are usually healthier people."

"People with stronger heart rate recovery are usually healthier people."

• Heart rate measurement on smart watches: Most smart watches measure heart rate using light, and it is fairly accurate but not perfect.
• VO2 max estimation on smart watches: They estimate VO2 max based on running speed and heart rate. At the population level it is quite accurate, but for individuals there can be large errors. Dr. Tso predicted this feature "will get better within a few years," but he advised that for now it is hard to fully trust without professional measurement.

5.4. Adjusting Exercise Intensity and the Importance of Breathing

Dr. Tso shared a simple way to gauge exercise intensity even without a mask: using the degree of your breathing. When we exercise, our body produces acid, and breathing increases in order to expel this acid.

• Zone 1 (low intensity): A level at which you can comfortably hold a conversation—your body can clear acid as fast as it produces it. 🏃‍♀️ At least 150 minutes per week is recommended.
• Zone 2 (moderate intensity): The point at which you become short of breath and conversation starts to become difficult. Your body starts producing acid faster than it can clear it. 🏃‍♂️
• Zone 3 (high intensity): A level at which you are so out of breath you can barely speak. This intensity is the most effective for improving VO2 max, but it can only be sustained for a short time (interval training). 🏃‍♀️💨 Usually recommended about 1–2 times per week.

Dr. Tso noted that most athletes combine 4–5 low-intensity sessions with 1–2 high-intensity sessions, and he advised that ordinary people would do well to use these proportions as a reference.

6. Conclusion

Wrapping up the talk, Dr. Tso emphasized the following three key points.

• VO2 max is a comprehensive fitness indicator and the best number for measuring aerobic exercise capacity.
• Fitness is deeply related to longevity. The better one's fitness, the more likely one is to live longer.
• Fitness can be improved through training. Any exercise is better than none, and you can also optimize your training with the help of an expert.

He mentioned that you can get VO2 max measurement and exercise physiology consultations at Stanford Health Care's performance clinic, but he added that for most people, "just knowing about exercise is enough, and actually doing something is what matters most."

"Just knowing about exercise is enough, and actually doing something is what matters most."

7. Q&A

After the talk, a variety of questions from the audience followed.

• Heart rate recovery: A fast drop in heart rate after exercise is evidence that the parasympathetic nervous system, the part of the autonomic nervous system responsible for relaxation, is strong, and it is a sign of good overall health and fitness.
• Optimal amount of exercise: Because individual variation is large, it is hard to know the exact optimal amount, but it is generally estimated that a few times the minimum recommended amount (5–10 hours per week) may be appropriate. Unless you are an elite athlete, there is no need to worry about the risks of excessive exercise.
• Effects of blood pressure medication and statins: Beta-blocker class blood pressure medications can lower heart rate and reduce VO2 max, but statins do not significantly affect VO2 max.
• Injury risk when starting exercise in midlife or later: When starting exercise in midlife, the risk of orthopedic injuries (musculoskeletal injuries) is greater than that of heart problems. Heart problems such as atrial fibrillation (AFib) are more common in elite athletes, but they may not be life-threatening.
• Atrial fibrillation (AFib) screening: If you have symptoms or are a high-intensity athlete, you may consider AFib screening. Atrial fibrillation is 5–10 times more common in athletes than in the general population.
• Atrial fibrillation and exercise: If you have atrial fibrillation, it is best to avoid overly strenuous exercise, and recently there are many cases where atrial fibrillation can be treated through catheter ablation, allowing a return to exercise.
• Heart size and longevity: Research on the relationship between athletes' cardiac enlargement and longevity or healthspan is still lacking, and more study is needed.
• Exercise for older adults: Research data on elderly athletes aged 80–90 and older is lacking, but age itself is not a reason to limit exercise, and he emphasized that exercising for enjoyment is a positive thing.
• HRV (Heart Rate Variability): Heart rate variability indicates how irregular the heartbeats are, and the better one's fitness, the higher the HRV.

The talk concluded by emphasizing the positive impact of exercise on health and longevity, and by stressing the importance of consistently practicing exercise that fits one's own health condition and goals. 🥳