Physiology Friday #112: Is 'Zone 2' Training Better than High-intensity Training for Mitochondrial Health?
The popularity of 'zone 2 training' has increased in recent years, but is there any evidence that this training intensity promotes superior adaptations to exercise?
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There has been a lot of hype lately (at least in the circles I inhabit) about “zone 2 training.” This is a result of many popular health and fitness podcasters and influencers speaking about “zone 2 training” and promoting this form of aerobic exercise to their audiences.
All of this is great — people are excited about exercise and hopefully motivated to explore the benefits that can come with this type of training (which I’ll discuss shortly).
However, on Wednesday, I took to Twitter to express how “fed up” I was with the buzzword that has become “zone 2 training.”
Brady Holmer @B_HolmerWithout a deep PubMed search… Methinks that HIIT (I.e zone 4/5+) probably produces comparable if not greater mitochondrial response. Another problem: most studies don’t/can’t investigate “zone 2.” Also: “Zone 2” isn’t magic, people 🪄 https://t.co/Vl3x9DQZYf
It’s not the actual training that I’m dissatisfied with, but the fact that the use of “zone 2” in the lexicon has become all too trendy. Zone 2 is one intensity of exercise among many that can be beneficial, and without proper knowledge of what “zone 2” is, the phrase becomes somewhat meaningless.
What is “zone 2 training”?
In exercise prescription, there are generally 5 (sometimes 6) “zones” of exercise that correspond to various types of intensity — 1 being the lowest and 5 or 6 being the highest intensity.
A “formal” definition of zone 2 is “the highest metabolic output or work that one can sustain while keeping lactate levels below 2.0 millimole/liter,” though many other definitions have been proposed. At zone 2, we are maximally stimulating our mitochondria’s oxidative capacity (ability to produce energy using oxygen). Above zone 2, we require additional energy production from non-aerobic (anaerobic) sources such as the breakdown of glucose.
Less metabolically-inclined definitions can include “aerobic threshold,” a pace/intensity that elicits an increase in breathing rate or a transition from nose- to mouth-breathing, or an intensity at which you could still hold a conversation. Some of these latter characterizations may be more helpful to imagine what zone 2 feels like.
In terms of heart rate at zone 2, it’s going to be somewhere between 60 and 70% of one’s maximal heart rate.
In short, zone 2 is fairly easy when it comes to exercise intensity — not a walk in the park, but something you could theoretically do “all day.”
The allure of zone 2 is that this exercise intensity not only promotes beneficial adaptations for metabolism and mitochondrial health and function but is also something that most people — whether they’re highly-developed athletes or relatively new to exercise — can adopt with little risk. Zone 2 is sustainable, “not-miserable”, and comes with a host of benefits.
What’s my problem then, with all this chatter about zone 2?
As I mentioned earlier, it has become a buzzword with little meaning absent any context. All intensities of exercise are beneficial — whether it be zone 1 or zone 5 — for different reasons. Zone 2 isn’t the only type of training, nor is it anything more than a rebranding of “moderate-intensity aerobic exercise.” In short, it may not deserve as much of the hype as it’s been getting, nor is it really “new.”
I’m also not sure that there is much evidence to conclude that zone 2 is any better for promoting mitochondrial adaptations or other benefits when compared to work-matched higher-intensity exercise (i.e. high-intensity interval training or HIIT). If this is true, all of this zone 2 promotion may be coming at the expense of neglecting a wide range of other exercise intensities.
So, what should you be doing if you are really concerned about enhancing your mitochondrial function to promote healthspan? Is zone 2 superior to other zones?
A few years ago, there was a “Crosstalk” published in the Journal of Physiology that asked whether exercise volume or exercise intensity was more important to promote mitochondrial adaptations in muscle.
This is not a perfect segue from the discussion above — where I’m asking which type of exercise is more effective. Rather, these dueling opinions focus on the relative importance of changing one variable or another (intensity or volume) for mitochondrial adaptations to exercise. But, we can still find some relevant information from both sides to inform our “zone 2 debate.”
