The Truth About Zone 2 Cardio, Mitochondrial Health & How REHIT Beats It (In Less Time!) With Ulrich Dempfle

My name is Ben Greenfield, and on this episode of the Boundless Life Podcast, we've spoken about HIIT and how HIIT is very effective.

Researchers then looked for the minimum effective dose of HIIT and found that 2 20-second sprints, so 40 seconds total at maximum intensity, creates a very robust, very strong training stimulus through that AMPK, AMP pathway, which actually also releases and activates PGC-1α, your body's master regulator of mitochondrial biogenesis, and creates a very robust stimulation to also develop more and more efficient mitochondria. And a number of other adaptations that very substantially improve your VO2 max, your cardiorespiratory fitness, with much, much less time.

Welcome to The Boundless Life with me, your host, Ben Greenfield. I'm a personal trainer, exercise physiologist, and nutritionist, and I'm passionate about helping you discover unparalleled levels of health, fitness, longevity, and beyond.

Ben Greenfield: It is a hot topic, I think, amongst many popular exercise, fitness, and health podcasts, this idea of what is often called Zone 2 training. Zone 2 training, it's all the rage right now. Everybody talking about Zone 2 for fat loss and Zone 2 for cardio and Zone 2 for longevity. But I've never really done a podcast in which I have addressed Zone 2. Or even compared and contrasted it to other methods of training, particularly for cardio. I realize that even I often during this podcast am accused of doing Zone 2 training on my treadmill while I'm interviewing my guests. Just a quick aside, walking on a treadmill would actually be something more like Zone 1. However, we'll address a lot of this on today's show with a guest who was on the podcast several years ago. So, it's been a little while. His name is Ulrich Dempfle. He is not only a guy who has a great deal of knowledge in the realm of cardiovascular training, but he's also the CEO and the co-founder of a company called CAROL. CAROL makes a bicycle, stationary bicycle that I use for for interval training, not for Zone 2 training but for interval training. And, Ulrich and I have been exchanging some emails back and forth about this idea of Zone 2. He has some interesting ideas and I thought it would be cool to have him back on the show. So, Ulrich, welcome.

Thanks so much for having me, Ben.

So, I know that in our first podcast, Ulrich, we talked a lot about the bicycle that you helped to develop and I'll certainly link to that in the shownotes. And, the shownotes for everybody listening in are going to be at BenGreenfieldLife.com/podcast. Bengreenfieldlife.com/zone2myth. That's bengreenfieldlife.com/zone2myth, like zone the number 2 myth. So, Oric, let's jump right in. What exactly is Zone 2 training? How do you describe it?

Yeah. So, Zone 2 training is a cardio workout at a slightly lower intensity as we would usually do it. So, where you'd still be able to hold a conversation and speak in full sentences even though uncomfortably. So, the intention is that you train at the point where your muscles still use predominantly fat as the fuel to power the exercise rather than carbs, glycogen. And, Because of the fairly low intensity, Zone 2 workouts tend to be longer and have to be longer to create a meaningful training stimulus. And the suggestion is from some of the leading proponents that sessions are or should be 60 to 90 minutes. And the outcome that, that we're trying to achieve with that would be increased mitochondrial density and increased mitochondrial efficiency so that over time, you can increase the intensity at which you can exercise while still using mainly fat and increase your capacity to clear lactate. And, that becomes really important for athletes to race harder for longer.

Yeah, I think that's a pretty fair description. It's kind of funny, back when I was studying exercise physiology, we didn't really use the term Zone 1, Zone 2, Zone 3, Zone 4, Zone 5, and in some cases depending on what zone training system you're looking at, not to make things too complex, Zone 6. Typically, we would look at what's called ventilatory threshold, meaning you'd have the first ventilatory threshold, where lactic acid starts to accumulate. You just noted that that also might be in Zone 2 where you can be moving but conversation is beginning to become uncomfortable as lactic acid just starts to accumulate. And then, you have a second point we call the ventilatory threshold 2 or VT2, and that is often synonymized, if I can kick out that word, with Zone 4, where lactic acid is beginning to accumulate or is accumulating at a much faster rate than it can be buffered. And, that's where we really get into the burn. In what you've seen in terms of looking at Zone 2, Oleg, have you come across the idea of Zone 2 defined as a percentage of your maximum heart rate or percentage of VO2 max?

Yes. So, there's several prescriptions as to the intensity you should train at, either linked to your maximum heart rate. So, can be some— and it really depends on the system you train with. Some people say from 65 up to 75, 80% of your maximum heart rate. to, to hit that zone 2 threshold. And the more precise, um, ways to find that threshold would be either through gas exchange analysis or really through blood lactate testing to find that point where you predominantly metabolize fat and not yet switch to carb and glycogen metabolism in your muscles.

