What if I told you there’s a pill that improves your mood, memory, and focus? What if I told you this pill also reduces your risk of cardio-metabolic diseases, boosts energy, and burns fat? What if this pill could even help you recover faster between BJJ or Boxing sparring rounds? You’d probably think, “No way a pill like that exists.” And you’d be right. It doesn’t. But there is a way to get all those benefits with one simple practice: Zone 2 Cardio.
My Personal Journey with Zone 2 Cardio
I started incorporating Zone 2 Cardio into my training 1.5 years ago after reading Peter Attia’s book Outlive.
It’s not like I wasn’t doing cardio before, I went on 10K runs & did high intensity boxing sessions. But back then, I had no clue about the zones. Looking back, I was constantly pushing too hard and leaving a lot of potential gains on the table. I wasn’t building my metabolic flexibility, meaning I wasn't teaching my body how to use fat as fuel.
And that’s where I see a lot of clients and friends go wrong. When they do steady-state cardio, they’re actually working too hard.
They rely only on glucose for fuel, missing the benefits of using fat for energy and improving both the quantity and quality of their mitochondria.
If only they would go a little slower.
Why Zone 2 Matters for Everyone
It's 2024, and you see more people embracing a hybrid approach to fitness, combining strength training with cardio. Hell, even Jeff Bezos is on board:
1/ Prioritizes Sleep
He gets 8 hours of sleep. He’s focused on it.
"I think better, I have more energy, my mood is better.
Let's say I sleep only 4 hours a day, now I got 4 'productive hours' back.
But is it worth it? No, because the quality of my decision will suffer."
This shift is partly thanks to Peter Attia, who has popularized Zone 2 training for longevity.
And where did Attia get his insights?
From the "father of Zone 2," Iñigo San Millán, a professor at the University of Colorado School of Medicine and a renowned expert in exercise physiology.
San Millán’s philosophy is simple: studying elite endurance athletes can help us understand and even prevent chronic diseases—much like how Formula 1 cars drive innovation for everyday vehicles. He famously stated in his 2014 TED Talk:
“Elite endurance athletes are the only population group 100% free from acquired cardio-metabolic disease." — TEDxMileHigh, Iñigo San Millán,
Why Are Elite Endurance Athletes Free From Cardio-Metabolic Diseases?
So why are the elite endurance athletes free from cardio-metabolic diseases? The answer lies in their exceptional mitochondrial health.Elite endurance athletes have the most developed mitochondria of any humans. Mitochondria, known as the power plants of the cell, are where we burn glucose, fat, and protein for energy.
These athletes also have the highest capacity for lactate clearance, which further enhances their performance. And where is lactate cleared? In the mitochondria as well. This combination not only allows them to perform at their peak but also protects them from chronic diseases.
Mitochondrial Dysfunction and Chronic Conditions
In contrast, people with chronic conditions often suffer frommitochondrial dysfunction. Conditions like type 2 diabetes, obesity, cardiovascular disease, Alzheimer's, and even cancer are all associated with poorly functioning mitochondria.
When mitochondria can't efficiently burn glucose, the pancreas releases more insulin to compensate. This can lead to a buildup of blood glucose, eventually causing insulin resistance and type 2 diabetes. Similarly, since fat can only be burned in mitochondria, dysfunction can result in fat buildup, contributing to obesity and metabolic issues.
Peter Attia backs this up, saying:
“I’m convinced it’s impossible to be healthy without also having healthy mitochondria, which is why I place a great deal of emphasis on long, steady endurance training in zone 2.”
— Outlive, Peter Attia MD
How Zone 2 Helps Your Mitochondria
Zone 2 training stimulates mitochondrial biogenesis—increasing both the number and size of mitochondria in muscle cells. This adaptation boosts the muscles ability to produce ATP (the cell's energy currency), making your body more efficient at using both fat and glucose for energy.
Additionally, Zone 2 training promotes mitophagy, a process that removes dysfunctional mitochondria. This cellular “cleanup” helps maintain a healthy population of mitochondria.
By focusing on Zone 2, you’re effectively upgrading your body’s power plants—leading to better energy management, improved metabolic health, and, ultimately, a longer, healthier life.
Metabolic Health: Professional Cyclists vs. Metabolic Syndrome Patients
What happens when you put athletes and metabolic syndrome patients on stationary bikes?
Iñigo San Millán and George Brooks explored this scenario in a study, revealing striking differences between professional cyclists and those with metabolic syndrome.
