‘New VO₂ max: 58.7ml/kg/min,” Bryan Johnson, the 45-year-old tech entrepreneur spending a reported $2m (£1.6m) a year on de-ageing himself, recently announced on Twitter. “Ranking me in the top 1.5% of 18-year-olds.” As a marker of the body’s ability to use oxygen, that’s pretty good, but maybe more impressive than the number is the fact that he put himself through the test in the first place. Any runner who has done it knows that gradually cranking up the pace on a treadmill while wearing a rubber mask isn’t exactly a great afternoon.

But does that stat actually mean anything? Is Johnson’s number good? And does anyone outside the winter biathlon circuit need to know their VO₂ max?

Let’s start with definitions. As a concept, VO₂ max was first studied in the 1920s by researchers who observed that oxygen consumption increases with running speed up to a certain limit, and then no more. At this point, your body has to rely on other sources of energy, and can’t sustain that level of effort for long. Your number, then, represents that peak: the maximum amount of oxygen (O₂), measured by volume (V), that your body can take in while exercising.

VO₂ max is often interpreted as an indicator of cardiorespiratory fitness and many exercise-training studies use it as a measure of effectiveness. Endless training plans offer to improve your VO₂ max, usually using excruciating three-minute bursts of fast-paced running interspersed with lower-intensity efforts. “Relative” VO₂ max, which allows comparison between athletes, is measured, as in Johnson’s tweet, in millilitres of oxygen per kilogram of body weight per minute.

VO₂ max is slightly tricky to measure properly, however.

Many smartwatches will give you a figure calculated by evaluating the relationship between your running pace and your heart rate, while old-school runners often rely on testing their pace over various distances and working out their results with a calculator. Most accurate, of course, is getting on a treadmill or stationary bike while wearing that rubber mask (usually available at professional sports facilities or high-end gyms), which allows testers to measure the mix of gases you’re breathing in and out, giving you the best picture possible of your oxygen consumption.

And, to be clear, 58.7ml/kg/min is pretty good. The highest recorded scores are in the 90s, most by Olympic-calibre cyclists and cross-country skiers, but the average sedentary man puffs in the 35 to 40 range. Women average slightly lower at 27 to 39. It’s not the only determinant of performance in endurance sports – 1969 world record marathon runner Derek Clayton reportedly only posted a 69, while slower runners were putting up numbers in the 80s – but it certainly helps.

How much does this all matter for your health? The surprising answer, considering that most people won’t know their number, is: probably quite a lot.

“There is an incredible amount of evidence indicating that your VO₂ max can predict your risk for cardiovascular disease,” says Brady Holmer, a runner and performance specialist who is conducting PhD research on cardiovascular physiology. “What’s more, the association of VO₂ max with morbidity and mortality is stronger than that for the risk factors usually considered, like cholesterol, BMI and blood glucose.” In one large-scale meta-analysis in 2022, for instance, subjects who ranked in the top one-third of aerobic fitness – as measured by VO₂ max – had a 45% reduced risk of death from any cause compared with individuals in the lower third.

But does increasing your VO₂ max somehow make you healthier, or is it simply an indicator of a healthily functioning body?

Probably a bit of both, Holmer explains. “VO₂ max is an assessment of whole-body function. Your maximal aerobic capacity is an integration of physiological systems. Your autonomic nervous system, heart and blood vessels, lungs and your mitochondria, among others, all influence your maximal aerobic capacity. You also need a strong body, as more muscle mass means you can extract more oxygen from your blood – and muscle mass and strength are both highly related to morbidity and mortality.”

If you have a high VO₂ max, in other words, you’ll probably also be benefiting from an array of other physiological metrics that mean you’re in good health – so our treadmill-pounding millionaire might be on the right track.

But to what extent can a VO₂ max be improved? Until quite recently, the common wisdom was that the measure is largely genetic: some people have a high number naturally, others don’t, and training might only give you a 10% bump. Studies are thin on the ground because training to specifically improve VO₂ max is difficult and lengthy.

Some coaches believe that long, slow workouts might work better over the long term to maximise your oxygen consumption than short, high-intensity ones. The take-home message is that if your training includes both styles of workout, you’ll be doing plenty to improve your health – even if you never actually put on the mask and learn your own number.