When it comes to fitness and even longevity, there is one number to rule them all: VO2 max.
The number measures the maximum amount of oxygen a person can use during intense exercise. A person awaiting a heart transplant might score below 10 while a world-class endurance athlete might be above 70 or even 80. Many health experts consider it the single best indicator of cardiorespiratory fitness, and research has found it correlates to longevity as well.
“The relationship between VO2 max and all-cause mortality is quite good,” says Dr. Michael Joyner, an anesthesiologist and fitness expert at the Mayo Clinic. “The odds of dying in the next 10 years are markedly low if your VO2 max is high.”
So what is VO2 max and why is it such a good measure of total health?
For starters, the test measures how multiple systems of your body function during hard exercise, including your heart, lungs, muscles, and veins, according to Dr. Kerry Stewart, a clinical and research exercise physiologist at Johns Hopkins School of Medicine. People who score well are less likely to have diabetes and high levels of LDL, or so-called bad cholesterol, and more likely to have high levels of HDL, or “good” cholesterol.
The American Heart Association in 2016 issued a scientific statement recommending that an assessment of cardiorespiratory fitness be considered a key vital sign in evaluating a person’s heart disease risk and overall health. It said that cardiorespiratory fitness was “was a potentially stronger predictor of mortality than established risk factors such as smoking, hypertension, high cholesterol, and Type 2 diabetes.”
High cardiorespiratory fitness is also linked to lower rates for certain types of cancer, according to the National Cancer Institute. It also appears to help people already diagnosed with cancer. “Research findings have raised the possibility that physical activity may have beneficial effects on survival for patients with breast, colorectal, and prostate cancers,” the institute said.
The positive news is that you can increase your VO2 score. How? By exercising. The harder you push yourself, the more you can improve your score. Even brisk walks around the neighborhood could lift the scores and health of millions of inactive Americans.
How can you measure your VO2 max? The most accurate tests are done in a laboratory where you are hooked up to a mask that measures oxygen consumption as you exercise. If you are on a treadmill, you are told to walk or run faster and faster until your oxygen consumption stops rising. If you go beyond this point, you will increasingly be using anaerobic energy—which doesn’t burn oxygen—and you will have trouble sustaining it for long periods.
Today, some fitness devices, including
Watch and Fitbit, can estimate your VO2 max score based on your heart rate and, in some cases, the speed you are moving. They aren’t as accurate as a laboratory VO2 max test, say doctors Joyner and Stewart. “They are reasonable estimates based on reasonable algorithms,” Joyner says.
VO2 is commonly measured in milliliters of oxygen consumed a minute per kilogram of body weight. An average adult might score 35. Endurance athletes can be twice that or even higher.
Sometimes, cardiorespiratory fitness is expressed in METs, a measure of metabolic energy. (METs are calculated by taking your VO2 max score and dividing it by 3.5.) One MET is the amount of oxygen a person uses while seated and at rest. If your exertion level rises to two METS, that means you’re using twice as much oxygen. A VO2 max of eight to 10 METs is considered healthy for an average adult, while one of less than five METs is considered worrisome.
“People who have more severe disease generally can’t achieve five METs worth of work,” Stewart said.
Women, because they tend to have less muscle per kilogram of body weight than men, usually score lower, though the best female endurance athletes still post high numbers. And just as with men, higher VO2 scores for women correlate to athletic performance and mortality.
VO2 scores decrease as people age because their maximum heart rate declines and they lose muscle mass and thus burn less oxygen. So a score that would be low for a woman in her 20s might be an elite score for a woman in her 70s.
To calculate VO2, The Apple Watch uses a combination of heart rate, weight, age, other personal information and walking or running speed to measure cardiovascular fitness. It is accurate within an average of one MET of an actual VO2 max test, according to Apple.
If you walk on sand or uphill or anything that makes walking or running more arduous, that will make your Apple Watch VO2 max score appear worse. Also, if you take medicines that slow your heart rate like beta blockers or calcium channel blockers, it will make your score appear better. The Apple Watch asks you to list any heart-slowing medicines, and then makes adjustments for them.
It used to be that you had to exercise to calculate VO2 max on an Apple Watch. But an Apple watch is now set up so that you can get a score from walking around. Your score appears in the Apple Health app, in the heart section under cardio fitness.
Fitbit also estimates your VO2 max score from your resting heart rate, age, sex, weight, and other personal information, and uses it to calculate a fitness level between one (poor) and six (excellent). If your Fitbit connects to GPS, you can obtain a more precise score by going on a run.
How hard is it to improve VO2 max score? That depends on your fitness level. Someone who is inactive could improve their VO2 max through moderate exercise and make a significant improvement in their health.
Fit people have to push harder. That means either exercising longer or exercising harder. Or both. The Center for Disease Control recommends that adults get 150 minutes of moderate exercise or 75 minutes of vigorous exercise a week. You can get additional health benefits by doing two or even three times as much exercise, but there are diminishing returns. Dr. Joyner says, “You get 70% to 80% of the benefits by following the CDC’s basic guidelines,” he says.
Lifting weights can also help your VO2 score. That’s particularly true for older people who have lost muscle mass and replaced it with fat. Weights can help reverse that process.
And one of the fastest ways to improve your VO2 max is to lose weight.
For those who are serious about improving their VO2 max, Joyner recommends that they add interval training to their routines. A popular approach now is 4×4 interval training, where you do four intervals of intense exercise, separated by rest between each interval.
“In general, for any person to reach their personal biological upper limit, they need to do interval training,” Joyner says.
Corrections & amplifications: Kerry Stewart is a clinical and research exercise physiologist at the Johns Hopkins School of Medicine who does work in cardiology. An earlier version of this article incorrectly called him a cardiologist.
Write to Neal Templin at [email protected]