Get Fit, Not Fried — The Benefits of Zone 2 Cardio

Most people skip zone 2 entirely

Brett McKay introduces the key problem: most people jump straight from rest (zone 1) to zone 3 cardio, skipping zone 2 entirely and missing out on significant health, fitness, and wellbeing benefits.

"When most people work out, they jump right from a resting state called zone one cardio to zone three cardio. But in skipping over zone two cardio altogether, they miss out on a significant range of benefits to their health, fitness, and overall wellbeing."

Training zones work like a dimmer switch

The dimmer switch analogy explains that training zones don't switch on and off abruptly but blend gradually. At any point, the body uses all energy sources in varying amounts, with fat dominating at lower intensities.

"In other words, a little bit of the fermentation going on, the lactate systems becoming involved, just to sort of generate that little bit of energy. And so zone two basically represents that point at which the metabolic demand is rising, but oxygen and fat stores are still, and obviously glucose stores as well, but aerobic systems are still responsible for the vast majority of our energy generation. Okay. And as we increase intensity, we shift to zone three. What's happening next? At zone three, what happens is you actually cross a point called the VT1, the ventilatory threshold one. And this is actually going to go a little bit into how we determine these zones. You'll notice at the VT1, your rate of respiration begins to increase. And one of the things that's driven by, you know, there are various, you know, chemoreceptors and everything else in the body that detect the sudden change in blood pH that's caused from an aggregate rise in carbon dioxide. That basically means that CO2 levels in the blood, basically your body, the muscle cells are generating, they're utilizing energy at a faster rate than your standard aerobic systems can supply that. So what starts to happen is your rate of respiration increases, your heart rate begins to increase, but we're already tapping more into kind of the anaerobic systems. We're tapping more into those fermentation systems. And the body is already at that point in this kind of delicate dance where it's not really able to sustain this indefinitely. We're already, again, anaerobic systems, when we don't use oxygen, they're a little bit less efficient. So the body is kind of on this slow path at that point towards a loss of ability to meet the energy requirements. So zone two, you can't really do forever. Other things will break down, but you can do zone two for an exceedingly long period of time because it's so sustainable. Crossing that ventilatory threshold represents, again, a slow accumulation of these metabolites and a gradual slow decrease in our body's ability to sustain a given level of intensity. And we're starting to make that shift from using fatty acids to starting to use glucose more to get that ATP. Exactly. And not just using glucose, but also using glucose in anaerobic metabolism, which is actually less efficient. We generate less ATP or less energy per unit of glucose when we're processing it through anaerobic systems than when we do when we're processing it through aerobic systems. So it was kind of likened a little bit to afterburners in a jet. Yes, we can produce a lot of energy that way, but we're using four times as much fuel per unit of energy produced. Oh, another difference between zone two and zone one and then zone three zone three, with zone two, zone one, when you're using fatty acids and oxygen, the mitochondria in your cells are creating the ATP. And I think in glycolysis, we're using glucose to create ATP. That occurs in your cytosol. It doesn't occur in the mitochondria. Is that right? Yes. Yeah. And a lot of cases, like say a lot of the lactate producing systems can occur in exactly in other systems. So this is no longer strictly a mitochondrial based process. And in fact, again, the process of conversion of lactate back to glucose or back to glycogen is much more systemic. This is less infinitely sustainable by those organelles. Yes. All right. So zone three, you're starting to use more glucose. What happens in zone four? Is it just that you're just using more and more glucose? When do you start using other energy sources? So, when we actually cross into zone four, that's where it gets a little fuzzier. The line between zone three and zone four is, as some people liken this to the ventilatory threshold two, it's when we actually start to get such a high utilization of these anaerobic energy systems that you start to really get, I