Tadej Pogačar has won the Tour de France multiple times. He's widely considered the most dominant cyclist of his generation — maybe one of the most gifted endurance athletes alive. His coach and sports scientist is Dr. Iñigo San Millán.
So when San Millán talks about how to train for VO2max, it's worth paying attention.
His central message: zone 2 training is not optional. It's the foundation. Everything else is built on top of it.
Who Is Iñigo San Millán?
San Millán is a sports scientist and assistant professor at the University of Colorado School of Medicine. He specializes in the metabolic and physiological aspects of exercise — specifically the cellular mechanisms that determine endurance performance.
He's also the person who made zone 2 training a household term in health and fitness circles. Before his appearances on Peter Attia's podcast — most notably a three-hour deep dive that has been watched 1.7 million times — "zone 2" was mostly elite sports jargon. Now it's on every fitness blog and podcast in existence.
What Zone 2 Actually Is (Not What You Think)
Here's where San Millán gets more precise than most people realize.
Zone 2 is commonly defined by heart rate: roughly 60–75% of your maximum. That's a useful approximation. But San Millán defines it by something more specific: fat max.
Fat max is the exercise intensity at which your body is maximally burning fat for fuel. Below that intensity, you're burning fat but not at peak rate. Above it, your body starts relying more heavily on carbohydrates (glycogen) as the primary fuel — and lactate begins to accumulate faster than your body can clear it.
At true zone 2:
- You're maximizing fat oxidation
- Lactate production ≈ lactate clearance (roughly 1–2 mmol/L blood lactate)
- You could sustain this for hours if needed
- You can hold a conversation, though you'd prefer not to
The problem: most people's "zone 2" is actually above their fat max. They're training slightly too hard, which blunts the metabolic adaptation. They feel like they're doing zone 2 because it's not exhausting — but their mitochondria aren't getting the optimal signal.
The only way to know for certain where your true zone 2 is? A lab-based lactate test. San Millán uses these with all his athletes. Without one, most people should train slower than they think.
Why Zone 2 Matters: The Mitochondria Story
San Millán's explanation of zone 2 is fundamentally a story about mitochondria.
Mitochondria are the tiny structures inside cells that convert oxygen into usable energy (ATP). Think of them as the engine cylinders in your body's energy system. More mitochondria = more oxygen-burning capacity = higher VO2max.
Zone 2 training at the right intensity sends a specific signal to your cells: make more mitochondria. This process is called mitochondrial biogenesis, and zone 2 is uniquely effective at triggering it in the slow-twitch muscle fibers that power sustained aerobic effort.
More mitochondria means:
- You can oxidize more fat at higher intensities (fat max shifts upward)
- You can clear lactate more efficiently (your threshold rises)
- You can sustain higher power outputs before hitting your ceiling
- Your VO2max goes up
This is why San Millán says you can't build the pyramid from the top. High-intensity work develops your peak — but the size of your peak is limited by the width of your base. Zone 2 is how you build the base.
What Makes Pogačar Exceptional
San Millán has described why Pogačar is so physiologically unusual, and it comes down to mitochondria.
Elite cyclists can produce enormous power outputs. What separates Pogačar is that at those high power outputs, he continues oxidizing fat and clearing lactate at extraordinary rates. His mitochondria work more efficiently than most humans can achieve — and they do so across a wider range of intensities.
This isn't just genetic. It's the product of decades of zone 2 training that built a massive aerobic foundation. The high-intensity work builds the peaks. The zone 2 work built the mountain those peaks sit on.
How to Actually Do Zone 2
San Millán is specific about implementation:
Duration per session: Minimum 45–60 minutes to trigger meaningful adaptation. Short zone 2 sessions don't provide enough of the sustained metabolic signal. This is one reason he's skeptical that brief HIIT alone can fully substitute — see the zone 2 vs. HIIT debate for the full argument.
Frequency: Ideally 3–4 sessions per week. More is better, up to a point. Elite cyclists do 15–20+ hours of zone 2 per week — but for most people, 3–4 hours weekly is a meaningful dose.
Intensity check: You should be able to hold a conversation — but it takes some effort. If you're talking easily, you might be below zone 2. If you're gasping mid-sentence, you're above it. A heart rate monitor helps: roughly 60–75% of max HR is a reasonable approximation.
Equipment: Anything works — cycling, running, walking (with effort), rowing, elliptical. San Millán tends to cycle because it's his domain, but the metabolic principle applies to any sustained aerobic effort.
The Role of High Intensity
San Millán isn't anti-HIIT. He's anti-ONLY-HIIT.
His model for elite athletes is roughly 80% zone 2 and 20% higher intensity work. The high-intensity sessions (what he calls zone 4 and 5) develop the top-end capacity that zone 2 builds the foundation for.
But without the foundation, the high-intensity work is limited. And many people — especially recreational athletes who are pressed for time — skip the slow work entirely and wonder why their fitness plateaus.
His recommendation for how often to do zone 5 (near-maximal) work: once per week, in controlled doses. Done correctly, it's a powerful stimulus. Done too often, it degrades the zone 2 base.
The Lactate Test: Knowing Your Real Zones
San Millán's gold standard for zone 2 determination is a lactate threshold test in a lab:
- Exercise at progressively increasing intensities
- Small blood samples taken from a fingertip at each stage
- Lactate levels plotted against intensity
- True zone 2 = where fat oxidation is maximal, typically 1–2 mmol/L lactate
This test costs money and requires equipment, but it gives you precise, individual data. San Millán argues that most people's heart-rate-based zone estimates are inaccurate enough to meaningfully affect training quality.
The VO2max testing guide covers more accessible options if a lab test isn't practical.
The Shortcut That Isn't
There's a version of zone 2 training that many people do wrong: they go slightly too hard, feel productive, and never develop the deep fat-oxidation capacity that makes zone 2 valuable.
If you leave zone 2 sessions feeling genuinely tired, you were probably in zone 3. Real zone 2 should feel almost embarrassingly easy. You should finish feeling fresher than you'd expect for the time spent.
That dissonance — "this feels too easy to be doing anything" — is actually a good sign. Resist the urge to push harder. The adaptation happens at the lower intensity, not despite it.
For the companion view on whether zone 2 is even necessary — Dr. Martin Gibala's compelling case for HIIT efficiency — see the zone 2 vs. HIIT debate. It's a genuinely interesting argument. And understanding both sides will make you a smarter athlete.