Yeah, the lactate test is actually something I used to do quite a bit when I was training for triathlon and coaching people for triathlon. We'd have a little blood lactate meter. I wish they'd make a CGM for lactate, a CLM I suppose it would be called. I don't think it exists. But basically, people do an exercise session, they ramp up at increasing intensities during that exercise session. You typically from resting, lactic acid values, see a slight bump in lactic acid. Typically, it'll approach about around 2 millimole or so in terms of the units that are being used. And, we would typically identify that as, okay, you've reached your first ventilatory threshold. We're now detecting more lactic acid than you'd see at rest. And, that might sometimes be called the onset of zone 2. And then, if you look at someone who maybe doesn't have access to a blood lactate meter or maybe they're not as obsessed with exercise as a physiologist, the question becomes, well, could you just approximate that based on a percentage? And, you alluded to that, Ulrike. And, I think back when I was doing exercise phys classes, it was generally 60 to 70% of your maximum heart rate. And, your maximum heart rate, if you don't know it, could be somewhat approximated by subtracting your age from the number 220. So, I'm 43, so my age-approximated maximum heart rate might be around 177. And so then, if I take 60 to 70% or let's say 65% of 177, that gives me my approximate Zone 2. And then, you alluded to the idea of fat versus carbs. I don't know what you think about this, Ulrich, but it seems like a lot of people think, well, there's an exercise intensity at which you're burning just fat and then there's an exercise intensity at which you're burning just carbs. But really, I think a better way to think about it is you're always burning a mix, but at Zone 2, that's where the actual amount of fat oxidation would peak. And when I say peak, depending on how fat-adapted you are, That's the point at which you're burning 50 to 70%-ish of your energy from fat with a little bit of carbohydrates providing the rest of the energy. So, as far as the approximate percentages, I think that's about where I understand that Zone 2 would lie. Have you come across kind of similar idea in what you've looked at as far as Zone 2 is concerned?

Yes. And this is one of the first issues with Zone 2 that it is actually not a clearly defined term and everybody talks about the same thing when they talk about Zone 2. In fact, if you did a keyword search on PubMed for Zone 2 training, you would find hardly any matches because it's not a defined term and it's not something that scientists study when they do trials and investigations in their labs. And so because it's really a popular concept, you'll find quite large variations in definitions. So you said 65 to 70 or 60 to 70, then medium 65. Others give quite different recommendations. So another approximation would be 180 minus age and then adding up to 10 if you're fit and well-trained and deducting up to 10 if you're not that well-trained. So there's a broad range of different definitions, and that makes it harder to, to really discuss it and to back it up with scientific evidence. And we see that, um, that that leads to recommendations being put out, um, that actually have quite little scientific evidence. And serious scientists— and we work with some of the leading researchers in that field— just stay out of the debate because it's not a concept that matches their research. So, they would research like lactic thresholds or ventilatory threshold 1, as you said, lactic threshold, ventilatory threshold 2, VO2 max level performance, all-out performance. Those are like clearly defined things that we have lots of data on. But, Zone 2 is unfortunately not something that we have lots of data on. And therefore, the recommendations are less clear and there's less rigorous debate around this.

Yeah, yeah, I agree. I wish that folks could be a little bit less loosey-goosey with what Zone 2 is and simply say, "Hey, you're at ventilatory threshold 1 or maybe slightly above it. And, once you've gotten close to ventilatory threshold 2, you're going to be out of it." And, that would simplify the concept for a lot of people. But, for the general layperson listening in, Yeah, Zone 2, you're out on a walk, you start to walk faster, it gets harder to talk, you're pretty much about in Zone 2. And then, you start sprinting, things start to burn, you really feel the lactic acid accumulating, maybe even get that metallic or bloody, as gross as it is to think about, taste in your mouth, you're pushing more towards Zone 4+ at that point. Now, you also mentioned, Ulrich, the amount of time that Zone 2 is generally considered to fall into in terms of how long one would be exercising. I think you said a typical session is 60 to 90 minutes.

So, that's the recommendation. So, if you listen to what some of the leading proponents of Zone 2 training put out, it's suggested one should do 60 to 90 minutes 3 to 5 times per week. Which is a phenomenal amount of time. And, you have to view that really in the context of most of us, most of Americans, 95% of Americans struggle to exercise and the number one reason being lack of time. So, having a recommendation for such enormous amounts of exercise volume at low intensity and making that the top priority for the cardio component of your workout program seems for us slightly hard to understand. And, in fact, we actually take some issue with that. We think it's the wrong recommendation for the majority of people.

Yeah. I actually talked about this a little bit in my book, the first book I ever wrote on endurance. It was called "Beyond Training." And, I discussed how there's two different ways to increase mitochondrial biogenesis. One is via pathway called PGC-1 alpha, and that indeed involves 60 to 90-minute sessions. You could make a case from some newer research that you'll see a little bit at 30 to 45 minutes, but generally 60 to 90 minutes. And then, there's another way to increase mitochondrial biogenesis via different pathway. I believe off the top of my head, it's called— I wrote the book, so I should know— the AMPK pathway.

Yes, correct.

Am I right?

Am I remembering that correctly? Yes, absolutely.

Absolutely. Okay. So, the AMPK pathway, and that's kind of like the cheat code way to do things, albeit possibly higher risk of injury if your joints aren't ready for it even though I think a bicycle is a safe way to do it. And, there's also a little bit more of a pain cave aspect. You have to be willing to say, "Okay, I'm going to hack time, but it also means I'm going to go to the pain cave." But, that's a different way to increase mitochondrial biogenesis. So, kind of like two different ways to do it. But, I think that one of the things that's interesting, I'd love to hear your take on this, is you see people saying, well, a lot of the world's top endurance athletes, they actually do a lot of training in Zone 2 and even use a so-called polarized training approach, right? Like, 80% of the training in Zone 2 and then 20% at very, very high intensities and not a lot of time spent in the middle, in the so-called, you know, like the gray training zone or the waste of time training zone. What do you think about that concept that a lot of endurance athletes, like professional endurance athletes, use a high amount of Zone 2 training?

First, it's a proven concept for professional athletes, for really elite athletes that works really well. But, you have to ask why do they do that this way? And, there are some really, well, simple explanations. One of them is you simply cannot do more than 5 to 7 hours at really high intensity each week. So, if you train 25, 35 hours per week, which those professional athletes do, then their maximum at high intensity is 5 to 7 hours. And, once those 5 to 7 hours are scheduled in for the week, they fill up the rest of the available time with low-intensity work. And that just works out at an 80/20 approach. But so, in discussions with some of the top researchers in the field, it's very clear that just because that works for a professional person that trains 30, 35 hours per week, it's not at all clear that that should be an optimal approach for a regular person who trains maybe a couple of hours or even less per week. And, that's something that has literally trickled down from the leading Tour de France teams into the mainstream.