wouldn't say an uncontrolled, but you get a steady accumulation of metabolites. In other words, the body is not clearing these metabolites. It is not able to sustain this level of intensity without an eventual crash. So we're using more anaerobic fermentation. We are getting to the point where the body is not able to keep up with a given level of work. And as a result, for example, the lactate levels, which are used, kind of go off into the stratosphere. And this is generally considered a level that the individual will rapidly fatigue at. In other words, there is constant accumulating fatigue throughout an interval. Whereas zone three, let's say a gifted marathon runner can run the entire marathon in their equivalent of a zone three. If they cross into a zone four, they'll probably begin cracking. Now, of course, for a marathon runner, the difference between zone three and zone four is actually very small, but that's a different story. But basically, at that point, when you cross into what we typically consider a zone four, that's unsustainable. We are using a lot more of very short-term energy systems, and we're using heavy amounts of anaerobic glycolysis, which causes rapid fatigue. And also, zone four and zone five, if that exists, there's a debate about that. You start using creatine and recycling ATP, basically, right? So, it's like when your body uses ATP to create energy, it loses a phosphate or something. And then... Correct. signaling ATP, basically, right? So it's like when your body uses ATP to create energy, it loses a phosphate or something. And then the body's like, okay, we can use that used up. It's called ADP now. And then we'll take creatine that's in the system and we can make ATP really fast, but it's not very efficient. Right, exactly. Because what happens at lower intensity is ATP is constantly being generated. We're generating a huge amount of ATP per molar, let's just say per gram of fat, we're generating a huge amount. And that's great when we're at rest because there's so much fat in our body, relatively speaking. We've got the equivalent of tens of thousands of calories of work that our fat can do. When it comes to glucose, when it comes to glycogen, we have much less. Our liver only really has about 400 to 500 calories worth of work it can do, whereas our muscles have several thousand. So even so, if we think about it over, let's say, a marathon, if we had to do that on strictly glucose, we would burn through our systems. But we would burn through that very quickly. Of course, if we were to do that even faster and go through a process of fermentation, let's say our muscles are capable of doing 2,500 calories worth of work with the glucose that's in them. That's through aerobic systems. If we do that through anaerobic systems, we're really only capable of doing anywhere between 500 and 600 calories worth of work with that same fuel. so not only as we go up in energy systems, are we looking at smaller pools of energy? We are also looking at, on aggregate, the body being able to do less work with the stores we have available. Okay. So just to recap here, zone one, zone two, you're primarily using fatty acids, you're using oxygen and the mitochondria to create the energy or ATP you need to do whatever. As you shift into zone three, you start using more glucose. And I think you made the point, this isn't like either or, like this is sort of like, it's a dimmer. I like that analogy. And then as you shift into zone four, it's more glucose and maybe even using recycling previously stored ATP. Do you think there's a zone five? Like what's your take on that? So a lot of that really depends. And you can already tell that since everything is a dimmer switch, we're talking about pretty fuzzy definitions here. And I think that's actually where a lot of confusion comes in. And I think something that people probably want to reiterate is how complicated this actually is. It's like there's no definitive switch between zone two and zone three, like even though we like to say there is. There's no definitive switch between zone three and zone four. Some people include a zone five and say, well, that represents the pure a-lactic systems, as they call it. We're talking about such a high level of intensity that we are just using the ATP-CP systems, for example. So that's just super high intensity, sprints of 10 seconds or under that's, you know, some people like to differentiate that. I don't particularly find it very useful because I think the entire purpose of planning in zones is to look at approximately, you know, stimulus, how long an athlete can sustain it and what it means metabolically."

