Science is supposed to be about consensus. But spend any time in the VO2max world and you'll find smart, credentialed, research-literate people disagreeing about some surprisingly basic questions.
That's actually a good sign. It means people are thinking carefully.
Here's a breakdown of where the major voices — Peter Attia, Andrew Huberman, Iñigo San Millán, Dr. Martin Gibala, Andy Galpin, and Rhonda Patrick — stand on the big questions. Where they agree is solid ground. Where they disagree is worth understanding.
Where They All Agree
VO2max Is the Best Longevity Predictor We Have
This one is unanimous. Every single expert mentioned above treats VO2max as the single most important measurable marker of longevity. Not cholesterol. Not blood pressure. Not BMI. VO2max.
The data behind this is enormous — a VA study of 750,000 people found VO2max predicted mortality at 2–5× the strength of traditional cardiovascular risk factors. Peter Attia covers this in depth in his exercise for longevity episode. Galpin leads with it in his VO2max training video. Huberman has cited it repeatedly across his podcast.
Consensus: VO2max is the most important fitness metric for health and longevity. Full stop.
Lactate Is Not Waste
Here's one where all the scientists agree but popular culture still gets it wrong: lactate is not the cause of muscle burning, and it's not metabolic waste. It's fuel.
Your muscles produce lactate constantly. At moderate intensities, it gets cleared and reused — by the heart, liver, and brain — as fast as it's made. At high intensities, production outpaces clearance, and you feel the accumulation. But the burning sensation itself comes from hydrogen ions and inorganic phosphate, not lactate.
San Millán has made this his signature topic. Galpin and Gibala both cover it. Even Rhonda Patrick has discussed it at length. The science here is settled — it's just not widely known yet.
Some High-Intensity Work Is Essential
Nobody in this group recommends doing only easy cardio. Everyone agrees that pushing into higher intensities — whether through intervals, tempo runs, or sprint work — is necessary to improve VO2max and preserve health.
The debate isn't whether to do hard work. It's how much, when, and what kind.
VO2max Is Trainable at Any Age
No one disputes this. The studies on older adults improving VO2max are consistent and convincing. Training in your 60s works. Training in your 70s works. Starting from scratch at 50 absolutely works.
The magnitude of potential improvement may change with age, but the mechanism doesn't.
Where They Disagree
This is where it gets interesting.
Zone 2 vs. HIIT: The Big One
This is the central debate in VO2max training right now, and it's a genuine disagreement between serious scientists.
The Zone 2 camp (San Millán, Attia):
Iñigo San Millán — who coaches Tadej Pogačar, the most dominant Tour de France champion of his generation — argues that zone 2 training is the non-negotiable foundation of aerobic fitness. Zone 2 is the intensity at which you maximize fat oxidation: a low, steady effort (roughly 60–75% of max heart rate) where you're working but could hold a conversation.
At this intensity, you develop mitochondrial density in your slow-twitch muscle fibers — the deep aerobic infrastructure that everything else builds on. You can't build a pyramid from the top.
San Millán recommends 45–60 minutes minimum per session to get the adaptation, multiple sessions per week. His three-hour deep dive with Peter Attia (1.7 million views) is the most thorough argument for this position.
The HIIT camp (Gibala, Rhonda Patrick):
Dr. Martin Gibala's controlled trials show that 20–25 minutes of high-intensity interval training, three to four times per week, produces equivalent VO2max gains to the zone 2 approach — in a fraction of the time.
His argument: for most people, who have 1–3 hours per week for exercise (not 6+), HIIT is more time-efficient without sacrificing outcomes. He even titled one of his Gabrielle Lyon interviews "Why the Science Doesn't Support Zone 2 Training" — which is deliberately provocative but reflects real data.
The actual answer:
They're not measuring the same thing. San Millán is optimizing for elite endurance performance, where zone 2 builds qualities that HIIT can't replicate at high volumes. Gibala is optimizing for VO2max improvement in time-limited people, where HIIT is genuinely more efficient.
For longevity and health: HIIT works. For peak endurance performance: zone 2 is irreplaceable. For most people: a mix of both is optimal — mostly easy, some hard.
The full breakdown is in the zone 2 vs. HIIT dedicated post.
How Much Intensity Is Enough?
Attia and Galpin generally recommend the "80/20 rule" — about 80% of training time at easy/moderate intensity, 20% at higher intensity. This mirrors what elite endurance athletes actually do.
Renaissance Periodization takes a different view: for most people without endurance performance goals, cardio is important but secondary to resistance training. They'd prioritize weight training and treat cardio as a supplement, not a foundation. Their critique of Huberman's workout program makes this case directly.
Jeff Nippard, the evidence-based bodybuilding educator, has a similar lean: he acknowledges the mortality data but frames VO2max as one of several health markers, with resistance training taking a larger share of most people's training time.
Neither position is wrong — they reflect different goals.
The Non-Responder Issue
Gibala has highlighted a finding that deserves more attention: approximately 40% of people show zero measurable VO2max improvement from six months of guideline-recommended moderate exercise.
San Millán and Attia don't emphasize this in their frameworks. San Millán's model assumes that sustained zone 2 work will improve mitochondrial function and aerobic capacity — and for most people, it does. But Gibala's research suggests a significant minority need a harder stimulus.
If you're in that 40%, moderate zone 2 training may not be enough on its own. This is the non-responder problem — and high-intensity training is the solution.
What "Zone 2" Even Means
This sounds like it shouldn't be controversial, but it is.
San Millán defines true zone 2 as a metabolic state — specifically, the intensity at which you maximize fat oxidation. This requires individual lab testing to determine precisely. His athletes get lactate threshold tests to find their exact zone 2 intensity.
Other practitioners define zone 2 loosely by heart rate ranges (roughly 60–75% of max) or by feel ("conversational pace"). These are reasonable approximations, but they can overestimate zone 2 — many people's "conversational pace" is actually above true fat max, meaning they're not getting the full metabolic adaptation.
San Millán would argue most people who think they're doing zone 2 are actually training slightly too hard, blunting the adaptation.
The Takeaway
The experts agree on the big things: VO2max matters enormously, lactate isn't waste, everyone needs some intensity, and you can improve at any age.
The disagreements are mostly about implementation — how much zone 2 vs. HIIT, how to define the zones, and whether non-response to moderate exercise is a major clinical issue.
For most people, the practical conclusion is reassuringly simple:
- Do some easy aerobic work regularly
- Do some genuinely hard work weekly
- Get off the couch consistently
That combination wins, regardless of which camp's exact ratios you follow.
For deep dives into each expert's specific approach, see: Andy Galpin, Iñigo San Millán, Andrew Huberman, and Rhonda Patrick.