You took the words out of my mouth, by the way. I was going to say the trickle-down effect, similar to what you see in bodybuilding. People are like, "Well, to build muscle, you got to do biceps, triceps on Monday or chest on Tuesday, shoulders on Wednesday, maybe legs and core on Thursday. And, what people don't tell you is that most of the bodybuilders gaining significant amounts of muscle using that approach are spending 2+ hours per day in the gym to adequately exhaust each of those muscle groups, which works if you're a gym junkie and that's your happy place, but most people don't have that amount of time at their disposal. And, you were just saying you see this trickle-down effect from, say, Tour de France cycling where people see these massively successful endurance athletes doing that amount of Zone 2 training, but that doesn't mean that unless you're a professional athlete, that approach is correct for the general population. One thing I should note, Ulrich, I'd love to hear your thoughts on this is I think one of the reasons a lot of successful endurance athletes have so much time in the bike saddle or on their feet running, so to speak, at Zone 2 is because they're racing for very long periods of time. You have to condition like your butt callus to sit in the saddle for that long, or you have to condition your running shoes, your joints to be on your feet for that long a period of time. But again, how well does that translate to the general population?

Yeah, exactly. So, if you do these stage races where you, for 3 weeks, spend every day 6 hours in the saddle, yes, absolutely. If you didn't do that with a lot of preparation, having sat in a saddle before, you'd get very sore very quickly and probably drop out. So, yeah, it perfectly makes sense for them. But the recommendation for the general public then to say, oh, you should do 80% in zone 2, and 20% in zone 5. And by the way, zone 2 should be your priority is very questionable because if you look at some of the scientific data that we can try to use to assess zone 2 training, there's some not very flattering things in there. So, if you were so good to actually follow government guidelines and you did 150 minutes of cardio each week, but you did it only at zone 2, 40% of people will not see an increase in their VO2 max, in their cardiorespiratory fitness. And, that would be a lot of time wasted in effect. Ben Greenfield: Well, not wasted.

I mean, those people would actually see an increase in mitochondrial biogenesis, but maximum oxygen uptake, this VO2 max. Yes.

Correct.

You wouldn't see a significant rise in that compared to using something like interval training.

That's correct. Yes. And, I should be careful here because exercise is so positive. I don't want to talk down any type of exercise. And, if you love Zone 2 and you listen to great podcasts or watch your favorite shows or you're actually outdoors with people, you like and you manage to do a substantial amount of Zone 2 training, it will be for sure good for you and improve those aspects of metabolic health and boost your mitochondrial fitness, your mitochondrial density and efficiency. But if your aim is to improve your VO2 max, which is a— probably the most important health marker, and you don't do Zone 2 training right, so maybe too short sessions, maybe not getting the right intensity or so, there is a risk that you don't get as much return on the time you invest than you would with other exercises.

Yeah. And, I think one thing to note here is that anytime we're talking about exercise research, we have to consider trained versus untrained. Populations. I think you mentioned that 40% of people will likely not see a significant increase in cardiorespiratory fitness or VO2 max with that 150 minutes of Zone 2 per week. But, the problem with that research is it doesn't do a good job differentiating between sedentary totally untrained individuals and people who are already exercising a bit. I think that If people are just getting started, they're going to see increases. And I'm not just saying I think, I mean research has shown those people are going to see increases in their VO2 max even with Zone 2 training, but it's not going to be the most efficient way to maintain or continue to increase VO2 max once you're actually kind of in that realm of being a trained individual.

Yeah, that's correct. That's correct. But then on the other hand, So, actually, a lot of research is done on relatively sedentary individuals because that is the broad— that's the majority of the population. There's actually relatively few people who are in great shape and the research on athletes. So, you have basically some research done on athletes, really highly trained, a lot of research done on your average Joe, and I guess less on moderately well-trained, well-trained athletes, not pro athletes because it's just a smaller group and maybe of less scientific interest, I guess. Yeah.

I think the other thing to bear in mind here just so people don't get confused even though I briefly alluded to it earlier is there's another term in exercise physiology known as non-exercise activity thermogenesis, moving at low intensities throughout the day, gardening, cleaning the garage, cleaning the garage with the weighted vest on, taking the stairs instead of the elevator, walking around the airport while you're waiting for a flight to depart rather than sitting there waiting to sit some more, which completely annoys me. All of that would not be considered Zone 2 training unless you're cleaning the garage like you've just snorted cocaine. For the most part, I don't even classify any of the things that we're talking about when it comes to Zone 2 training as fitting into that category. Even me, like I mentioned earlier, walking on a treadmill while I'm talking to you, this is an easy conversational pace for me, obviously. It's not something I would be considering a way to increase mitochondrial biogenesis or VO2 max. But, I will say that moving at these low intensities during the day, I think the biggest value for that approach, that NEAT approach, is number 1, you control blood glucose very effectively, so you have lower amounts of glycemic variability. And, number 2 is you just burn some extra calories. So, I think moving at these low intensities during the day versus traditional Zone 2 training, those need to be chunked into different categories.

Absolutely. No, that's right. Those are different things and Zone 2 is substantially higher than I think sometimes is suggested. So, what sounds as low intensity and, "Oh, you can still talk," once you do that for 60 to 90 minutes, that is actually quite hard. So, you will definitely sweat. Well, your butt will hurt if you're on a bike. And, if you go for a run, your ankles and feet might hurt. So, Zone 2 training, if done as proposed, even though it's relatively low intensity becomes also quite strenuous.

