Inside every cell there are clocks that show how old the body has become. You can read these clocks from chemical marks on DNA, like pencil marks on a doorframe that show how a child grew. Scientists decided to test whether ordinary exercise could make these clocks run slower.
78 adults without serious illnesses exercised three times a week for six months. The workload was measured in MET-hours: think of it as a package combining intensity and time. An hour of brisk walking is about 3 MET-hours, an hour of running is 7. It turned out that the more of these MET-hours a person accumulated, the more their biological clocks slowed down. To wind the clock back a year, you needed about 110 MET-hours over six months, which is roughly 37 hours of running or 110 hours of walking. That is not all that much: standard doctor's recommendations for the same period fall right within this range.
On top of that, those who moved more saw improvements in body composition, heart function and even gut bacteria. This means that movement helps several body systems at once. Previously it was unclear exactly how much training was needed to get this effect. Now there is a benchmark: ordinary physical activity, the kind doctors recommend, may already be enough to slow the aging of cells.
Elite athletes, such as Tour de France cyclists, can accumulate 250-400 MET-hours per week, which is 29-47 times the load needed to slow epigenetic age by one year under the most conservative model.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.