Alex Hutchinson Zone 2 Training

I have an article in today’s New York Times about so-called “zone 2” exercise. It’s a hot topic these days thanks primarily to Peter Attia’s promotion of it, and my aim in the article is to offer an accessible introduction to what it is, why it’s supposed to be good for you, and what the current state of the evidence is. I hope you’ll head over and give it a read (using this gift link if you’d like).
 
To write it, I spoke to many more scientists beyond those quoted, and had a chance to dig into some of the nuances of the topic in a way that I couldn’t fit into the article. So I’d like to use this newsletter as a place to “empty my notebook” and share a few additional thoughts about zone 2. This gets a little detailed and technical, so start with the NYT article and proceed with caution!
 
Why are mitochondria so important for health?
 
In the article, I mention Iñigo San Millan’s belief that mitochondria are an early warning signal of metabolic problems that may eventually progress to type 2 diabetes or other conditions. I found his logic interesting. Normally we test for prediabetes by looking at glucose levels in the bloodstream. But the problems start inside the muscle cell long before they show up in the blood.
 
To take glucose from your bloodstream and burn it for aerobic energy requires a series of biochemical steps: first you transport it from the blood across the cell membrane into the muscle cell, then you convert it to pyruvate, then you transport it into the mitochondria, and then you finally oxidize it via the Krebs cycle.
 
San Millan has a preprint in which he compares healthy sedentary people with moderately active people who meet the public health recommendations of 150 minutes of exercise per week. If you compare glucose in the bloodstream, no difference. Their GLUT4 transporters, which move glucose from the bloodstream into the cell, are all working fine. But if you look inside the cell, the sedentary group already has problems transporting pyruvate into the mitochondria and burning it for energy. Eventually, San Millan says, this dysfunction is going to create a traffic jam that leads to higher glucose levels in the blood—the road to type 2 diabetes—but that impending problem might not show up in blood tests for another decade or more.
 
Where does lactate fit in?
 
In the article, I describe zone 2 as the exercise intensity which maximizes fat-burning. Why does fat-burning drop when you push harder than zone 2? One reason is that this is the point at which your slow-twitch muscle fibres can no longer deliver enough power, so you have to start recruiting fast-twitch fibres. These fast-twitch fibres have less mitochondria (which can burn both carbs and fat aerobically), and rely more on burning carbs anaerobically, which produces lactate as a byproduct.
 
This means that rising lactate levels are another indicator of where zone 2 is. It varies from person to person, but typically levels of 1.7 to 2.0 mmol/L correspond to zone 2.
 
Lactate isn’t the evil waste product we used to think it was. In fact, you can reuse lactate as fuel in—you guessed it—your mitochondria. So from a performance perspective, another reason to beef up your mitochondria is that it enables you to reuse lactate at a faster rate, effectively increasing your lactate threshold.
 
Do you need to train in zone 2 to improve zone 2?
 
In San Millan’s view, zone 2 is both diagnostic test and treatment. As a diagnostic test, your power output (or running pace) in zone 2 is a proxy for how well your mitochondria are working, he says. This is not necessarily a mainstream view: he mentioned that peer-reviewers have objected to this claim in some of his papers.
 
As treatment, the idea is that training at zone 2 is the best way to improve zone 2. This is such a logical concept that I almost didn’t give it a second thought. But Brendon Gurd, the head of the Muscle Physiology Lab at Queen’s University, isn’t convinced. To him, the assumption that simply using something is the best way to make it adapt is a kind of folk physiology. We don’t assume that the best way to improve your ability to lift ten pounds is to simply lift ten pounds over and over. Instead, we lift 20 pounds, or 100 pounds, with the goal of triggering molecular signals that tell our muscles to adapt.
 
Similarly, there’s plenty of muscle biopsy evidence (which I touch on briefly in the NYT article) that mitochondria respond best to higher intensity even if they’re already working at full capacity at lower intensities. “I like to think of mitochondria as responding to energetic stress,” Gurd’s grad student Kristi Storoschuk told me. “We have to disturb homeostasis so that our mitochondria get that signal to grow and become better and stronger.” The classic evidence is this 2006 study comparing sprint intervals and long steady training, and finding similar signalling responses and mitochondrial adaptation.
 
Is it possible to go too easy?
 
It’s not as easy to answer this question as you might think, because none of the rigorous muscle biopsy studies have prescribed exercise at exactly zone 2. If you extrapolate from the existing data, the general picture is higher intensity and higher volume both give you more mitochondrial adaptation. At the other end of the spectrum, according to both Storoschuk and David Bishop of Victoria University in Australia, there may be a minimum intensity threshold below which you don’t get any mitochondrial adaptations. So zone 2 isn’t just a synonym for “easy.”
 
As a corroboration of this point, I was interested to see recent comments from cyclists on UAE Emirates, the team San Millan works with. Here’s Pavel Sivakov, for example: “I train a lot in the famous ‘zone 2’ that is known in all endurance sport. The big difference is that I don’t ride easy anymore, like below 200 watts. It’s always constant pressure on the pedals… It’s not easy mentally… It’s hard for me now to train with other guys because I never go easy. I always have to press on.”
 