Yeah. This is, I think, back to the calorie burning piece, what does confuse a lot of people because many of the people that we're talking to right now, Ulrich, they're not training for an Ironman per se or wanting to break a 3-hour marathon. Many of them are concerned about body composition, fat loss, et cetera. Now, you and I already both said this, that you see higher amounts of fat oxidation when training at Zone 2. And, as you get harder and harder with your training, you shift more and more towards carbohydrate utilization. Why is it that Zone 2 is so often hyped as something that is the most effective way to improve body composition or burn fat? And, what are your thoughts on that approach?

Why is it hyped that way? I don't actually know. I guess if you talk about it's the exercise where you burn most fat, to fuel the exercise, the immediate conclusion, well, it must be great for fat loss because it burns fat. But that's just not the case. It's as effective or as ineffective for fat loss as other forms of exercise because your— yes, your muscles during the exercise use mainly fat for fuel. But your body is quite capable of regulating different energy sources dynamically. And if you use more, say, sugar or carbs during a workout, that will just lead to increased fat oxidation after the exercise. And for weight loss, everybody should be entirely clear that nutrition and your overall energy balance is just much more important. And so, while exercise is clearly very helpful, the primary role is really to make you fitter and stronger rather than to help with weight reduction.

Ben Greenfield: The way I think about this is we have this zone 2 often called the fat-burning zone, which it is, I mean, compared to lactate threshold or very high intensities. But, that doesn't mean it's the best fat loss zone or weight loss zone, meaning that I could be in Zone 2 burning, let's say, 60% fat and burning maybe, I don't know, 200 calories per hour. And, if I exercise at higher intensities, let's say I'm burning 800 calories per hour but only 40% fat, I'm burning a much higher proportion of not only overall calories but overall fat. And then, as you just alluded to, I don't think you use this term per se, but my post-exercise oxygen consumption because I'm sucking wind afterwards and the body temperature is elevated dictates I continue to burn more calories longer after a high-intensity session or an interval session versus a Zone 2 session. That's kind of the way I think a lot of people incorrectly address this is they assume I'm in fat-burning zone, therefore this is the best way to lose fat.

Yeah, absolutely. No, for fat loss and weight loss per se, nutrition is just the 1, 2, and 3. It's so important. Where I think exercise can really help, yes, obviously it increases your calorie consumption and therefore helps with your energy balance, But in terms of improving your metabolic flexibility, improving your insulin sensitivity, your ability to access fat from storage, and therefore being able to also reduce calorie intake and, for example, have longer fasts without starving your brain of energy. I think that becomes a really important, like, adjunct and supportive role where exercise helps. Whereas if you're insulin resistant and metabolically inflexible, it just becomes much harder to reduce calorie intake, have periods of fasting, and your brain is— that's at least how I look at it— is screaming at you when you're not eating enough.

Yeah. Yeah. I should say I have a lot of clients who are on fat loss programs. I have many of them doing a little bit of fasted Zone 2 in the morning. And, I'm very straightforward with this. I tell them, "Look, this isn't going to improve your fitness or burn calories as quickly as you doing high-intensity interval training in the morning." But, what I found is I can get an overweight person to roll out of bed and do 20 to 30 minutes of Zone 2, brisk walk around the neighborhood, even walking the kids to school or taking the dog for a walk with a weighted vest, et cetera, consistently day after day because there's very little cognitive resistance to getting up and going for a brisk walk. Whereas, for a lot of people to get up and within an hour of waking hit it hard, lift weights, do high-intensity interval training, they're just that much less likely to do it. I think you even mentioned earlier the best kind of cardio is the kind that you're going to actually do. So, I still use Zone 2 training with a lot of my clients not because I'm convinced it's the superior method but because I can get people, especially for a morning session, to do that fasted. And then, I simply have them either have a quick fueling session and move into weight training or interval training or depending on their time limitations and schedule, do the weight training and interval training later on in the day. And, I'm a big believer in this idea of a two-a-day workout session anyways if you want to maximize your results is do something in the morning, do something in the later afternoon or the evening. And, in an ideal scenario, you're able to do something consistently every single day, preferably in a fasted state in my opinion, that will kind of spark fat burning early on. So, I don't think Zone 2 training is all bad, I just think that If your goals are efficiency and your time, you're kind of needing to hack time, that interval training, which we'll talk about here briefly, is going to be a little bit superior.

Yeah. I think the time efficiency is a really important consideration for many people, but you're absolutely right. The exercise that you can do consistently is obviously the exercise that's best for you. And then, if Zone 2 is something you can do, as you said, like in a fasted state, very much preferable to after like a meal or sugary drink or anything like that. If you want to do it also, it's much better to do it standalone or in the beginning of your session. And, if you do Zone Training, then, Ideally, you'd want to hit the right level of intensity and neither go too low or too high so that you really stay in that maximum fat oxidation window. And then, work that long enough, ideally, yeah, 60 minutes or more to get the benefit it offers.

Yeah. Although, I should note that I think most of the research on Zone 2 training, they're looking at cumulative volume per week. Not the actual session length, 60 to 90 minutes. I don't know how that has become so popular, but really it's looking at the cumulative session per week or the cumulative volume per week. But, it's difficult to get enough Zone 2 training volume in unless you actually are doing 60 to 90-minute sessions. One thing I'm curious about though that we haven't touched on yet with Zone 2, Ulrich, is women versus men because women have more Type 1 muscle fibers, They tend to oxidize more fat at submaximal intensities. They're not just men who smell nicer than men. Women have completely different physiologies. And, I'm just curious if there's anything you've found in the type of approach with a woman versus a man when it comes to zone 2 intensities.

