04: Much Ado About Zone 2

Some people say Zone 2 is the best way to train for fitness and health. Others say women should avoid it altogether. In this episode, I dig into what the evidence actually says about Zone 2 cardio by breaking down a recent review paper on the topic.

You'll learn what Zone 2 actually is, where many of the claims about it come from, what the research says about fitness adaptations like mitochondrial function and fat metabolism, and the practical pros and cons of moderate- versus higher-intensity cardio.

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EPISODE RESOURCES

Watch on YouTube: Simplify Cardio: What Matters for Results

Read the review paper Much Ado About Zone 2

Listen to The Real Science of Sport podcast episode featuring one of the authors of Much Ado About Zone 2

TIMESTAMPS

00:00 Why Zone 2 has become so controversial

02:25 What Zone 2 actually is and why it's difficult to measure

04:20 Where the claims about mitochondrial function and fat burning come from

06:40 What the review paper found about Zone 2 versus higher intensities

08:00 Why Zone 2 is really moderate-intensity exercise

09:45 The CDC and WHO cardio recommendations

10:45 Does Zone 2 improve mitochondrial function and fat metabolism?

13:35 Why higher intensities often improve fitness more efficiently

14:15 The practical problems with trying to stay in Zone 2

15:50 The pros and cons of moderate versus vigorous cardio

18:55 Why the best cardio is the kind you'll actually do consistently

Transcript

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If advice about zone two cardio has you confused, it's because the advice is all over the place. So let's clear it up. When we're consistent, we make progress. And when we make progress, we gain momentum. With momentum, consistency is met with less resistance. And before long, we're stronger, fitter, we feel better. And our entire relationship with exercise is different. Because we're different. We're not people who struggle to exercise anymore. We're people who struggle to imagine our lives without it. I'm Laurel Beaversdorf. I help people build strength and endurance in ways they can actually stick to. If you've ever wondered what to focus on, what to ignore, or how to make strength and cardio feel doable with the time, energy, and equipment you have, you're in the right place. And you've probably heard it. One side says zone two is the best, most optimal way to train, that it's the optimal intensity for things like increasing mitochondrial density and improving mitochondrial function, or that it's best for fat burning. And if you go harder than zone two, you might be overly stressing your body while also missing out on these benefits. And then the other side says almost the opposite. Specifically targeted toward women is this idea that women should avoid zone two, that it's not effective, that it's not going to create positive changes to their fitness, that it's somehow just the wrong type of stress for their bodies. So, you know, you end up with these two really extreme conclusions. Only do zone two or don't do zone two at all. And as it turns out, neither align with the evidence. So to help us make sense of this, we're going to use a specific paper that was making the rounds on social media several months ago. I had been hearing about it, and then one of the authors of the paper was actually on a podcast that I listened to, The Real Science of Sport, and went into detail about the paper itself. I'll link that episode in the show notes. But the title of this paper is called Much Ado About Zone 2, a narrative review assessing the efficacy of zone 2 training for improving mitochondrial capacity and cardiorespiratory fitness in the general population. And yes, the title is a reference to Shakespeare's play Much Ado About Nothing, which, if you're not familiar, is a comedy about two couples whose relationships are shaped by misunderstandings, mixed messages, and a lot of unnecessary drama before things eventually get sorted out. And honestly, that's a pretty good parallel for what's happening with Zone 2. There is a lot of confusion, a lot of strong opinions, and a lot of people talking past each other, but not a whole lot of clarity. So, side note, I was an actor for a while in my early 20s. I did some Shakespeare, but never much ado about nothing. So this is actually my first time spending time with it through exercise science. So we're going to use this paper to cut through the noise and look at what the evidence actually shows about zone two, including its benefits, its limitations, and where it actually fits. So first of all, we need to define what zone two is. It comes from heart rate training zones, and it is typically described as about 60 to 70% of your maximum heart rate. So your heart rate is how many beats per minute. And if we measure that maximum, what's the fastest your heart can beat in a minute? 