Zone 2 is the sweet spot for endurance and metabolic health

Zone 2 is identified as the sweet spot for building endurance and improving metabolic health, where the body primarily uses fat as its main energy source. The talk test confirms you're in zone 2 if you can hold a conversation but need to pause every few sentences.

"In other words, a little bit of the fermentation going on, the lactate systems becoming involved, just to sort of generate that little bit of energy. And so zone two basically represents that point at which the metabolic demand is rising, but oxygen and fat stores are still, and obviously glucose stores as well, but aerobic systems are still responsible for the vast majority of our energy generation. Okay. And as we increase intensity, we shift to zone three. What's happening next? At zone three, what happens is you actually cross a point called the VT1, the ventilatory threshold one. And this is actually going to go a little bit into how we determine these zones. You'll notice at the VT1, your rate of respiration begins to increase. And one of the things that's driven by, you know, there are various, you know, chemoreceptors and everything else in the body that detect the sudden change in blood pH that's caused from an aggregate rise in carbon dioxide. That basically means that CO2 levels in the blood, basically your body, the muscle cells are generating, they're utilizing energy at a faster rate than your standard aerobic systems can supply that. So what starts to happen is your rate of respiration increases, your heart rate begins to increase, but we're already tapping more into kind of the anaerobic systems. We're tapping more into those fermentation systems. And the body is already at that point in this kind of delicate dance where it's not really able to sustain this indefinitely. We're already, again, anaerobic systems, when we don't use oxygen, they're a little bit less efficient. So the body is kind of on this slow path at that point towards a loss of ability to meet the energy requirements. So zone two, you can't really do forever. Other things will break down, but you can do zone two for an exceedingly long period of time because it's so sustainable. Crossing that ventilatory threshold represents, again, a slow accumulation of these metabolites and a gradual slow decrease in our body's ability to sustain a given level of intensity. And we're starting to make that shift from using fatty acids to starting to use glucose more to get that ATP. Exactly. And not just using glucose, but also using glucose in anaerobic metabolism, which is actually less efficient. We generate less ATP or less energy per unit of glucose when we're processing it through anaerobic systems than when we do when we're processing it through aerobic systems. So it was kind of likened a little bit to afterburners in a jet. Yes, we can produce a lot of energy that way, but we're using four times as much fuel per unit of energy produced. Oh, another difference between zone two and zone one and then zone three zone three, with zone two, zone one, when you're using fatty acids and oxygen, the mitochondria in your cells are creating the ATP. And I think in glycolysis, we're using glucose to create ATP. That occurs in your cytosol. It doesn't occur in the mitochondria. Is that right? Yes. Yeah. And a lot of cases, like say a lot of the lactate producing systems can occur in exactly in other systems. So this is no longer strictly a mitochondrial based process. And in fact, again, the process of conversion of lactate back to glucose or back to glycogen is much more systemic. This is less infinitely sustainable by those organelles. Yes. All right. So zone three, you're starting to use more glucose. What happens in zone four? Is it just that you're just using more and more glucose? When do you start using other energy sources? So, when we actually cross into zone four, that's where it gets a little fuzzier. The line between zone three and zone four is, as some people liken this to the ventilatory threshold two, it's when we actually start to get such a high utilization of these anaerobic energy systems that you start to really get, I wouldn't say an uncontrolled, but you get a steady accumulation of metabolites. In other words, the body is not clearing these metabolites. It is not able to sustain this level of intensity without an eventual crash. So we're using more anaerobic fermentation. We are getting to the point where the body is not able to keep up with a given level of work. And as a result, for example, the lactate levels, which are used, kind of go off into the stratosphere. And this is generally considered a level that the individual will rapidly fatigue at. In other words, there is constant accumulating fatigue throughout an interval. Whereas zone three, let's say a gifted marathon runner can run the entire marathon in their equivalent of a zone three. If they cross into a zone four, they'll probably begin cracking. Now, of course, for a marathon runner, the difference between zone three and zone four is actually very small, but that's a different story. But basically, at that point, when you cross into what we typically consider a zone four, that's unsustainable. We are using a lot more of very short-term energy systems, and we're using heavy amounts of anaerobic glycolysis, which causes rapid fatigue. And also, zone four and zone five, if that exists, there's a debate about that. You start using creatine and recycling ATP, basically, right? So, it's like when your body uses ATP to create energy, it loses a phosphate or something. And then... Correct. signaling ATP, basically, right? So it's like when your body uses ATP to create energy, it loses a phosphate or something. And then the body's like, okay, we can use that used up. It's called ADP now. And then we'll take creatine that's in the system and we can make ATP really fast, but it's not very efficient. Right, exactly. Because what happens at lower intensity is ATP is constantly being generated. We're generating a huge amount of ATP per molar, let's just say per gram of fat, we're generating a huge amount. And that's great when we're at rest because there's so much fat in our body, relatively speaking. We've got the equivalent of tens of thousands of calories of work that our fat can do. When it comes to glucose, when it comes to glycogen, we have much less. Our liver only really has about 400 to 500 calories worth of work it can do, whereas our muscles have several thousand. So even so, if we think about it over, let's say, a marathon, if we had to do that on strictly glucose, we would burn through our systems. But we would burn through that very quickly. Of course, if we were to do that even faster and go through a process of fermentation, let's say our muscles are capable of doing 2,500 calories worth of work with the glucose that's in them. That's through aerobic systems. If we do that through anaerobic systems, we're really only capable of doing anywhere between 500 and 600 calories worth of work with that same fuel. so not only as we go up in energy systems, are we looking at smaller pools of energy? We are also looking at, on aggregate, the body being able to do less work with the stores we have available. Okay. So just to recap here, zone one, zone two, you're primarily using fatty acids, you're using oxygen and the mitochondria to create the energy or ATP you need to do whatever. As you shift into zone three, you start using more glucose. And I think you made the point, this isn't like either or, like this is sort of like, it's a dimmer. I like that analogy. And then as you shift into zone four, it's more glucose and maybe even using recycling previously stored ATP. Do you think there's a zone five? Like what's your take on that? So a lot of that really depends. And you can already tell that since everything is a dimmer switch, we're talking about pretty fuzzy definitions here. And I think that's actually where a lot of confusion comes in. And I think something that people probably want to reiterate is how complicated this actually is. It's like there's no definitive switch between zone two and zone three, like even though we like to say there is. There's no definitive switch between zone three and zone four. Some people include a zone five and say, well, that represents the pure a-lactic systems, as they call it. We're talking about such a high level of intensity that we are just using the ATP-CP systems, for example. So that's just super high intensity, sprints of 10 seconds or under that's, you know, some people like to differentiate that. I don't particularly find it very useful because I think the entire purpose of planning in zones is to look at approximately, you know, stimulus, how long an athlete can sustain it and what it means metabolically."