You're absolutely right in pointing that out. And, I think What this leads to is, again, it shifts the priority of zone 2 further to the back. So, women have more slow-twitch fibers, have greater mitochondrial density at kind of on average than men, and have greater metabolic flexibility. So therefore, the the targets you're trying to achieve with zone 2 training are maybe slightly lower priority for women than for men. And then, if you consider other factors affecting like perimenopausal women where it does become fairly like harder to build and retain muscle mass and where long endurance exercise can actually have a catabolic effect. The priority and what you should do in your program, whether Zone 2 should be the base and the first is very, very questionable. So, we think some high-intensity interval training would be actually much more beneficial and more important. And then, there's a lot of other things around resistance and strength training, maybe also mobility and flexibility training. That would deserve a higher spot in your priority list of what you want to do. And, of course, the big zone 2 advocates say, well, you just have to do it all. You have to do zone 2, you have to do HIIT, you have to do resistance, mobility, flexibility, and so on, sleep enough. But, that's for most people not a very realistic approach. And so, if you have to prioritize, then, Zone 2 would just not be the first thing. Like, yes, if you have many, many hours per week, then it's a, it's a good addition. But if you're limited in what you can do, then there's other things that will give you just a much higher return for the time you invest.

Yeah, yeah, you mentioned it's not realistic. And again, we have to differentiate. Um, I think about it like this: a lot of people think that Zone 1 is Zone 2. I am a huge fan, like I mentioned earlier, of Zone 1 training spread all throughout the day. If you want to call it training, I just call it movement. And then, high-intensity interval training and weightlifting for the actual fitness sessions. So, don't consider the fact that Ulrich just said, "Well, Zone 2 training along with all the other stuff is not doable," if you're not defining Zone 2 training as actual ventilatory threshold 1. The part where it's getting difficult to talk, carry on a conversation, you're obviously exercising, et cetera. That is not as compatible with trying to also do weightlifting and interval training as doing Zone 1 and doing the interval training and resistance training. So, make sure that if you're listening, you have your definitions set up properly when it comes to what is Zone 2 and what is Zone 1. It's very similar to the idea that a lot of people I don't think even definitionally have interval training set up properly in their minds. I see a lot of people doing, let's say, a classic Tabata set, 20 seconds hard, 10 seconds easy, and their 20 seconds hard appear to be 75% intensity when they're actually supposed to be, pardon the expression, balls to the wall, lactic acid bleeding out your eyeballs and ear holes to actually get the benefits of that Tabata set. So, a lot of people think that Zone 2 is Zone 1, and a lot of people also don't push hard enough when it comes to the actual interval training. Now, the other thing I wanted to note, Ulrike, that you made a point about that I think if I understand this correctly is what you're saying is that women are naturally predisposed to do a really good job burning fat. They're probably going to feel even better than a man in terms of what they're naturally inclined towards. To be doing Zone 2 training, but that doesn't mean that's what they should be doing. It means that they're already really good at that stuff, so they should be focusing on the stuff that they're not as good at, primarily muscle maintenance, muscle building, interval training, and the type of stuff that's going to be more favorable for long-term health, longevity, fitness, et cetera, versus just sticking to what you're good at because you feel really good doing Zone 2 training and your body burns more fat than a man's, you should just do that form of training, right?

That's absolutely right.

Yeah. Okay. So, hopefully, people have not been bored to death so far about us taking a deep dive into Zone 2 training. We've mentioned interval training a few times. The last time that I interviewed you, you talked about reduced— sorry, it's R-E-H-I-T. What's it stand for again? Reduced exertion, high-intensity interval training?

High-intensity interval training. Yes, exactly. REHIT.

Okay. So, get into that. Is that something that you coined? Is it research-based? Like, where'd this idea come from?

We've spoken about HIIT and how HIIT is very effective and it's got a lot of research to show the effectiveness, but it's very hard to do. You set balls to the wall. So, the rate of perceived exertion is very high and then people find it actually hard to perform and stick to. And so, that is an issue with with HIIT. And researchers then looked for the minimum effective dose of HIIT and found that 2 20-second sprints, so a short workout with just very casual warm-up—

not 220 seconds, 20-second sprints.

Exactly. So, 40 seconds total at maximum intensity creates a very robust, very strong training stimulus through that AMPK pathway, which actually also releases and activates PGC-1 alpha, your body's master regulator of mitochondrial biogenesis, and creates a very robust stimulation to also develop more and more efficient mitochondria and a number of other adaptations that, yeah, very substantially improve your VO2 max, your cardiorespiratory fitness with much, much less time. And because two 20-second sprints are so short, even though you go to maximum intensity and you go all out, it becomes actually a fairly doable workout, one that you can do in a fasted state, that you can do in the morning. And yes, 2 times 20 seconds, the 20 seconds feel— they get quite long, but you always see the light at the end of the tunnel. And especially if time is a— as it is for most people— is a big barrier, That is just a solution that helps many, many people or can help many people to achieve their cardio fitness goals.

Ben Greenfield: Okay. So, walk me through this. Let's use the 2:20 second even though I know there's a few other flavors of HIIT like 3:10 seconds, et cetera. Walk me through what that actually looks like. How long do you need to warm up to actually be ready? To go hard for those 20 seconds? How long do you recover in between those two 20-second efforts? And, how hard does it feel? How hard are you working? I just asked you a three-part question, I know, but fill me in.

