Imagine two 50-year-olds. One has the body, energy, and resilience of a 40-year-old. The other has those of a 65-year-old. Their birthdates are the same—but their biological age, which determines how long and in what condition they will live, is completely different.

Most people exercise for a reason that goes far beyond what they see in the mirror: to live longer—and, above all, to live better. Yet few can say, with hard data to back it up, whether their routine is actually bringing them closer to that goal. That is precisely what oxygen—and the way your body uses it—allows us to measure.