Elite athletes spend 80% of training in zone 2

Elite endurance athletes spend about 80% of their workout time in zone 2 at a conversational pace, while recreational exercisers blow past it into zone 3. The paradox: going slow makes you fast.

"And also, I think that something that contributes to that is just like the fitness literature, right? You pick up men's health and it talks about HIIT workouts. Everyone's got to be doing HIIT workouts. Oh, man. Yeah. Yeah. I can't tell you when, again, like back when I first started doing all this stuff, HIIT was everything. And the whole know, the whole idea was, hell, I remember, you know, working with a lot of strongman athletes and talking to them. Because again, you know, again, early on in my career, I was talking to other powerlifters. Well, what do you do for cardio and strongman? What do you do for cardio? Oh, yeah, I push heavy sleds and, you know, tire flips. And, you know, that's my cardio right there. And, you know, obviously, we know that's still good stuff. But to them, that was kind of the minimum for cardio. Anything easier than that is just a waste of time. Oh, so let's dig into more about zone two. So how do we figure out what our zone two is? How do we know? Because you mentioned, so you said like zones are more about the energy sources that we're using, the metabolites, the changes in hormones that's happening in our body. So how do we figure out if we're in zone two or not? Yeah, absolutely. Yeah, because there are a lot of systems out there. Some people use heart rate. And while that's good for some, the problem with using heart rate is, first of all, everyone's max heart rate is different. The whole 220 minus age or any one of those systems are really only good for about the middle 60 to 70% of the population. And even then, there's kind of a plus or minus eight or seven standard deviation on that. What I mean by that is let's say with 220 minus age, it tells me my maximum heart rate should be 178. Well, that could be plus or minus eight. So my max heart rate could be anywhere between 186 and 170. If I'm trying to calculate 70% of that, that's going to give me a crazy number. So I tell people, forget about the heart rate zones. Don't worry about that. Some people say, well, you should use a lactate meter and check that. That doesn't work either. The easiest way I tell people is, look, when we talk about zone two, we're talking about a metabolic shift. So let's find the most obvious metabolic shift that happens with the most profound impact that you can see. And that would be your rate of breathing. So when I have people do a zone two test, which I also call a VT1 test, again, talking about the first ventilatory threshold, what I have them do is basically just continue to first start out at a walk, and then every minute increase their pace ever so slightly, tracking their heart rate and tracking their pace, until they get to a point where they can no longer speak a 15 to 20 word sentence without interruption. When they get to that point, it represents that inflection point where the rate of respiration is increasing, which corresponds to a change in blood pH. And the reason that's so accurate is because that change in blood pH right there, and that change in the need to breathe and respiration rate almost perfectly corresponds to those various metabolic shifts that represent the shift from zone two to zone three."