So, the warm-up is anything from 20 seconds to 2 minutes. Whenever you're ready, you can initiate this all-out sprint. And this is because it's on a bike that has multiple safety features and is in itself a very, very self-safe exercise modality. So, you can literally start the exercise and go to highest intensity, to maximum intensity after as little as 20 seconds. But, most people spend a minute up to 2 minutes warming up.

I'm a long warm-up guy, by the way. If I do the CAROL, I know you have a setting on there to half the warm-up time. I always need the full warm-up. I've always been that way. For me to have a good workout, I got to have a good warm-up.

That's all right. And, of course, personal preferences is important, but if you're really pressed for time, you could cut that short, the warm-up, and you will probably perform a little bit better in your sprint if you warmed up a little bit longer. But in terms of getting a good training stimulus, you can achieve an amazing amount with very, very little warm-up. Then in the sprint, you want to hit your peak power, and this works by accelerating to really high cadence and then dialing in the optimal resistance at the optimal time, and then just pushing for as hard as you can. And that's what we've optimized our bike for. It does that automatically. You don't have to turn any buttons. We use AI to find the, the perfect resistance for you every single time. And we can do that because we have a ton of data. So we have now millions of rides from tens of thousands of riders. And that allows us to run really sophisticated models to create a perfect workout for you. So, that's your first 20-second sprint. Then, you have up to 3-minute recovery. That allows some of your really fast energy stores to replenish. So, your phosphocreatine system.

Yeah, creatine phosphate, some ATP.

Exactly. It needs like up to 4 minutes to recharge. 3 minutes, you'll be in a good position to do a second sprint. Again, you go all out for 20 seconds. The first 10 seconds feel fine. Then, the next 5 seconds feel a little bit harder. The last 5 seconds, you have to bite and you need to fight through, but you see the light at the end of the tunnel and it's so quick that it's practically over.

I literally just count to 10 over and over again very quickly.

Yes. And then, you have a 3-minute cooldown. That is the time you need for your heart rate to come down a little bit and most importantly for your blood pressure to normalize. So, after those all-out sprints, you create well, you mobilize a lot of glycogen in your muscles, it gets broken down into some metabolic waste products, lactate, and actually water from your bloodstream shoots into your muscles, feels all quite pumped, and your blood pressure drops a little bit. And then, kind of light pedaling after the sprint helps you move the water out of the muscles again and for your blood pressure to normalize and you're finished.

Yeah, yeah. Clear some lactic acid too.

Yeah, exactly. And so, that means you can perform this workout. If you're really in a rush, you can do it in as little as 5 minutes and it takes up to, if you do each period to the full length, up to 8 minutes 40 seconds.

You've actually, I think, seen me at Health Expos where you've had a booth there and I'll drop in because I'm super busy at these things. I'm like, "All right." I got 5, 10 minutes, let's get the workout in. And, it does suck and you're feeling it during, but then it's pretty good after. As a matter of fact, if people are watching the video version, and again, go to BenGreenfieldLife.com/Zone2Myth for the show notes and I'll put the video there as well. The bicycle you see right behind me when I'm recording podcasts, that's the exact bicycle that we're talking about. Ulrich, you mentioned two things. You mentioned safety features, built-in safety features, and you mentioned the ability to be able to use AI to set your actual resistance. First, what safety features are you talking about?

So, first, an exercise bike is a very, very safe mode to train in itself. So, if you tried the same on a treadmill, I think it would be outright dangerous.

Yeah, you'd be on a viral TikTok video potentially of the guy flying off.

Absolutely. Yes, absolutely that. And that just doesn't happen that much on an exercise bike. And then, we've got a number of features to make it as safe as possible. So, we have— it's called a freewheel clutch. So, you can stop pedaling whenever you want. It's not that the flywheel is driving your legs. So, if during the sprints, you get out of the rhythm, out of sync, your feet won't get yanked around. We track your heart rate, of course, during the exercise. And if, for example, your heart rate during the warm-up is abnormally high, we would give you warnings. If your heart rate doesn't recover fast enough after the sprints, the bike would intervene and give you a warning or even aboard the ride, if that depends a little bit on the settings. So in gyms or some of our bikes are in offices, it's more restrictive. If you use it at home, you have choice over that. You can have that less restrictive. But there's these safety features built in to make it as safe as possible. And I mean, we've got now We've launched our bikes in 2018 and we've got now millions of rides. And, I'm very happy to say that we've not had a single severe adverse incident.

That's great. Although, you need the "What kind of pre-workout drink did I crank before I got on here" settings so that if someone's heart rate is elevated, the CAROL bike knows that they just doubled up on the espresso that morning.

Yeah. And, if it's your bike at home, you can control the settings. So, we're not nannying or mothering our users. You have choices there.

I know. I personally like to recover at a slightly higher wattage than what you had built into the bike. So, I just went in and adjust the wattage. So, I like to feel— when I'm recovering, I like to feel more resistance versus it being completely easy. So, I add a little bit of resistance for my recoveries. But, yeah, that's all modifiable. What about the AI piece? Obviously, that word is thrown around all the time these days. I think often overused and abused. So, when you say AI, what do you mean?

The technical deep field is reinforcement learning. So, it's the same way, say, Google trained the Deep Blue computers to beat the best Go players. So, it's a form of neural networks and we can run those across the data of, yeah, now millions of rides from tens of thousands of riders and identify what a perfect ride looks like. And so, if you do those all-out 3-hit sprints, you hit your peak power after a few seconds, like very early on, 3, 4 seconds, and then you fatigue throughout the sprint. And there is such thing as an optimal shape of that curve. So, if it's too hard, you will fatigue too quickly and it becomes unpleasant and you grind to a halt. If it's too soft and if the resistance is too soft, you'll just push through and you will never reach your peak because if it is your peak, you will only be able to hold it for a second. So, we know what that optimal shape of the curve looks like by running those models. And, we don't just know it generically but we know it for somebody who looks exactly like you. And therefore, we can tailor workouts for somebody who's 75 and maybe hasn't done a lot of exercise and just wants to start now. Or somebody who's like a 30-year-old CrossFit athlete at the peak of his fitness, we have the ability then to create a workout that's just optimal for that individual and also that keeps adjusting as you get fitter and stronger. Or if you— so, I had a bit of a skiing accident earlier this year, had to take a break from training. I was actually very grateful that my carrow bike dialed back the resistance when I got back on it.