60 to 70% of your maximum heart rate. That would be considered zone two cardio. But most people don't actually know their true maximum heart rate. And the most precise way to determine zone two would involve lab testing. Because actually, zone two is really more about the amount of lactate in your blood. Blood lactate. So all this to say zone two sounds like this precise target, but it's not actually that practical because making it a precise target would require measurement that we don't have access to. Okay. A more practical gauge would be using something like the talk test, right? Whether you can sing, talk in full sentences, talk in short phrases because this aligns with your respiratory rate, which tends to align pretty well with rising blood lactate levels, right? So we can also use a scoring system called RPE, rating of perceived exertion, to note, you know, on a scale of one to ten, how hard does this feel. By the way, I just recorded a YouTube episode all about this. I'll link it in the show notes. It's called Simplify Cardio, what matters for results. But when people extol the benefits of zone two, they're usually suggesting that zone two is best for improving things like mitochondrial function and fat metabolism. Let's talk about where this idea comes from. So this observation likely comes from elite endurance athletes and observing the fact that they do a lot of zone two training. But here's the thing elite endurance athletes do a lot of training, period. So included in this is a lot of zone two training, but also a lot of higher intensity training as well. They are doing huge volumes of work. And since all of that work can't be high intensity, a good bit of it, in fact, a majority of it, is typically moderate intensity or zone two training. Now, when we talk about mitochondrial function as a benefit of zone two training, let's talk about what that means. Mitochondria are the powerhouse of the cell. You probably learned in biology class, right? Mitochondria are responsible for turning oxygen into fuel that your body can use so that it has energy. And improving mitochondrial function means improving your ability basically to sustain activity. Let's talk about what fat burning or fat metabolism means. So fat burning in this context doesn't mean weight loss, it means using fat as a substrate as a way to make fuel during exercise. Another substrate is carbohydrates, so fat burning versus burning carbs. These are different ways to metabolize energy for fuel. And training can improve how well your body uses both. This is often called metabolic flexibility. So in real life, these adaptations show up, right, when we have better mitochondrial function, better fat metabolism. These adaptations show up as having more energy, of being able to go longer before fatigue sets in. Your usual pace starts to feel easier, you're able to go farther or faster at the same effort, and you recover more quickly, you feel less wiped out after activity. These are all signs your fitness is improving. And as we know, fitness tends to map pretty well onto health and longevity. So when people claim that zone two is the best way to improve on these adaptations, mitochondrial function, fat metabolism, what they're often suggesting is that it's the best way to improve fitness and by extension health, that zone two is the optimal intensity for improving mitochondrial function or fat metabolism. And in this paper, it uncovered the fact that there isn't actually strong evidence for this claim in the literature. Quote, our review failed to uncover substantive evidence supporting claims that zone two is superior to higher intensities for improving mitochondrial and fat oxidative capacity, a result possibly driven by the lack of studies explicitly examining zone 2 training as it is commonly characterized. So it could be that there just aren't enough studies examining this question, but basically it found that zone 2 is not superior for improving mitochondrial adaptations, and that higher intensity training often produces equal or greater changes, and that fat utilization improves across intensities, not just in zone two. They also found that higher intensity consistently led to greater improvements in overall cardiorespiratory fitness. So the idea that you need to stay in zone two to get these benefits isn't true. However, this does not make zone two irrelevant or useless. So as we said, zone two is essentially moderate intensity cardiovascular exercise. And in research, as I mentioned briefly, it is defined by blood lactate levels. Specifically, it's defined as effort below the first lactate threshold where lactate production in the blood is rising as intensity increases. But below that first lactate threshold, lactate production and clearance are balanced. So this means you're able to clear lactate about as quickly as it's created. And this means effort feels pretty steady, pretty controlled. You feel like you're working above resting levels for sure, but you can still talk in full sentences, and you likely feel like you could sustain that effort and pace and keep going for a while. And this is what public health guidelines like the CDC and the WHO refer to as moderate intensity exercise, or what runners will often call their easy pace. But it's always relative. So this is really important. For one person, zone two might look like a walk that is brisker than their normal walk. For another, it could be a steady jog. For another, it could be what looks like a pretty fast-paced run. So what zone two looks like can be very, very different depending on the person and their fitness status. Just like what's a heavy weight for one person might be a lot bigger or smaller weight than what's a heavy weight for another person. Heavy isn't a fixed heaviness, right? It's not a fixed number. And zone two isn't a fixed pace or speed, right? Zone two is really about an effort level. Now, zone two clearly improves health and fitness. This is why public health guidelines recommend around 150 minutes or more of moderate intensity exercise per week as the minimum. However, they also recommend 75 minutes of vigorous intensity exercise per week. You