Dennis’s watch says his VO2max is 39. It’s one of the numbers fitness culture loves most: cited on podcasts as the single best predictor of how long you’ll live, posted as a badge of honour, chased with interval sessions that leave you on the floor. So when Dennis sent me a new study on VO2max and longevity, I asked him what he was really after. His answer:
“I saw the study online and since I am an MVP (minimum viable product) person when it comes to [sports] I thought [it] would be worth checking if doing more is better or what the science says where the sweet spot lays.”
Dennis
Spoken like a data scientist with two kids and a finite number of hours. It’s also exactly the right question. The answer turned out to be more interesting, and more encouraging, than the headline version.
The study: nature’s own randomized trial
The paper is Kjaergaard and colleagues, published in the Journal of Clinical Endocrinology & Metabolism (online 2024, print 2025). It uses a method called Mendelian randomization, and it’s worth thirty seconds to understand why that matters.
We already know that fit people live longer. The trouble is that fit people also tend to be leaner, eat differently, smoke less and earn more. Untangling which of those things does the work is nearly impossible from observation alone. Mendelian randomization sidesteps the problem by using genes. You’re dealt gene variants at conception that nudge traits like body fat or aerobic capacity slightly up or down, and that deal is essentially random with respect to your lifestyle. If the genes that raise a trait also raise lifespan, that’s evidence the trait itself is doing something. It’s a bit like a randomized trial that nature ran for us.
The researchers used genetic data on roughly 70,000 UK Biobank participants whose VO2max was estimated on a bike test, and on the lifespans of about a million parents as the longevity measure. Their hypothesis was blunt: VO2max predicts survival because it travels with the real causes, not because it is one.
What it found
The hypothesis held. Three things came out as causal drivers of a longer life: lower body fat percentage, not developing type 2 diabetes, and being physically active. Lean muscle mass helped too, but its effect disappeared once body fat was accounted for, which suggests muscle protects you largely by keeping you lean and metabolically healthy. The activity signal was the least clear-cut of the three, but it held up across several checks.
VO2max itself showed no causal link to lifespan. Instead, it behaved like a result: lower body fat, more muscle, more activity and higher haematocrit (more oxygen-carrying red cells) all pushed VO2max up, while genetically higher VO2max didn’t change anything downstream. The authors’ own conclusion is practical: it’s hard to imagine raising VO2max in any way that doesn’t run through body composition and activity anyway.
In other words, VO2max is the scoreboard. The engine is what you do every week, and what you carry around your waist.
Before anyone cancels their runs
This study does not say cardio is pointless. Training raises VO2max and lowers body fat, improves insulin sensitivity and keeps you active, which are exactly the things the study found to matter. The point is where the benefit comes from, not whether it exists. A few honest caveats, because Dennis is a scientist and so, I suspect, are some of you:
- Weak genetic signal for VO2max. Only a handful of gene variants reliably predict it, so “no effect found” partly means “not enough power to see a small one”.
- Inborn isn’t trained. The genes capture things like natural heart size and trainability. Whether the VO2max you build behaves the same way is an assumption, not a finding.
- Per kilogram means body weight is baked in. VO2max was divided by total body weight, so carrying more fat mechanically lowers it. Fat and VO2max are tangled together by the arithmetic.
- No information on dose. Physical activity was a self-reported yes/no. The study can’t tell you how long or how hard to train.
That last point is the one Dennis cares about most: where’s the sweet spot? For that we need different studies.
Finding the sweet spot
Three large bodies of evidence fill the gap. All three are observational, so they show associations rather than proof, but they point in the same direction, and it’s good news for minimalists.
Aerobic volume. A Harvard analysis of 116,221 adults followed for 30 years (Lee et al., Circulation, 2022) is close to a dose-response curve for exactly Dennis’s question. Simply meeting the standard guidelines (150 to 300 minutes of moderate activity a week, or 75 to 150 of vigorous) went with roughly a fifth lower mortality. Doing two to four times that added only a little more on top. The lowest mortality sat at about 150 to 300 minutes of vigorous or 300 to 600 minutes of moderate activity, and beyond that the curve went flat: no extra benefit, but no sign of harm either. Adding vigorous work only helped people doing less than about five hours of moderate activity a week. Intensity, in other words, is a way to get the benefit in less time, not a requirement.
Running. A study of 55,137 adults (Lee et al., JACC, 2014) found runners had about 30% lower all-cause mortality than non-runners, and the benefit looked similar whether people ran a little or a lot, slow or fast. The strongest effect belonged to people who kept running over the years.
Strength. A meta-analysis of 16 cohort studies (Momma et al., BJSM, 2022) linked muscle-strengthening work to roughly 10 to 20% lower mortality, with the biggest effect at about 30 to 60 minutes a week, and the best results when strength and aerobic training were combined. Above an hour a week the longevity curve didn’t keep improving. That’s a statement about lifespan, not about lifting: if you want shoulders, you’ll train more than that, and that’s fine.
So, to answer Dennis directly: for living longer, more is better up to a point, and that point arrives early. The minimum viable dose already buys most of the benefit. The sweet spot is a few hours a week, not a second job.
What I’m doing with this for Dennis
Put the pieces together and the longevity prescription is surprisingly calm: a few hours of mostly easy running each week, a little quality work, regular strength training, and above all, steady progress on body fat, kept up for years. No suffering required. It’s also, almost exactly, the shape of the hybrid plan Dennis is already on, which is a nice moment for any coach. His goals go beyond living longer, a better runner and a stronger build, and those will ask for a bit more than the minimum. But the foundation is the same, and it’s within reach of a busy dad.
So I’m not going to make his watch’s VO2max a target. I’ll keep watching it as a progress marker, and here’s the fun part: because it’s measured per kilogram, every bit of fat Dennis loses will nudge that 39 upward even before his heart adapts. When it climbs, we’ll celebrate, but we’ll know the real wins happened earlier: in the easy run he kept easy, the gym session he showed up for, the waist measurement that dropped. That’s the engine. The number just reports on it.
If you take one thing from this: stop chasing the metric and build the habits behind it. Move most days, lift twice a week, and look after your waistline. Your scoreboard will take care of itself.
This is a coach’s reading of the research, not medical advice. If you have a health condition, talk to your doctor before changing how you train.
— Coach Bob
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