If I could pick one number to tell me how someone is likely to age, it wouldn't be their weight, their cholesterol, or their blood pressure. It would be their VO2 max.
VO2 max is the maximum amount of oxygen your body can use during hard exercise. In practical terms, it's a measure of how well your heart, lungs, blood, and muscles work together as a single system. And across a large body of research, it tracks with all-cause mortality more tightly than almost any other single fitness marker.
Why one number carries this much weight
It's worth being precise about why this matters, because it isn't magic. VO2 max is a composite. To move a lot of oxygen, you need a heart that pumps efficiently, lungs that transfer gas well, blood that carries it, vessels that deliver it, and mitochondria in your muscle that actually use it. A single number that summarizes all of that is going to correlate with a lot of things, because it reflects a lot of things.
That's also why it's hard to fake. You can improve a cholesterol panel in a few months with a medication. You cannot meaningfully raise VO2 max without actually changing the underlying system.
What the numbers actually look like
Averages shift by age and sex. Broadly, men in their thirties tend to average somewhere in the low-to-mid forties, women in the mid-to-upper thirties, measured in milliliters of oxygen per kilogram of body weight per minute. From there, the figure declines roughly ten percent per decade in people who don't train.
The part I find most useful clinically is that the decline is not fixed. A meaningful share of the age-related drop appears to track with reduced training volume rather than aging itself. Lifelong exercisers routinely test higher in their sixties than sedentary people do in their twenties. You are not simply watching a number fall. You are watching the result of what you did or didn't do with it.
The goal isn't a high VO2 max for its own sake. It's staying far enough above the functional floor that ordinary life never becomes exercise.
How to actually raise it
The training that moves VO2 max is well established and not complicated. A large aerobic base at conversational intensity, often called zone 2, builds mitochondrial density and capillary networks. Then a smaller amount of genuinely hard interval work raises the ceiling. Most people who plateau are doing neither well: their easy work is too hard and their hard work isn't hard enough.
A workable structure for most people is roughly 150 to 200 minutes per week at an easy, sustainable pace, plus one or two sessions of higher-intensity intervals. That's it. The rest is consistency over years, not intensity over weeks.
Where measurement fits
A true VO2 max test is done in a lab with a mask and a treadmill. Most people won't do that, and mostly don't need to. Consumer wearables now estimate it from heart rate response and recovery. Those estimates carry real error, and I'd treat any single reading skeptically.
But the trend is still useful. If your estimated VO2 max is drifting down over a year, that's a signal worth acting on regardless of whether the absolute number is precisely right. Direction matters more than decimal places.
The systems view
What I find compelling about VO2 max is that it behaves the way a well-designed system metric should. It's hard to game, it reflects the health of multiple subsystems at once, and it responds predictably to the right inputs. Most health metrics fail at least one of those tests.
If you're only going to track a handful of things, this belongs on the short list, alongside strength, body composition, and metabolic markers. Not because the number itself is the goal, but because it's an honest readout on whether the underlying system is getting better or worse.