Why VO2 max tracks how well you're aging

VO2 max is the most oxygen your body can use per minute at full effort, measured in millilitres per kilogram of body weight. It depends on your heart, lungs, blood and muscles working together, which is why one number says so much.

The largest study on it followed 122,007 people who took a treadmill test at the Cleveland Clinic. Over about 8 years, people in the bottom quarter of fitness for their age and sex were five times as likely to die as the top 2%. The gap between below-average and above-average fitness was as large as the gap between smokers and non-smokers. These were patients sent for a test, and the treadmill estimated their fitness rather than measuring it directly, but other large studies point the same way.

VO2 max falls with age, and the fall speeds up. A US study that retested the same people for years found it dropped 3 to 6% per decade in their 20s and 30s, and more than 20% per decade from their 70s. Active people declined at about the same rate. What exercise changes is where you start, so fitter people have further to fall before stairs and hills get hard.

Training raises it at any age. In trials with healthy adults under 45, a few months of steady cardio raised VO2 max by about 5 ml/kg/min, and intervals did slightly better. In 41 trials with sedentary people over 60, endurance training raised it by 16% on average.

Ageon uses the VO2 max from your Apple Watch when you have one. Without one, it estimates VO2 max from a recent run, a 5K time or three quick questions, and marks your score as Estimated. Ageon compares your number with 16,278 lab treadmill tests from people your age and sex in the US FRIEND registry.