Right. It could detect that your peak power was dropping more rapidly during the sprint and adjusted that for future sprints. I think of this very similar to a lot of these variable resistance training weight training machines now like the Tonal, the ARX. They will detect if the amount of force or power in some cases that you're producing drops off and adjusts the weight as you go through the set to allow you to maintain good form and/or to maintain proper time under tension or exhaustion by the end of the set. So, similar concept but for cardio instead.

Correct. Yes, that's exactly it.

Okay. Now, bringing this full circle, obviously, we've established that REHIT is good. We've looked at some of the pros and the cons of Zone 2 training. But, have you actually matched up the two in a comparison or in a trial to see— we could theorize all day long, but are there any studies or research that actually look at re-hit at very low amounts of time per week, Zone 2 at higher amounts of time per week, which wins out?

Yeah. So, that has been done. We have randomized controlled trials. Showing the performance of REHIT with very limited time investment. So, 3 times a week, 8 minutes versus 150 minutes of— and that was moderate-intensity continuous training, which that is what most people would understand as Zone 2. And we had, yeah, very marked differences in the results. So, VO2 max in a REHAB group that spent less than half an hour a week went up by over 12% in 8 weeks. And, in the moderate intensity in the zone 2 group, we only saw improvements of 6.9%. So, we did see those improvements. As you said, if you're setting free, you get an initial benefit, But also, the rate of non-responders was higher. So, yes, the average in that group got an improvement, 6.9% is meaningful, is great, but much less and with much more time.

You mean there were fewer non-responders to the REHIT training versus the Zone 2 training?

Yes, yes, absolutely. The, um, the research we've seen, and this is the, the leading research in, uh, kind of the most credible research in that field, suggests if you do moderate intensity continuous training, Zone 2, 150 minutes per week, that 40% of, um, sedentary people would not see an increase in VO2 max. And this has been reflected in that study too.

Untrained people would see an improvement, right, with Zone 2 or with REHIT, but trained people would see more of an improvement with REHIT. Is that what you're saying?

There's less people who don't see an improvement. So, you see a greater improvement. So, the average is higher. Yeah, the average is almost twice as high and the rate of non-responding is lower. So, you'd have a much lower share of people who don't respond compared to Zone 2 training where you have a higher share of people who don't respond.

That makes sense. But although back to what we said earlier, if you were to break that out between trained versus untrained individuals, untrained individuals, you'd probably see a pretty good response rate to either modality, I would imagine.

The other thing is the whole concept of non-responders has shifted slightly in the exercise research world. Ben Greenfield. So, a few years ago, it was the understanding that, yes, there is just a certain population, a certain share of the population that doesn't respond terribly much well to cardio training and that you have something like 10 to 20%, they can do almost what they want but they don't respond. That has shifted slightly. And it's from non-responder more to slow responder. So, they need more time. And if they stick to it and continue to do it, then eventually they will also, or the majority of those will also see a response. So, that has been— yeah, the mainstream opinion has really changed from there are non-responders to, well, they're mainly slow responders.

Yeah. And then, that improvement in VO2 max, I mean, I know I've actually personally had people push back and be like, "Well, the CAROL bike's only been around since 2018. How'd they actually study the claims that it adds 2 years of healthy life?" It's actually healthy life expectancy because what you guys did, you said, "Okay. Well, we see a 10% improvement in VO2 max. We know that that approximately correlates in exercise science research to 2 additional years of healthy life expectancy." Therefore, if this is what the CAROL is doing this quickly, we can approximate that you're increasing lifespan/healthspan by approximately 2 years with that increase.

That's right. So, the research on VO2 max and what its relative importance is for longevity and life expectancy is ultra robust. So, there are studies that followed tens of thousands, even hundreds of thousands of participants over long periods of times and where those endpoints were established and where we can see that VO2 max is the most important predictor for life expectancy, the strongest correlate for life expectancy. And therefore, if you have an improvement in VO2 max, it's quite reasonable to assume that this will have an effect of what you can expect in terms of your lifespan.

Yeah, higher than say lactic acid tolerance. I wouldn't say higher than mitochondrial health, but VO2 max can be ascertained within 20 minutes in an exercise physiology lab. Assessing mitochondrial health is still a little bit loosey-goosey. I recently interviewed the folks from testing company called MeScreen. They've got a decent test for mitochondrial efficiency and mitochondrial health, but I think you can say, and I'm not sure if you agree with this, Ulrich, that if you're seeing VO2 max improvements, you are very likely seeing increases in mitochondrial biogenesis and efficiency without necessarily having to do a fancy blood test to assess mitochondrial efficiency or health.

Absolutely. And, If you think about the two sides of VO2 max, one of it is oxygen delivery. So, that's your lung capacity, your peak flow, and your stroke volume of your heart, your plasma volume, blood capillarization. So, everything that brings oxygen to your muscles, that's one part of VO2 max. But, the other part of VO2 max is around oxygen utilization. So, how much oxygen can you burn and how much oxygen can you metabolize? And, that is exactly mitochondrial density and mitochondrial efficiency. And so, it's a very, very large part in terms of what drives VO2 max. And so, yeah, you're absolutely right. And so, they're almost Yeah, two things of the same bigger total and mitochondrial health and mitochondrial function is critical for life expectancy. I'm absolutely sure about that. Yeah.

