A pretty damning analysis on Zone 2 exercise training was just published, and it concluded that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity [1].
Yet Zone 2 training has been discussed at length by voices like Dr. Peter Attia as a sweet spot that gives us some of the strongest benefits from exercise.
So should you and I incorporate Zone 2 exercise into our training protocol, or should we prioritize other forms of exercise? The answer is nuanced, so let’s take a look.
Table of Contents
- The story of the benefits of Zone 2 training
- What the evidence shows
- Final takeaway and strategy
-
Reference list
The Story of the Benefits of Zone 2 Training
Let’s first explain what Zone 2 exercise is and why it’s thought to be so helpful—before moving on to the controversial criticisms raised by this new analysis.
“Zones” in exercise are a way to describe intensity. Not everyone defines Zone 2 exactly the same way. For Dr. Peter Attia, it’s the level of exercise we can sustain while keeping our lactate level below 2 mmol/L [2].
As we exercise, our muscles produce lactate as a byproduct of energy production. As intensity ramps up, lactate builds up faster than our bodies can clear it. This leads to fatigue and that burning feeling in muscles. With Zone 2, the goal is to keep intensity at a level where our bodies can keep up with the lactate produced—so it doesn’t accumulate. That’s a level of intensity we can sustain for a long time.
Since it’s difficult to measure lactate while out jogging, people often rely on the “talk test.” Zone 2 is the intensity where you’re working but can still maintain a conversation comfortably.
So what’s so special about Zone 2? According to proponents, it’s the zone where we get the greatest benefit in two areas: mitochondrial capacity and fatty acid oxidation (FAO) capacity [1].
Mitochondrial Capacity
Mitochondrial capacity is a broad term for mitochondrial health and function. These “powerhouses of the cell” are critical for metabolic health and athletic performance. They’re involved in how well we can use sugar and fat as energy sources and play a big role in aging [1].
Fatty Acid Oxidation Capacity
Our bodies can draw energy from two sources: glucose and fat. Fat oxidation refers to how efficiently we can burn fat for energy. The term “oxidation” just means the process uses oxygen—like fire. So fat oxidation capacity describes how well we can transform fat stores into energy.
Higher fatty acid oxidation is linked to better athletic performance, heart health, and insulin response. It allows us to draw from fat reserves and be less dependent on carbs during exercise [1].
There’s broad agreement that exercise improves both mitochondrial function and fat oxidation capacity [3][4].
What the Evidence Shows
But here’s where we run into controversy: which kind of exercise is best to gain these benefits? The authors of this new analysis poke some serious holes in the Zone 2 hype.
What Elite Athletes Do
A common pro-Zone 2 argument is that elite athletes train heavily in this zone and have excellent mitochondrial and FAO profiles. So copying their strategy must be beneficial.
But the authors raise two issues:
1. Elite athletes also do high-intensity training, so it’s unclear whether their benefits come from low or high-intensity work [1].
2. They train a lot—often over 20 hours per week—while many of us struggle to hit 2 hours. That drastically changes the calculus for what type of training is most efficient for us [1].
Mitochondrial Function and Zone 2
Let’s look specifically at mitochondrial function.
Exercise can signal the body to strengthen and create new mitochondria through various pathways. But does Zone 2 stimulate these pathways?
The evidence shows minimal or non-existent activation of mitochondrial signaling pathways during Zone 2 exercise [1]. For example, one key signal is stress to cellular energy systems. Zone 2 doesn’t cause much of this stress—unless the exercise goes on for a long time (like 2 hours) [1].
Some studies do show small activation after very long Zone 2 sessions, but if we’re only working out 2 hours per week, this isn’t helpful.
Looking at Outcomes
The researchers also looked at direct outcomes, not just pathways. A meta-analysis showed that for non-endurance-trained individuals, exercise intensity around Zone 2 did not improve mitochondrial function [1].
That said, some evidence does suggest Zone 2 can initiate mitochondrial changes. But the key point is this: it’s not the most effective way to do so.
In contrast, exercise above Zone 2 clearly stimulates more robust mitochondrial changes—including stronger signaling and larger functional adaptations [1].
Fat Oxidation and Zone 2
What about fat oxidation?
There’s only one study that looked directly at Zone 2 and FAO—and it showed improvements after a year of training [5]. Other studies suggest improvement, but they didn’t measure lactate, so we don’t know if participants were truly in Zone 2.
When comparing intensities:
- One study in obese men found low-intensity exercise improved fat oxidation more than high intensity [6].
- But another found high-intensity was better [7].
- A meta-analysis of 13 studies found both moderate and high-intensity improved fat oxidation similarly [8].
In short: it’s unclear which intensity is best for improving fat oxidation, but there’s no strong evidence that Zone 2 is superior [1].
Final Takeaway and Strategy
So both of the claimed benefits of Zone 2 exercise—mitochondrial gains and fat oxidation—are in doubt for non-elite athletes. In fact, when it comes to mitochondrial health, the evidence suggests the opposite: higher intensity wins.
This has practical implications.
Time Efficiency Matters
Elite athletes can combine lots of Zone 2 with some high-intensity work. But for the rest of us, Zone 2 can crowd out time we could spend on higher-payoff exercise. And it’s those higher intensities that deliver bigger health benefits.
Cardiorespiratory Fitness (CRF)
Let’s talk about VO2 max—how much oxygen your body can use during intense exercise. It’s a strong predictor of health outcomes. One study found that people with the highest VO2 scores had an 84% lower risk of death compared to those with the lowest [9].
VO2 max has a much stronger link to health and longevity than mitochondrial health does [1].
So if we want to focus on the biggest lever for improving our health, cardiorespiratory fitness is the one to pull. And here too, Zone 2 falls short.
Studies show that in healthy, active people, CRF only increases with intensities above Zone 2 [1].
The Practical Strategy
Dr. Inigo San-Millan, a Zone 2 proponent, says optimal results require at least 1 to 1.5 hours of Zone 2 training, four times per week [10]. That’s a big time investment.
So here’s the more strategic path:
- Prioritize higher-intensity exercise, done safely.
- Start slow if you’re new, to avoid injury.
- Consider power training, combining heavier weights with quick movements.
- Aim for 75 minutes of vigorous activity per week.
- If you have time beyond that, then consider adding Zone 2.
Reference List
1. https://link.springer.com/article/10.1007/s40279-025-02261-y
2. https://peterattiamd.com/category/exercise/aerobic-zone-2-training/
3. https://pubmed.ncbi.nlm.nih.gov/31674658/
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC5766985/
5. https://www.thieme-connect.de/products/ejournals/abstract/10.1055/s-2007-965158
6. https://journals.physiology.org/doi/full/10.1152/japplphysiol.00030.2001
7. https://onlinelibrary.wiley.com/doi/10.1002/oby.21206
8. https://www.sciencedirect.com/science/article/pii/S1728869X23000424
9. https://www.internationaljournalofcardiology.com/article/S0167-5273(14)00394-5/abstract
