Two of the smartest people in exercise science walk into a bar. One coaches the best cyclist in the world. The other has published hundreds of peer-reviewed studies on interval training. They order drinks and immediately start disagreeing about the best way to improve your VO2max.
This is essentially what's been playing out publicly between Iñigo San Millán and Dr. Martin Gibala — and it's one of the most interesting debates in fitness science right now.
Here's the breakdown.
The Zone 2 Case (San Millán, Attia)
Iñigo San Millán is not speaking from theory. He coaches Tadej Pogačar — the man who has dominated professional cycling in a way not seen in a generation. When Pogačar climbs mountains that make other professional cyclists crumble, San Millán's training philosophy is what built that engine.
His core argument: zone 2 training is the non-negotiable foundation of aerobic fitness, and no amount of high-intensity work fully substitutes for it.
Zone 2 — sustained low-to-moderate intensity exercise, roughly 60–75% of max heart rate, where you can hold a conversation with mild effort — develops mitochondrial density in slow-twitch muscle fibers. These are the endurance fibers that power all sustained aerobic effort.
More mitochondria means:
- Higher fat oxidation capacity (you burn more fat at higher intensities)
- More efficient lactate clearance (your threshold rises)
- Higher VO2max ceiling
The adaptation is real, measurable, and unique. San Millán argues that HIIT can't fully replicate it because HIIT doesn't spend enough time in the specific metabolic zone where mitochondrial biogenesis is maximally triggered in slow-twitch fibers.
His prescription: minimum 45–60 minutes per zone 2 session, multiple times per week. This is what elite endurance athletes do — typically 15–20+ hours per week of zone 2 — and San Millán argues that the ratio (roughly 80% easy, 20% hard) is informative for everyone.
Peter Attia agrees with this framework in his exercise for longevity discussion. If you have the time, zone 2 is the base.
The HIIT Case (Gibala, Rhonda Patrick)
Dr. Martin Gibala's lab at McMaster University has spent decades running controlled trials comparing training protocols. His CV is extraordinary — hundreds of peer-reviewed publications on exercise physiology.
And his data says something uncomfortable for the zone 2 camp: 20–25 minutes of HIIT, three to four times per week, produces equivalent VO2max improvements to three to six hours per week of zone 2 training.
Not slightly inferior results. Equivalent.
In his interview with Dr. Rhonda Patrick — 567,000 views and counting — Gibala walks through this research carefully. His argument:
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The mechanism works: HIIT drives mitochondrial biogenesis too. The stimulus is different (higher intensity, shorter duration) but the downstream signal to make more mitochondria is triggered.
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Non-response is a serious problem: Roughly 40% of people get zero VO2max improvement from six months of moderate exercise. This population doesn't show up in the zone 2 research because non-responders quit — but they're real. High-intensity training essentially eliminates non-response.
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Time is a real variable: Most people don't have 3–6 hours per week for cardio. If the choice is between "perfect zone 2 protocol they'll never follow" and "shorter HIIT protocol they'll actually do," the latter wins on real-world grounds.
He made this argument directly in a Gabrielle Lyon interview provocatively titled "Why the Science Doesn't Support Zone 2 Training." (He later clarified this was specifically about zone 2 as the primary stimulus for VO2max improvement in time-limited people — not that zone 2 is useless.)
The Actual Answer (More Nuanced Than "Who Wins")
Here's what the debate looks like when you zoom out:
They're optimizing for different outcomes.
San Millán is coaching elite endurance athletes whose goal is maximal performance at sustained high outputs. For that population, zone 2 builds qualities that HIIT at any dose cannot fully substitute — the specific mitochondrial adaptations in slow-twitch fibers that underpin hours of high-intensity effort. An elite cyclist doing 20 minutes of HIIT instead of 20 hours of zone 2 per week would absolutely fall apart.
Gibala is researching what produces VO2max improvement in time-limited general populations. For that population — which is most of us — the evidence is clear that HIIT is more time-efficient without meaningfully inferior outcomes.
The right question is: what are you trying to achieve?
- Performance goal (endurance sport, competitive athletics): Zone 2 is irreplaceable. High volume, slow foundation. Then add intensity on top.
- Longevity and health goal, limited time: HIIT works. Three sessions per week of 20 minutes is enough to meaningfully improve VO2max and dramatically reduce mortality risk.
- Optimal for both: A mix. The "polarized" approach elite athletes use — mostly easy (zone 2), some hard (zone 4–5), very little in between — is the most validated framework across populations. See the Norwegian training method for how world champions implement this.
The Zone That Nobody Should Be In
One point Galpin and others emphasize: there's a zone everyone should avoid as their primary training zone — roughly zone 3, or "moderate-hard."
It's too hard to recover from quickly, which limits how often you can train. But it's not hard enough to produce the maximal adaptive stimulus that true zone 4–5 HIIT delivers.
Most recreational athletes do most of their training in this zone — and wonder why they plateau. They feel like they're working hard. But they're neither building the aerobic base (zone 2) nor pushing the ceiling (zone 4–5).
The fix: go easier on easy days. Go harder on hard days. Leave the grey zone alone.
A Practical Summary
| Goal | Best Approach |
|---|---|
| Maximum endurance performance | 80% zone 2, 20% hard intervals |
| VO2max improvement, limited time | 3× HIIT/week (20 min each) |
| General health and longevity | Any consistent combo; some hard work weekly |
| Never respond to moderate exercise | Add at least 1 HIIT session immediately |
For the full Andrew Huberman protocol (which uses both zones): his VO2max approach. For Rhonda Patrick's Tabata-focused version: her protocol. For San Millán's full argument: zone 2 deep dive.
The debate is real and worth understanding. But it shouldn't paralyze you. Both approaches work better than doing nothing — and doing nothing is the actual enemy.