Professional cyclists can produce a power output of 4 watts per kilogram—impressive by any standard—while primarily burning fat for fuel. In contrast, individuals with metabolic syndrome struggle to produce even 1 watt per kilogram, relying almost entirely on glucose from the first pedal stroke. Their bodies have virtually no capacity to access fat stores, making them metabolically inflexible—able to use glucose but not fat.
The Importance of Metabolic Flexibility
This difference in energy usage is significant. While endurance athletes can easily tap into their fat reserves, those with metabolic issues are stuck in glycolysis, burning glucose and producing large amounts of lactate. As a result, people with obesity or metabolic problems often have elevated resting lactate levels, a sign that their mitochondria are struggling to maintain even baseline energy levels.
It’s a paradox: those who need to burn fat the most are unable to do so effectively. Meanwhile, lean, well-trained athletes thrive on their metabolic flexibility and healthier mitochondria.
The Warburg Effect: A Similar Process in Cancer Cells
This process mirrors how cancer cells generate energy through the Warburg Effect. The Warburg Effect is a metabolic phenomenon where cancer cells prefer glycolysis for energy production, even when oxygen is available. Normally, oxygen would support a more efficient process called oxidative phosphorylation, but cancer cells bypass this, opting for faster ATP production through glycolysis.
Why Cancer Cells Choose Glycolysis
This shift allows cancer cells to generate energy quickly and produce the building blocks for new cells, supporting rapid growth. However, this method is less efficient than the energy production seen in healthy mitochondria, resulting in a high production of lactate.
The Common Link: Mitochondrial Dysfunction
In both cancer cells and individuals with metabolic syndrome, the reliance on glycolysis reflects dysfunction in normal mitochondrial energy production. This highlights the importance of metabolic flexibility and healthy mitochondria for overall health, as they enable the body to use both glucose and fat efficiently, reducing the risk of chronic diseases.
Brain, Learning & Mood Benefits of Zone 2 Cardio
Doing Zone 2 cardio, like cycling indoors during winter, isn’t just about physical fitness—it’s a powerful way to boost your brain and mood.
Research shows that even 20-30 minutes of aerobic exercise can significantly increase BDNF (Brain-Derived Neurotrophic Factor) in young men, which helps grow new brain cells and supports the survival of existing ones. This means better memory, focus, and attention.
Zone 2 exercise also releases dopamine, serotonin, and noradrenaline, all crucial for lifting your mood and maintaining focus. Studies show that aerobic exercise can improve attention for at least 2 hours post-workout and enhance reaction times—an essential benefit for athletes and busy professionals alike.
But the benefits don’t stop there. Consistent Zone 2 training helps build a stronger hippocampus (your brain’s memory center) and prefrontal cortex (key for decision-making). Regular sessions—like spinning 3 times a week for 45 minutes—has been show to lead to lasting improvements in long-term memory and executive function, helping you think more clearly and stay sharp.
Aerobic Base: The Key to True Endurance
We now understand how important steady-state Zone 2 training is for building endurance. But most of us are busy and may not have hours to dedicate to it. So naturally, you might ask:
“I’m already doing strength training. Isn’t that enough?”
“I even incorporate HIIT workouts. Isn’t that enough for cardio?”
Why Strength and HIIT Aren’t Enough
Firstly, you often see a phenomenon with bodybuilders—they are fit, but not necessarily healthy. Strength training is extremely important, but it doesn't fully engage your aerobic system, which is essential for cardiovascular health and endurance.
Secondly, while HIIT training is excellent for improving VO2max and high-intensity performance, it doesn’t replace the benefits of a strong aerobic base. Building your aerobic capacity through Zone 2 training provides a foundation that enables your body to handle higher intensities more effectively.
Why Elite Athletes Prioritize Zone 2 Training
There’s a reason elite cyclists, runners, and alpinists spend about 80% of their training in Zone 2—building that aerobic foundation. This low-intensity, steady-state training enhances mitochondrial function, improves fat-burning capacity, and helps maintain endurance over long periods. It’s the base upon which more intense efforts, like HIIT, can be built.
