Top Exercise Scientist Dr. Inigo San Millan Made Zone 2 Cardio Famous. Here's What It Actually Does
You’ve probably heard it by now: zone 2 cardio is the workout trend promising longevity, fat loss, and a stronger heart at a pace gentle enough to keep a conversation going. It’s traveled from elite cycling teams all the way to your social media feed, and it’s not slowing down.
If you’ve been paying attention to fitness content lately, you know that the best time of day to work out is just one piece of the puzzle. How you train matters just as much as when. And right now, zone 2 is at the center of that conversation.
But recent science is starting to complicate the story in ways worth understanding before you restructure your entire routine around it. Here’s a full breakdown of what zone 2 cardio is, who’s behind it, what a major 2025 research review found, and how to use it in a way that actually makes sense for your life.
What Is Zone 2 Cardio, Exactly?
Zone 2 is steady aerobic exercise performed at 60% to 70% of your maximum heart rate, which most people estimate by subtracting their age from 220. At this effort level, blood lactate hovers around 1.7 to 2.0 mmol/L, meaning your body is efficiently clearing the byproducts of exercise rather than letting them build up. In practice, that looks like a brisk walk, easy bike ride, light jog, swim, or rowing session. Incline walking counts. A casual cycle to the coffee shop counts. The variety is part of the appeal.
The most reliable way to check if you’re there is the talk test. If you can speak in full, relaxed sentences but couldn’t comfortably sing, you’re in zone 2. You don’t need a wearable to start, though a heart rate monitor helps you stay consistent once you’re ready to dial things in.
Why Zone 2 Cardio Is Everywhere Right Now
The trend has a clear origin point. Iñigo San Millán, PhD, professor at the University of Colorado School of Medicine and performance coach to Tour de France champion Tadej Pogacar, has spent decades arguing that zone 2 is the training intensity that does the most for your mitochondria, the tiny structures inside your cells responsible for producing energy. His podcast appearances made that argument go wide.
The broader wellness world picked it up from there. Longevity researchers and fitness influencers began framing zone 2 as the cornerstone of a healthy, long life, particularly because VO2 max, your body’s capacity to use oxygen during exercise, is one of the strongest predictors of how long and how well you’ll live. Zone 2, the pitch goes, is how you build the aerobic base that supports a higher VO2 max over time.
It’s also worth noting that zone 2 is often called the “fat burning zone.” Your body does preferentially use fat as fuel at this intensity, but as Houston Methodist’s fitness experts point out, burning fat during exercise doesn’t automatically mean you’ll reduce your overall body fat percentage. The two aren’t the same thing.
What the Latest Science Says About Zone 2
This is where things get more nuanced. In June 2025, a narrative review published in Sports Medicine by Storoschuk, Moran-MacDonald, Gibala, and Gurd took a hard look at the evidence behind zone 2’s biggest claims. Their conclusion: the data doesn’t support zone 2 as the single best intensity for improving mitochondrial function or fatty acid oxidation in everyday people.
The core issue is a volume mismatch. The popular 80/20 rule, spending 80% of training time in zone 2 and 20% at high intensity, was built on data from elite athletes logging 12 to 20 hours of exercise per week.
Most people are working with 4 to 6. At that volume, higher-intensity training turns out to be more time-efficient for building VO2 max and mitochondrial capacity. Zone 2 remains genuinely well-suited for beginners, older adults, people returning from injury, and anyone who needs a sustainable low-stress way to stay active.
Zone 2 vs. HIIT: Do You Have To Choose?
You don’t. These two approaches aren’t competing with each other; they’re complementary. Zone 2 builds your aerobic base, improves metabolic flexibility, and is easy on your joints. High-intensity interval training, or HIIT, builds cardiorespiratory fitness faster and in less total time, but it demands more recovery and can’t be done every day.
For most people, a balanced weekly routine looks something like two to three zone 2 sessions of 30 to 60 minutes, one or two higher-intensity workouts, and at least two strength-training sessions. That structure takes the 2025 research seriously while still letting zone 2 do what it genuinely does well.
How to Start Zone 2 Cardio Without Overthinking It
- Start with the talk test. If you’re gasping, you’ve gone too hard. Slow down.
- Don’t chase pace or distance. Work to maintain the effort level, not a number on a screen.
- Incline walking, easy cycling, and rowing are all great low-impact entry points.
- Many beginners are surprised by how slow true zone 2 feels. That slowness is the point.
- Add a heart rate monitor once you’re ready to be more precise, but don’t wait for one to begin.
Zone 2 cardio is a real and useful tool. Used as part of a well-rounded routine, it can genuinely support your cardiovascular health, your metabolic flexibility, and your long-term fitness. Used in isolation, you may be leaving a lot of benefits on the table.
This article was created by content specialists using various tools, including AI.
