After I finished researching and writing this issue, I did the obvious thing and tested myself. I ran 1.43 miles in 12 minutes (the Cooper test) in 75-degree weather, which works out to a VO₂ max of about 40 mL/kg/min, right around average for my age. That was a reality check. I look like I'm in shape, but my cardiopulmonary fitness has real room to improve.

And that's the whole point of this issue: the number doesn't care how you look, and most of us have never measured it. VO₂ max, your cardiorespiratory fitness, predicts how long you'll live better than cholesterol or blood pressure, and every 3.5 mL/kg/min gain is tied to a 13–15% lower risk of dying.

I've always pictured VO₂ max as that fit person running on a treadmill with a mask on. VO₂ max is a measure of cardiorespiratory fitness: how much oxygen your body can actually use during hard exercise, and it's the single best number we have for both endurance and how long you'll live. The surprise is that some people who look out of shape have far more endurance, a higher VO₂ max, than someone who's thin. We tend to assume that if a person is thin they're generally in better shape, and that's often not the case.

And it matters more than you'd think. When researchers pooled ten long-term studies, the adults with the lowest VO₂ max (the bottom fifth for their age and sex) had about double the risk of dying over the follow-up, regardless of their weight. Overweight and obese adults who had a higher VO₂ max, by contrast, showed no significant increase in death rates compared with lean people of the same fitness (Prog Cardiovasc Dis 2014). In plain terms, a heavier person with a high VO₂ max generally outlives a thin person with a low one.

That doesn't give extra weight a free pass. A higher VO₂ max blunts the mortality risk of obesity, but it does not erase the conditions that often travel with it, type 2 diabetes, high blood pressure, fatty liver, joint disease, and some cancers, which carry their own risks whatever your VO₂ max. Think of VO₂ max and body weight as two separate levers: carrying extra weight still matters, but VO₂ max is the stronger predictor of how long you'll live, and you can raise it at any age.

Watch the whiteboard explainer:

VO₂ max is the maximum amount of oxygen your body can use during all-out exercise, the gold-standard measure of cardiorespiratory fitness (Circulation 2013).

"Cardiorespiratory fitness" is just your whole system working together: your lungs taking in oxygen, your heart and blood delivering it, and your muscles using it. VO₂ max is the one number that captures that entire chain, reflecting how hard your heart can pump, how much blood you carry, and how well your muscles pull oxygen out of that blood (NEJM Evidence 2025). It's reported in mL/kg/min, milliliters of oxygen used per kilogram of body weight each minute. The higher the number, the bigger your aerobic engine.

On its own, low fitness raises your risk of dying more than familiar warning signs like high blood pressure, high cholesterol, or extra weight, and not just because fit people tend to be healthier in other ways (JACC 2022). The numbers are hard to ignore:

The most widely used US reference standards come from the FRIEND Registry, built from over 22,000 directly measured tests (Mayo Clin Proc 2022). Here are the median (50th-percentile) values by decade from treadmill testing, find your row:

For both sexes the decline is steady, averaging about 10% (roughly 4 points) each decade on treadmill testing. In the chart below, the solid lines are the median for each age; the dashed lines mark roughly the 90th percentile, a useful "excellent for your age" target to aim toward:

A measured result below 85% of predicted for your age and sex is considered below average; below 70% signals low fitness worth a doctor's attention (Eur J Prev Cardiol 2026).

The gold standard is a cardiopulmonary exercise test (CPET), exercising to exhaustion in a lab while a mask measures every breath (NEJM Evidence 2025). Most people will never have one. The good news: two simple tests that can be done outside estimate VO₂ max closely enough to find your bracket and track your trend.

Run as far as you can in 12 minutes on a flat course or track, then measure the distance in meters.

It lines up reasonably well with a lab test, but it only works if you truly push to your limit. One thing to know: the run tends to make less-fit people look a little worse than they really are, and very fit people a little better (PLoS One 2015).