could do that in place of the 150 minutes of moderate exercise, or in combination to basically achieve a total of 150 moderate minutes. So vigorous minutes count double, in other words. And this is what this narrative review is suggesting as well, which is that higher effort exercise tends to drive larger improvements per minute or larger improvements. And this is both reflected in the CDC and the WHO guidelines, and it's also what this paper is driving at. All right. So let's summarize. And according to this paper, few studies have explicitly investigated the impact of zone two training on outcomes that it's typically purported to support, right? That not a lot of studies have explicitly investigated the impact of zone two training on mitochondrial outcomes. And although the collective evidence is mixed, it does appear capable of activating signaling pathways that initiate these mitochondrial adaptations. However, exercise performed below 60% of maximum work rate is not expected to improve mitochondrial content or mitochondrial respiratory capacities. Are quotes from the paper. So in other words, the signal is not as strong, right? The signal that we get to adapt from zone two cardio or moderate intensity cardio is not as strong as it is when intensities are higher. And so again, that's reflected in the CDC and WHO guidelines. The paper is quoted as saying exercise performed above zone two may be superior for inducing mitochondrial adaptations, directly contradicting the notion that zone two training is the optimal intensity for such outcomes. And then when it comes to fat metabolism, yes, zone two can help your body use fat as a substrate better, but so can other types or intensities of training. And zone two is not clearly better. Quote, zone two training does appear to increase fat oxidative capacity, but this is likely limited to sedentary or untrained populations. So if you take someone who is untrained and they start doing cardio, they do appear to increase their fat oxidative capacity. They make uh a pretty big change. However, if you were to take people who are already engaging in cardiorespiratory exercise, you might not see that big of a change. It wouldn't drive the change as much as higher intensity exercise would. So, in other words, zone two probably doesn't produce greater increases in fat oxidation than higher intensities do. And then finally, harder efforts often improve your fitness more per minute. They send a bigger signal than moderate intensity efforts. This is because higher exercise intensities result in greater improvements to cardiorespiratory fitness and the evidence-linking maximal aerobic capacity measured with VO2 max with cardiometabolic disease and all-cause mortality risk is robust. So, in other words, the fitter you are, the lower your all-cause mortality risk is, and higher intensity exercise tends to make you fitter per minute that you engage in it than moderate intensity exercise. So here's the thing: zone two can drive positive adaptations to fitness. There is evidence to suggest that engaging in zone two cardio prevents the onset of chronic disease and early death because it is a recommended intensity to be engaging with, according to the CDC, but you have to engage in twice as much of it than vigorous intensity exercise. According to this paper, what should you do about zone two? It is not clearly better for fat metabolism, nor is it clearly better for mitochondrial function. And exercise performed above zone two at a higher intensity may be superior. That is the thread running through the paper. So now that we have some evidence to go by, let's wrap this up by looking at the pluses and minuses of engaging in moderate intensity exercise, which we could call zone two. Okay, the first is a downside. Zone two is often presented as this very precise target, but accurately identifying it would require measuring blood lactate levels, and that is impractical. Staying in it consistently is also difficult, especially if you're untrained. You can start engaging in cardio and it feels moderate, but then your heart rate rises quickly and suddenly you feel like you're working really, really hard and you have to slow down or walk. So trying to stay in zone two can become really frustrating. And this is why I don't recommend that when people come to me for cardio programming that they try to stay in some heart rate target range, right? Which is often the proxy that we use to know, you know, if we're engaging in zone two cardios, we just use our heart rate and try to find something between 60 and 70% of our max. People don't know their max. There are equations that you can use to estimate your max. They're not totally accurate. But at any rate, it doesn't really give us useful information, in my opinion. Far more useful is to simply figure out how hard your hardest effort, right? Try to imagine what your hardest effort feels like and try to find something that's like a four, five, or six, maybe a seven. Somewhere in there is moderate. Now, if we zoom out from the label and just think about modern intensity, the trade-offs become clearer. So moderate intensity cardio is actually quite accessible for a lot of people. It feels easier to stick with. It works, I think, well for people who are new to cardio or who are dealing with some injury or who simply don't want to feel overwhelmed while they're engaging in cardio. They want to have a little bit more room to maybe listen to a playlist or a podcast or have a conversation with someone. And it's also moderate intensity