Lessons from Alpinists: The Ultimate Endurance Test
In 2017, I read about professional alpinists and their training. Climbing Mount Everest is one of the most demanding endurance challenges. You can't shortcut your way through HIIT for something this extreme. One quote from the book Training for the New Alpinism stuck with me:
“Without a strong base of aerobic support, training at higher intensity will never allow you to maximize your fitness potential.” — Training for the New Alpinism, Steve House, Scott Johnston
Practical Benefits of a Strong Aerobic Base
A strong aerobic base makes life more enjoyable. You’ll recover faster between BJJ or boxing sparring rounds, and you’ll have more stamina for activities like tennis or badminton.
It also means you can go on hikes or tackle mountain trails, join a local run without extra preparation, and climb stairs without your heart rate skyrocketing.
The best part? Some coaches, like Dr. Andy Galpin, don’t even consider Zone 2 as “training” since it requires minimal recovery time.
cc: From Training for the New Alpinism (modified to Iñigo San Millán’s Zones)
Understanding The Zones of Training
cc: From Training for the New Alpinism (modified to Iñigo San Millán’s Zones)
While zones might sound complicated, they’re really not. They just show which energy system your body’s using.
Most fitness trackers stick to a 5-zone system, but Iñigo San Millán breaks it down into 6 zones. This approach digs deeper into how your body shifts between burning fat, carbs and producing lactate.
cc: YouTube: Iñigo San Millán’s - Describing Training Zones
Zone 1 – Recovery & Daily Movement
Zone 1 is your walk in the park or sitting while reading this article.
It’s the intensity you hit while doing daily activities, using a mix of fats and carbs for energy.
Walking over 7,500 steps a day is essential for overall health, but relying on Zone 1 alone isn’t enough for serious improvements in mitochondrial function.
“But I thought 10,000 steps is all I need?” Not quite. While Zone 1 uses fat as fuel, it does so at a very low rate because the body’s energy demand is minimal. It doesn’t create the metabolic stress needed to trigger significant mitochondrial adaptations. Mitochondrial biogenesis—growing more and stronger mitochondria—requires higher levels of aerobic demand, which is where Zone 2 comes into play.
Zone 2 - Maximum Fat Burning & Mitochondrial Health
Zone 2 is where maximum fat burning happens. While you're still burning carbs in all zones (as Iñigo San Millán's chart shows), in Zone 2, your body reaches its peak for using fat as fuel. This is important because fat can only be burned in the mitochondria, making Zone 2 crucial for both athletic performance and metabolic health.
The more effectively your mitochondria work, the better you can use fat for energy, making you metabolically flexible—able to switch between using fat and glucose as needed.
The better your mitochondria work, the more efficiently you can use fat for energy, making you “metabolically flexible”—able to switch between fat and glucose as needed.
What Defines Zone 2 Training?
Lactate level is the key measure for Zone 2 training. Keeping your blood lactate below 2.0 mmol allows your body to continue burning fat without building up lactate, which would shift you into Zone 3.
Most fitness trackers put Zone 2 at 60-70% of your max heart rate, but Peter Attia suggests aiming for 70-80% for a more accurate range. For example, my WHOOP shows my Zone 2 as 125-135 bpm, while Attia’s personal range for Zone 2 is 134-142 bpm. Some athletes can maintain Zone 2 at 140-150 bpm due to their better lactate-clearing capacity.
How to Know if You're in Zone 2?
The simplest way is the talk test: if you can speak in full sentences, you’re likely in Zone 2. If you’re too out of breath to chat, you’ve probably drifted into Zone 3 and need to slow down. Breathing through your nose is another good indicator.
For those who want more precision, head to a lab. In Warsaw where I live I found a lab that measures VO2 max, max heart rate, and lactate threshold for just $100. It’s a worthwile investment, and I’m scheduling my test soon.
Zone 3 – The “No Man’s Land”
Zone 3 is often called a no man’s land. At this intensity, your body shifts from burning fat to relying more on carbs, and lactate levels start to rise. It’s too intense to get the fat-burning and mitochondrial benefits of Zone 2, but not intense enough to build the high-intensity capacity you’d develop in Zone 4.
Many casual joggers, cyclists, and swimmers unknowingly spend a lot of time in this zone, missing out on the unique advantages of either going slower (Zone 2) or pushing harder (Zone 4).
Zone 4 – The "Turbo" Zone
Zone 4 is what Iñigo San Millán calls the “Turbo” zone. It’s where elite athletes can push their limits, sustaining high efforts for 15-30 minutes—this is where races are won. At this intensity, lactate builds rapidly, and the body shifts more towards anaerobic energy pathways. But Zone 4 isn’t just about going all-out.