How does this relate to polarized training?
 
Zone 2 is often linked to the idea of polarized training, which (in very broad strokes) refers to the idea of spending about 80 percent of your training time going easy and 20 percent going hard. To San Millan, the ubiquity of this 80/20 pattern across the endurance world is one of the strongest pieces of evidence in favour of the idea that zone 2 is uniquely effective. Sure, the biopsy data might suggest that higher intensities are better, he says, but there’s a reason that elite endurance athletes don’t just hammer high intensities all the time: “They tried it and it didn’t work.”
 
Like most competitive runners, my training has generally followed this 80/20 pattern. But does my easy mileage, which I tend to do at a relaxed conversational pace, really count as zone 2? Or do I need to be “pressing on” like Pavel Sivakov?
 
I checked in with Stephen Seiler, the originator of the polarized training concept, and his daughter Siren Seiler, a runner and coach. One of the key points they highlighted is that cyclists have a much broader range of “easy” efforts than runners. “When a runner goes easy, they generally end up in high zone 1 or low zone 2 anyway, whereas a cyclist can chill at 50 percent HRmax without imposing technical challenges,” Siren Seiler told me. “So a cyclist has to ‘push’ a bit harder to maintain zone 2.”
 
How precise does zone 2 need to be?
 
In one of his podcast conversations with San Millan, Peter Attia talks about his focus on staying exactly in zone 2 when he works out: “I can’t do zone 2 on the road, he says. “I can really only do it on the trainer. I just can’t stay at a constant level on the road, with starting and stopping and wind and hills and stuff like that.”
 
Given the polarized training literature (and my own training experiences), I have no problem believing that large volumes of easy training are a great idea. I have a harder time with this idea that you need to be at a very precise intensity to get the benefits. San Millan definitely takes a more relaxed approach than Attia: “I’m not religious about it,” he told me. He rides outside, and if he comes to a hill, he doesn’t get off his bike and walk up it just to avoid getting out of zone 2.
 
What’s the problem if you push too hard? According to San Millan, it means your lactate levels will creep up, which will suppress fat-burning in the mitochondria. If you’re well-trained, you’ll recover quickly from the hill or surge and be back in zone 2 within a minute or two. If you’re less fit, it might take ten or 15 minutes before you clear the lactate and get back into zone 2.
 
I also asked Stephen Seiler about this, and he sent me the draft of a journal paper that will soon be published in Sports Medicine by a group of researchers and elite cycling coaches. The primary focus of the paper is agreeing on a definition of zone 2, given all the conflicting zone systems and definitions out there. But this passage also caught my attention:
 
Finally, experts highlighted that the physiological responses to increasing intensities are better explained as a continuum instead of adaptations that are turned on and off by crossing specific intensity zone demarcations established from “fresh” testing. Accordingly, they did not expect large differences in the adaptations produced by the upper end of zone 1 and the lower end of zone 3 when compared to what they expected from zone 2.
 
So from their perspective, the difference between zones 1.5, 2, and 2.5 are a matter of degree, not an on/off switch.
 
What’s the verdict?
 
My NYT editor and I spent a lot of time going back and forth about the overall tone and message of the article. I didn’t want it to come across as “zone 2 is a miracle,” but I also didn’t want it to be “scientists debunk zone 2.” Neither perspective would be accurate, I don’t think.
 
The evidence in favour of zone 2 is a combination of observational and mechanistic. Elite endurance athletes do a lot of zone 2-like training, and San Millan says the regular people who visit his lab seem to improve their mitochondrial function in proportion to how much zone 2 they do. The chain of logic connecting mitochondrial function to long-term metabolic health makes sense. What’s missing is training studies: have some people do zone 2, other people do other training, and see who improves the most.
 
The evidence against zone 2 comes from the training/biopsy studies I mentioned above. But it’s important to note that none of these studies actually prescribed zone 2 training. The pattern of higher intensity producing bigger gains is clear, but intensity is usually prescribed as a percentage of VO2max rather than based on fat-burning or lactate profile. Moreover, it’s important to think carefully about what’s actually being compared. Should it be the same duration of exercise? The same number of calories burned? The same subjective effort for the overall session? The same willingness to exercise again the next day?
 
What I take from it, in the end, is an argument in favour of the idea that accumulating relatively large volumes of easy training is a good idea – alongside other types of training, including hard VO2max-style intervals and strength training. Whether one intensity is slightly more “optimal” than another one is almost certainly less important than doing a mix of intensities and finding a sustainable pattern that you enjoy. I find this line of research really interesting... but I won’t be getting a lactate meter.
 
Okay, my notebook is now officially empty. Big thanks (and congratulations) if you made it this far. And hey, if you enjoyed this, please consider pre-ordering my new book, The Explorer’s Gene: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, which comes out next month. Publisher’s Weekly calls it “an enlightening if overstuffed examination of why humans are ‘drawn to the unknown,’” by which they mean that it’s just as dedicated to pursuing answers and following threads all the way down the rabbit hole as this email is!