cardio, a great fit if cardio is something that you want to be doing on a daily basis, because you will probably recover quite efficiently from it. Now, the downside, which we've been hinting at all episode, is that it is a less efficient way, it is a less time-efficient way to drive fitness changes. It generally produces smaller improvements per minute compared to higher effort work. So if your time is limited and you don't have a lot of time to engage in cardio, but you want to meet the minimums set out by the CDC, this is where higher intensity efforts might be really valuable to you. Higher intensity cardio is more time efficient. It tends to drive larger improvements to cardiorespiratory fitness per minute per session. It works well for people who are short on time. It's also sometimes more interesting to people. They enjoy pushing themselves, they like structured workouts, and oftentimes uh workouts will be structured into intervals of higher versus more moderate intensity efforts or lower intensity efforts, so it can kind of make the time pass a little quicker. However, higher intensity cardio is more fatiguing. It's hard to repeat it as frequently. You might feel like you need a day or two off in between higher intensity bouts of cardio. It can also feel intimidating, unpleasant, or overwhelming if you are new to that feeling of getting your heart rate up, of breathing really hard, of you know, feeling your body working at more like a seven, eight, or nine out of ten, right? So here's the good news, which is that this really should come down to preference. Zone two cardio is not the end all be all, but it's also not worthless. It can drive fitness changes, they aren't profound. But if you accumulate enough time engaging in it, they can be. The key is not choosing the perfect intensity or staying exactly within a range, but accumulating a sufficient amount of work over the week. Vigorous counts as double. If you prefer moderate effort and you can do more of it consistently, that's a really great option. Moderate intensity cardio can be engaged with probably every day, and that adds up. However, if you prefer higher intensity efforts, you like that feeling of your heart beating really hard in your chest and your breathing being very rapid, and you like to, you know, feel like you really worked. These higher intensity cardio efforts are a lot of bang for their buck, and they're gonna drive fitness more time efficiently. In many cases, for most people, a mixture is often ideal, right? Mixing in higher intensity efforts with more moderate or lower intensity efforts within the same workout, or having sessions that are more vigorous or higher intensity in nature with sessions that are more moderate intensity or uh moderate in nature. And this adds some variety to how we're working out. It also allows us to efficiently drive changes to our fitness on certain days and then build in recovery days around that. You might even be able to engage in moderate intensity cardio on days that bookend a vigorous intensity day because the moderate intensity cardio doesn't drain you as much. It allows you to kind of keep recovering while you engage in it. So, in many cases, a mixture is best. What I see with my clients is that moderate effort helps them to build up volume and consistency. Higher intensity cardio tends to be more attractive to clients who really like to push themselves or have a hard time staying engaged with cardio or they don't have a lot of time. But most of my clients actually work with a mixture of intensities. So instead of asking yourself, should I do zone two or something harder, or is zone two right for me, or how do I stay in zone two? A better question is well, what combination of effort levels can I engage in consistently in a high enough volume, right, measured in time, right, according to the CDC and the WHO, how can I get my my minimum, right, or maybe beyond that to keep improving my cardiorespiratory fitness, to keep my fitness high and to support my health and longevity because that's ultimately what matters, right? Is getting the work done. You have a ton of choice and you can choose to work out in a way that fits your schedule, that fits your preference, that fits your lifestyle, and that you will actually do, right? What will you actually do? All right. I hope this episode was helpful in clearing up some confusion around zone two, is it the holy grail? Is it bad for us? It's neither. It's just an exercise intensity. It's one that we can't really very easily precisely know or measure. Therefore, we can kind of just let it go. Check out my YouTube video, Simplify Cardio, what matters for results. If you would like some very practical guidance on how you can start to make cardio feel more doable, more accessible, and how you can start to feel successful with cardiovascular exercise. And stop just winging it, right? Stop just thinking I should do more cardio, but then have zero plan for how to make that happen. Because this video, I think, really goes a long way toward getting you started on being more intentional with cardio, but keeps it really, really simple. Thank you so much for joining me. I hope to have another episode for you in probably more like three weeks instead of two. But thank you for tuning in. Thanks for listening. Um, thank you for the eight people who have left a five-star rating, that means a lot, and for those of you who've left a written review, I really appreciate it. Alright, everybody, happy exercising, and I will be back in your ear soon enough.