Walk one mile as fast as you can, and write down your time in minutes and your heart rate the moment you finish. Best if you're older or out of shape, since it doesn't require an all-out effort, and it's been tested in adults aged 30–69 (Med Sci Sports Exerc 1987).

This estimate lines up closely with a lab test (Med Sci Sports Exerc 1987).

Intensity is the key lever: high-intensity interval training (HIIT), short bursts of hard effort with rest in between, raises VO₂ max the most (Sports Med 2015).

Two things to keep in mind about HIIT's edge. Those 5.5 and 4.9 numbers compare each workout with doing no exercise, not with each other. When intervals and steady cardio are tested head-to-head, HIIT wins by only a little, about 1 mL/kg/min. And that small edge shows up mainly in men, not in women, so for women the two approaches work about equally well (Scand J Med Sci Sports 2026).

What counts as "moderate"? Moderate aerobic activity is about 64–75% of your maximum heart rate, the pace where you can talk but not sing. Estimate your max with 208 − (0.7 × age), which is the same for men and women. For a 50-year-old, that's a max of about 173 bpm, so moderate works out to roughly 111–130 beats per minute, the range to aim for on your easy, steady sessions.

One caution: if you have heart disease, start supervised, ideally through cardiac rehab, not solo at the gym (JACC 2021).

Raising VO₂ max really comes down to two things inside your muscle, and different workouts build them differently.

First, the mitochondria, the tiny engines inside your muscle cells that combine oxygen with fat and sugar to make energy. Training grows them by roughly 23–27%, and all three styles (intervals, sprints, and steady cardio) do this to a similar degree (Sports Med 2025). More engines means your muscles make more energy aerobically and fatigue less, so stairs, hills, and carrying groceries feel easier.

Second, the capillaries, the tiny blood vessels that deliver oxygen-rich blood to those engines. Here the workouts differ: steady, moderate cardio is the biggest driver, growing these vessels about 13%, versus about 7% for HIIT and no real change for all-out sprints (Sports Med 2025). Engines are useless without fuel lines, which is why this plumbing matters as much as the engines themselves.

When you're out of shape, the main limit is in the muscle itself, too few capillaries and mitochondria, and that muscle side explains about 89% of the difference in VO₂ max between people (J Appl Physiol 2016). Once you're trained, your muscles can use more oxygen than your heart can deliver, so the limit shifts to your heart, how much blood it can pump each beat and each minute. That's where intervals earn their edge, training the heart to pump about 14% more blood per beat (Sports Med 2022).

You probably know a parent, spouse, or friend over 50 who's been told their slowing down is "just age." Share this, knowing your VO₂ max, and that you can still move it, is worth a real conversation with their doctor. That's how Pareto Life grows.

References

Tucker et al., J Am Coll Cardiol 2022 · Clausen et al., J Am Coll Cardiol 2018 · Barry et al., Prog Cardiovasc Dis 2014 · Ekblom-Bak et al., Prev Med 2019 · Fletcher et al., Circulation 2013 · Brazile et al., NEJM Evidence 2025 · Kaminsky et al., Mayo Clin Proc 2022 · Santana et al., Eur J Prev Cardiol 2026 · Mayorga-Vega et al., PLoS One 2015 · Kline et al., Med Sci Sports Exerc 1987 · Tanaka et al., J Am Coll Cardiol 2001 · Martin et al., PLoS One 2025 · Milanović et al., Sports Med 2015 · Mølmen et al., Sports Med 2025 · van der Zwaard et al., J Appl Physiol 2016 · Rosenblat et al., Sports Med 2022 · Hov et al., Scand J Med Sci Sports 2023 · Yamagishi & Babraj, Scand J Med Sci Sports 2017 · Arnett et al., J Am Coll Cardiol 2019 · Bozkurt et al., J Am Coll Cardiol 2021 · (HIIT subgroup) Scand J Med Sci Sports 2026