Below is a bulleted list of some of the main points of each side of the debate.
High-intensity exercise increases exercise-induced metabolites like hydrogen ions, ADP, AMP, phosphate, creatine, and lactate to a greater extent than moderate-intensity exercise. More “metabolic stress” = more adaptations.
Higher-intensity exercise elicits greater activation of signaling and gene-expression pathways involved in mitochondrial biogenesis (i.e. AMPK, PGC-1ɑ). Some studies have even found that a 6 x 20 second “all-out” sprint interval protocol enhanced gene expression to a greater degree than 50 minutes of moderate-intensity cycling.
Studies on HIIT and sprint-interval training (SIT) provide strong evidence that these higher-intensity exercise protocols increase mitochondrial content and mitochondrial enzyme activity more than moderate-intensity exercise despite having a much lower exercise volume.
From a pragmatic point of view, HIIT/SIT are more efficient — one can increase mitochondrial content/enzyme activity with a much lower time commitment vs. low- or moderate-intensity exercise.
Evaluating research findings from 56 studies, a greater training volume was associated with larger increases in mitochondrial enzyme activity and mitochondrial volume — an association that became stronger when studies involving sprint-interval training (SIT) were removed.
No associations were found in the analysis between exercise intensity and changes in mitochondrial content.
I’ll be honest, this side of the argument was less convincing. The authors make a case that increasing exercise volume leads to more improvements in mitochondria — which seems obvious — without really providing much evidence from comparative studies that moderate-intensity exercise (i.e. high-volume) is more effective than lower-volume HIIT. In other words, increasing the volume of both HIIT and moderate-intensity training leads to more mitochondria, but this doesn’t necessarily prove that volume independent of intensity is more important.
Thoughts and musings
Let’s circle back to the original point of this discussion — does zone 2 deserve as much special treatment as it’s been getting?
In my opinion — no. There is not much evidence that zone 2 promotes more adaptations to the mitochondria compared to higher exercise intensities. In fact, it may be quite the opposite — higher intensity exercise (i.e. zone 4 or 5) is likely superior.
At a higher intensity exercise, you also need much less time (sometimes 6- to 8-fold less) to achieve similar or superior benefits to moderate-intensity exercise. For many people, time efficiency is a priority. If one is only willing to devote 1-2 hours per week to aerobic exercise, HIIT should definitely be on the menu.
This is NOT to say that zone 2 is unimportant. A balance of low- and moderate-intensity exercise and high-intensity training each week is important to promote a wide range of metabolic and cardiovascular adaptations.
The main argument in favor of zone 2 is that it may be a more viable option to promote healthspan and longevity — a lower exercise intensity is health-promoting and also less stressful on the body. Zone 2 may also be better for burning fat compared to higher zones. Lastly, most are going to be able to adopt zone 2 quite easily compared to jumping into a high-intensity training protocol.
I only wish to “dispel” the myth that zone 2 is some magical zone that deserves all of our attention. For elite athletes training 15-20+ hours each week, zone 2 is likely more important to fill in a large portion of training volume that simply can’t all be high-intensity (nor should it be). The average person with a voluntary or involuntary cap on their weekly training volume might need to be less concerned about “overdoing” intensity.
For time-strapped individuals who want to maximize their work output to time ratio, HIIT may be the perfect solution. Spending time in zone 2 may compromise desired training adaptations in a time-limited training program.
Finally, we must always remember that physiology does not exist in “zones” but rather, on a spectrum. We don’t neatly move from zone 1 to zone 2 to zone 3 but rather, exist in a liminal state between zones most of the time.
When it comes down to it, we need some easy activity, some moderate activity, and some really heavy activity each week. If we do that, our health will reward us. Let’s not get lost in the “twilight zones.”
If you want a very in-depth discussion on all things zone 2 and to explore some of the inspiration for this post, I highly recommend that you check out Dr. Peter Attia’s set of interviews with Dr. Inigo San-Millan (you can listen to Part I and Part II here).
Thanks for reading. See you next Friday,