Lung volume in addition to blood parameters and oxygen utilization at the cellular level being kind of like the third component of VO2 max, just basically like how big are your lungs, how much volume can you move. So, yeah, if you address all three of those, then you're going to address VO2 max. Ulrik, I know we're getting close on time. Pretty straightforward model here. I know you guys are direct-to-consumer. So, someone can order a Carrow bike. And, when they order it, the bicycles are not the lightest bicycles out there. As far as delivery and setup, any additional features that you guys have baked in as far as and ease of setup and use for someone who wants this on, let's say, a second-floor apartment building or something?

We're quite happy if you order one. The standard would be in a box and you assemble it yourself, and it's actually quite easy. Or we're very happy to do assembled service to a room of choice. So, that is not the thing that's very difficult about the bike. No, not at all. We can do that.

Okay, cool. Regular outlet, no special 220-volt, just plugs into a standard outlet and you're good to go, right?

Absolutely. Yeah. And maybe just also to, in case there's any question, the bike can do a lot more than just Rehit. In fact, even though we're not the biggest fans of Zone 2 and we think that Zone 2 has been overhyped, maybe a big claim now, but I would argue that our bike is probably the best tool you can use If you want to do Zone 2 perfectly at the moment, so we have the zone-based training where the bike will keep you automatically at kind of in the Zone 2 and you can just go and, yes, you can watch a movie and listen to your favorite podcasts and so on.

Heart rate monitor ties to the power and resistance on the bike based on, I don't know if you guys are using 220 minus age percentage of that or 180. Minus age for approximating zone 2. But, basically, I can set that up, start riding, and the bike will automatically keep me at that heart rate without me having to worry about pedaling easier or pedaling harder.

Exactly that. And, the bike will also allow you to do a number of different fitness tests first so that you actually know what your maximum heart rate is, you know what your VO2 max, your FTP is. And then, there is— this is not yet available, but it will be available very shortly because Zone 2 is a big thing, a feature to lead you very precisely to exactly your Zone 2 threshold based on heart rate variability. So, this is quite cool new research that has come out that you can use not just heart rate or heart rate variability, but you can use the complexity of your heart rate variability. So, this is another higher order. To quite reliably find where your lactate threshold, where your VT1 is.

Oh, that's interesting. You mean you're using heart rate variability as a correlate to predict lactate threshold?

The complexity of the heart rate variability. This is one step further. So, you have heart rate, you have heart rate variability, and then you have the complexity of heart rate variability. And, you can analyze that, it's, it's called fractal analysis, and you have a, um, a metric, uh, detrended fractal analysis, um, that you can use to very reliably find the point, um, where your lactate threshold is. And if you, if you think about other ways to measure that, you would need either a prick test and, and draw some blood That's not very practical. You can use gas analysis when you have a device like Pinoy or Cosmed or so to look at your respiratory exchange rate. But this is all not very practical for everyday use. And using heart rate variability to find that threshold is actually very possible if you have the right analytics built in and have the right heart rate sensor. Heart rate monitor that tracks that, that's capable of tracking that and comes as a standard with each of our bikes. And therefore, yeah, I'm quite convinced that Carobike, as ironic as it might seem, is actually the most powerful tool to do Zone 2 training right if that's what you're into.

Well, I'm not a fan of spending 90 minutes on a bike, but that's super interesting about the test for VO2. Functional threshold power and lactic analysis. I assume— I have no Wi-Fi at my home, so whenever I want to kind of update my Carole, I just tether it to my phone. I'm assuming that that would just be something that will roll out as a software update once it's ready?

Yes, absolutely. So, the analytics are largely completed. And so, we're looking probably the next 2 or 3 months, that will be a software update to make that available to all our members.

That's incredible. Alright, you got to remember to email me if— Yeah, absolutely. I want to be the first to try. Well, cool. This is fantastic. I know we're about out of time, but again, if folks go to BenGreenfieldLife.com/Zone2Myth, I know we have some kind of a discount code or something for Carol. I'll make sure I put it in there. I'll link to my previous episode with Ulrike as well and also anything else related that I've podcasted about when it comes to cardio in Zone 2. So, Ulrich, thank you so much, man. This has been incredible and yet another fascinating discussion. Thank you. Alright, folks. Well, if you don't want to build up a butt callus on the bike, you've now learned how to avoid doing that. And, until next time, I'm Ben Greenfield along with Ulrich Demphlé from CAROLBIKE signing out. Again, the URL is BenGreenfieldLife.com. Bit.ly/Zone2Myth.

To discover even more tips, tricks, hacks, and content to become the most complete boundless version of you, visit BenGreenfieldLife.com. In compliance with the FTC guidelines, please assume the following about links and posts on this site. Most of the links going to products are often affiliate links of which I receive a small commission from sales of certain items, but the price is the same for you, and sometimes I even get to share a unique and somewhat significant discount with you. In some cases, I might also be an investor in a company I mention. I'm the founder, for example, of Kion LLC, the makers of Kion-branded supplements and products, which I talk about quite a bit. Regardless of the relationship, if I post or talk about an affiliate link to a product, it is indeed something I personally use, support, and with full authenticity and transparency recommend in good conscience. I personally vet each and every product that I talk about. My first priority is providing valuable information and resources to you that help you positively optimize your mind, body, and spirit, and I'll only ever link to products or resources, affiliate or otherwise, that fit within this purpose. So, there's your fancy legal disclaimer.