Training in this zone helps your body become more efficient at moving lactate from fast-twitch muscle fibers to the mitochondria in slow-twitch fibers, making it crucial for competitive performance.
As intensity increases, lactate production ramps up. There comes a point where your body can’t clear lactate as quickly as it’s produced—this is known as the lactate threshold. Zone 4 training pushes you right up to that limit, helping you build the ability to maintain higher intensities without fatiguing as quickly.
Zone 5 – VO2 Max Training
Zone 5 is all about maximizing your aerobic and anaerobic capacity. This high-intensity zone involves 2-4 minutes of sustained effort, pushing your body’s ability to utilize oxygen to its peak (VO2 max). The goal is to challenge your heart, lungs, and muscles to operate at their maximum capacity.
Zone 5 training is most effective when paired with a solid foundation of Zone 2 work. As highlighted in Training for the New Alpinism
“Athletes who supplemented a huge base of aerobic endurance training with event-specific high-intensity work had much better outcomes than those relying solely on high-intensity training.” — Training for the New Alpinism, Steve House, Scott Johnston
In other words, building your base with Zone 2 allows you to get the most out of those hard pushes in Zone 5.
I see this in my boxing sessions. When I do HIIT, it's 8 rounds on the heavy bag—3 minutes each, going all out. That’s Zone 5, pushing my limits and leaving me gassed. But it’s my Zone 2 work that makes the difference between rounds. It helps me recover faster, catch my breath, and put in some high quality work.
Zone 6 - Pure Sprint (Anaerobic)
Zone 6 is all about those all-out efforts—think 10-20 second sprints. This is pure anaerobic work, relying on quick bursts of energy from stored phosphocreatine and carbs. There’s no time for oxygen to fuel your muscles here. I’ve done sprints in the past, but they’re not a regular part of my training now. Check out my friend @granthardwell, though—he’s been incorporating them consistently and seeing great results:
How Often & How Much Zone 2 Training Should You Do?
Me Doing Zone 2 Cardio
“Consistent Zone 2 exercise delivers significant metabolic benefits by boosting mitochondrial health without over-stressing the body. The benefits of Zone 2 prove you don’t have to push yourself to the limit for effective metabolic workouts. It often feels surprisingly easy, but the research speaks for itself: sustained moderate exercise increases the number of mitochondria, enhances glucose uptake, improves heart efficiency, and lowers the risk of almost every chronic disease.” — Good Energy, Casey Means MD
How Much Zone 2 Should You Aim For?
The general recommendation is to aim for at least 150 minutes of Zone 2 exercise per week. This can be broken down in different ways:
Iñigo San Millán’s Recommendation: 3 sessions of 60-90 minutes each week can significantly improve mitochondrial function—4 sessions are even better.
A Good Starting Point: Even starting with 2 sessions of 30 minutes is a solid entry point.
My Routine: I personally do Zone 2 twice a week for 60-75 minutes.
Important Caveat: For any Zone 2 session, aim for at least 30 minutes to allow fat oxidation to reach its peak, helping your body effectively use fat as fuel and supporting long-term metabolic health.
Fitting Zone 2 Training Into a Busy Schedule
“Gosh, 150 minutes sounds like a lot—how do I fit it into my busy schedule?”
For me, Zone 2 cardio isn’t wasted time. When I’m on the stationary bike, I use it as productive time—reading, listening to podcasts, or even taking calls. Since Zone 2 allows you to speak in full sentences without being out of breath. It counts as work time for me, so it doesn’t feel like it takes away from my day.
Consistency is Key for Lasting Benefits
Remember, Zone 2 isn’t a temporary fix—it’s a lifelong practice. Even elite athletes who go sedentary for just two months can experience a 40% drop in mitochondrial capacity. The gains you make from Zone 2 training can fade quickly if you stop, which is why it’s important to make it a consistent part of your routine.
As Peter Attia puts it:
“Can you do it for the rest of your life?”
A Bonus to Zone 2: It Feels Good
After a Zone 2 session, you might notice a surge in appetite—and a well-deserved dinner afterward feels even more satisfying. It’s one of those workouts that leave you feeling refreshed rather than exhausted, making it easier to stick with for the long haul.
TL;DR: Whether you’re an elite athlete or just starting your fitness journey, incorporating Zone 2 into your routine can transform your metabolic health, improve endurance, and provide a solid foundation for overall well-being.
Disclaimer: None of this is to be deemed medical advice of any kind. Thank you, Ed
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