“Current Evidence Does NOT Support Zone 2 Training”

A pretty damning analysis on zone 2 training was just published, and it concluded that the current evidence does not support zone 2 training as optimal intensity for improving mitochondrial or fatty acid oxidative capacity. 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 training into our protocols, or should we prioritize other forms of exercise? Well, the answer is a bit nuanced. So, let's take a look at the research.

So, first, I want to explain what zone 2 training actually is and why it's thought to be so helpful, and then we'll look at some of the controversial criticisms that have been raised in this new analysis. So, zones in exercise, they're a way to describe intensity. So, not everyone defines zone 2 exactly the same way. So, for Peter Attia, for instance, he talks about how it's the level of exercise that we can sustain while keeping our lactate levels below 2 millimoles per liter. As we exercise, our muscles produce lactate, and it's a byproduct of the processes that produce energy. And as intensity ramps up, lactate, it starts to build up faster than our bodies can clear it away. And that leads to feelings of fatigue and burning in our muscles. And with zone 2, the goal is to keep the intensity at a level where our bodies can keep clearing away the lactate produced so that it doesn't build up. And that's at a level of intensity that we can sustain for a long time.

Now, obviously, that definition is a bit technical, and we can't easily check our lactate levels when we're going out for a jog. So, people often rely on the so-called talk test instead. So, zone 2 is an intensity where you're working out, but you're able to maintain a conversation comfortably.

So, what's so special about zone 2? Well, according to its proponents, it's the zone where we can get the greatest benefits in two areas. So, that's mitochondrial capacity and fatty acid oxidative capacity. So, let me explain both of these.

So, mitochondrial capacity is a broad term for mitochondrial health and function. Mitochondria, they're the powerhouses of our cells, as people like to say in podcasts like this, and they play very important roles. So, they're central to our metabolic health and athletic performance. They're linked to how well our bodies can use sugar and fat as energy sources, and they're even a bigger player in the aging process.

So, what about fatty acid oxidation capacity? Well, our bodies can draw from two different energy sources. So, on the one hand, we can use glucose. On the other hand, we can burn fat. And that's what we're talking about when we say fat oxidation. And the term oxidation, it just indicates the process uses oxygen. So, in that respect, just like when we burn something in a fire, it takes plenty of oxygen to break down that material and release its energy. So, our fat oxidation capacity describes how well our bodies can transform fat stores into energy.

But why does that matter? Well, a higher fatty acid oxidation capacity is linked to better athletic performance. It's linked to better heart health and insulin responses. It helps us draw on our fat reserves and be less dependent on carbohydrate metabolism for fuel when we exercise. And we've got lots of evidence that exercise is a powerful way to boost both mitochondrial health and function, as well as increasing fatty acid oxidation capacity.

So, here are some things that virtually everyone agrees about. Boosting mitochondrial health and function, as well as increasing fatty acid oxidation capacity, are both fantastic for athletic performance and for our overall health. And exercise does both. But here's where we run into some controversy: what type of exercise is best to gain these benefits?

So, the authors of this new analysis, they poke some holes in the zone 2 hype. So, a common argument for proponents of zone 2 training goes something like this: When we look at elite athletes, they do large volumes of zone 2 training. They've also got great mitochondrial health and high fatty acid oxidative capacity. So, copying their training strategy might be a good way to obtain similar benefits. But the authors raise two worries about this type of thinking.

First, elite athletes, they also incorporate some high-intensity exercise. So, it isn't clear that their low or high-intensity training is more important for the benefits that we're interested in here when it comes to our own health. And the second worry is that most of us aren't elite athletes. So, those elite athletes, they might be training for over 20 hours a week, while many of us, we struggle to fit in 2 hours a week. So, we'll see in a moment that that difference has a huge impact in terms of the type of exercise strategies that make the most sense to maximize our gains.

But leaving that aside for a moment, what about the claims that zone 2 training is best for mitochondrial health and fatty acid oxidative capacity? Because it's often a key selling point for this type of exercise. Well, here's where the authors of this new analysis raise some serious doubts.

So, let's have a look at mitochondrial function to start with. So, when we exercise, we can stimulate our muscles and send signals to the body to strengthen our exercise capacity in terms of mitochondria and create new mitochondria. So, that signaling happens along with several other different pathways. So, the question is, does zone 2 training stimulate the signaling?

So, in the analysis, the authors look at the research when it comes to several of these pathways, and in each case, they find that the evidence shows minimal or non-existent activation, and sometimes we don't have the data to say one way or the other. So, consider one example: mitochondria can be stimulated in response to a strain on cellular energy systems, and when muscles experience this strain, they essentially say to the cells, "We need more power plants to provide this energy." But existing evidence, as discussed in the new analysis, shows that zone 2 training provides minimal stress to our cellular energy systems. In other words, this level of intensity might not be enough to stimulate mitochondrial development.

But this is the important point that changes if it goes on long enough, as in elite athletes who are doing a lot of zone 2 training can see changes in their mitochondrial development. But for us, if we're not doing a huge amount of it, we might see minimal changes. So, for instance, one study found that there was evidence of significant stress to cellular energy systems, but only after 2 hours. But if we're only exercising for 2 hours in total a week, that's not particularly helpful.

But the authors of this new analysis also come at the question in a different way. So, they looked at measures of improvements in function instead of just measures of signaling pathway stimulation. So, let me clarify a couple of differences within an analogy. So, suppose I said that new fertilizer made plants grow taller by stimulating their root growth. So, you could check my claim by looking at the roots, and that's like examining the pathways. Or you could measure the plants to see if they actually did grow taller, and that's like examining the outcomes.

So, what were the outcomes? At the end of the day, can we see improvements in mitochondrial function after zone 2 exercise? Well, there are a few studies looking at this question directly, and so far, the evidence is mixed. But a meta-analysis assessed the relationship between exercise intensity and mitochondrial impact. So, for non-endurance trained individuals, intensity around zone 2 didn't seem to improve mitochondrial function. In contrast, they argue that we've got lots of evidence showing that exercise above zone 2 stimulates mitochondrial function more. So, studies show that high-intensity interval training, for instance, stimulates significant changes. It strongly initiates key signaling pathways. And the authors of the analysis conclude the evidence points towards intensities above zone 2 as being the most helpful in stimulating mitochondrial changes. That's the exact opposite of what we often hear. And we'll have a look at the approach to exercise that's the most strategic to our own health shortly.

But what about fatty acid oxidation capacity? Well, at this point, there are limited studies addressing the impact of zone 2 training directly. In fact, the review authors found only one study that measured the rates of fatty acid oxidation after repeated zone 2 training. In that study, one year of zone 2 training significantly raised the maximum fat oxidation rate during exercise. Now, there have been a couple of other studies looking at fat oxidation after exercise, but it's really tricky to figure out if those people were actually exercising at zone 2 because their lactate levels weren't checked. So, the participants, they may have been working above zone 2. And that's what makes this data so difficult because we're not always sure which zone people are exercising in.

But when we look at studies that compare different levels of intensity for their effects on fatty acid oxidation, the results are conflicting. So, some studies found that lower intensity exercise does have a greater benefit. So, for instance, one study divided obese men into two groups. Both of them exercised for 12 weeks, and one group worked at a low intensity, and the other group worked at a high intensity. And by the end of the study, the low-intensity group had a higher fatty acid oxidation capacity during exercise, but there were no changes in the high-intensity group. But then there are yet other studies showing superior results for high-intensity exercise. So, one trial used high and low-intensity groups, and it found improvements in fatty acid oxidation capacity that was greater in the high-intensity group. And then there's a recent meta-analysis that examined 13 studies looking at fatty acid oxidation capacity. And its authors found that both high-intensity and moderate-intensity training improved fat oxidation, and the impact was similar.

So, at the moment, it isn't clear which exercise intensity is best for improving fatty acid oxidation capacity, but it doesn't look like we can say that zone 2 is superior, and that's the critical point.

In summary, it seems that both the claimed benefits of zone 2 exercise are suspect for non-elite athletes like many of us. It doesn't look like it's more effective in stimulating mitochondrial health, and it certainly doesn't look like it makes a bigger impact on fatty acid oxidation capacity. In fact, when we look at the evidence around mitochondrial health, it looks like the opposite is true. And this has got some very practical implications for most of us. And here's why.

So, as I pointed out earlier, most of us are not elite athletes. And for those of us who are, it can make sense to combine large volumes of zone 2 exercise with smaller amounts of high-intensity activity. So, they can gain benefits associated with both. But for the rest of us, the problem is that zone 2 exercise can easily crowd out the limited time that we have to do exercise at higher intensities. And it's that high-intensity exercise that's got a much bigger payoff when it comes to our health. And as we've already touched on, this is in relation to mitochondrial function.

And I want to look at one other area: cardiorespiratory fitness. So, this is about how well our heart and lungs can work together to process oxygen and pump blood. And a key measure of this type of fitness is VO2 max. It's the maximum volume of oxygen that we can use during intense exercise. And it's strongly linked to health outcomes. So, for example, one study with patients with heart disease, they monitored the participants for deaths over an 8-year follow-up period. And those with the best VO2 max scores had an 84% lower mortality risk than those with the worst. In fact, the link between VO2 max and things like heart disease and all-cause mortality is much stronger than their link with mitochondrial health. And what this means is that if we're going to focus on anything for improving our health, cardiorespiratory fitness is a good target. But the data favors higher intensity exercise as having a more significant impact in this area. Again, for non-elite athletes who are struggling with time to fit in in terms of the exercise protocols.

So, what's the key takeaway? Well, if we're not elite athletes, then we're likely to have limited time for exercise during the week. And experts in elite training say that we'll see the best gains if we spend one to one and a half hours of zone 2 training at least four times a week. But for most of us, that's a lot more than what we've got time for. And that means that we need to focus our energy where we're going to get the biggest payoff. And if we're spending the majority of our time in zone 2, then we're missing out on significant gains that we could be achieving at higher intensities. And the better shape that we're in, the more this is going to be true. When the body is pushed, it responds most strongly.

So, the advice that I give to my patients is to focus on high-intensity exercise done safely. So, if you're starting out, we need to build up slowly, otherwise you're going to risk an injury. And this also includes power training when we're combining higher weights with quick movements. Again, we want to make sure that this is done safely. And the standard recommendation is to aim for at least 75 minutes of vigorous-intensity exercise a week. And if we've got time beyond that, then yes, it might make sense to add in some zone 2 training. But for some people, the 75 minutes that I've just mentioned can feel like a stretch. And the reality is that life is so busy that fitting in that exercise can be a real struggle.

So, in this next video here, I look at some fascinating data on how much a difference even minimum amounts of exercise can make. And if we focus on the right kinds of movements, small steps can lead to massive payoffs. So, have a look to get both inspiration and practical ways to start moving more.