Hot Take VS. Stone Cold Science: The Zone 2 vs HIIT debate, Naked and Exposed.

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Transcript

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The fitness sector on the internet has opinions about zone two cardio. Loud ones, emboldened ones. But this episode, it has something better, data. Hey, rebels. Welcome to Blasphemous Nutrition. Consider this podcast your pantry full of clarity, perspective, and the nuance needed to counter the superficial health advice so freely given on the internet. I'm Amy, the unapologetically candid host of Blasphemous Nutrition and a double-degreed nutritionist with 20 years experience. I'm here to share a more nuanced take on living and eating well to sustain and recover your health. If you've found most health advice to be so generic as to be meaningless or so extreme that it's unrealistic, and you don't mind the occasional F-bomb, you've come to the right place. From dissecting the latest nutrition trends to breaking down published research and sharing my own clinical experiences, I'm on a mission to foster clarity amidst all the confusion and empower you to have the health you need to live a life you love. Now, let's get started Welcome back to Blossomy's Nutrition. I am Aimee, licensed nutritionist and certified personal trainer. A former boyfriend once classified me as a zone two cardiovascular event, and I have the screenshots to prove it. Recently, a certified personal trainer posted something here on Substack that got a lot of attention. She called it a hot take, bold into the text and everything, saying that, "Zone 2 cardio is the most under-prescribed intervention in women's health." Not high intensity interval training, not even HRT. Slow, conversational paced cardio, 45 minutes, three times a week. And the comments lit up. They didn't go in and disagree, but they came in with resounding approval. Finally, somebody said the popular thing yet again, because it's been two whole days. Zone 2 is the answer. We don't have to work as hard, and yet we get all the benefit. Hard exercise is bad for us. Hooray. Except it isn't. Not entirely. And the framing of "this, not that, pick a side", is precisely the problem. Two people in the comments wrote something that I keep seeing elsewhere in the Zone 2 conversation. One said, "I swear my body does not believe in Zone 2. I'm either walking at a Zone 1 pace or I'm jogging, and it's always at Zone 3 even when I try and do the slowest shuffle possible. What's the secret for someone who doesn't have a gym membership and does all of her cardio out in the streets?" Another person said, "I keep hearing this, but it's so hard for me to work out at this speed. I'll shoot from one to three without any amount of effort. Any suggestions?" So my response to both of them was this, "Just ignore the drama. Stop focusing on the minute differences. Move your body in a way that feels good and sustainable because honestly, nobody's nona lived to 102 worrying about Zone 2 cardio." But I do wanna address the science because the full picture is more interesting, more useful, and considerably less tidy than a hot take permits. This is the direct response to a post that started scads of comments and to the broader habit in the fitness and nutrition industry of neglecting nuance in favor of a fistfight between two things that were never actually competing in the first place. Before I dive into the science, though, I do wanna make sure that we are speaking the same language because these two training modes are actually quite different and specifically defined. This nuance often itself gets lost in reels and hot takes, but the difference matters enormously, especially once hormones start shifting. Zone 2 is aerobic exercise performed at roughly 60 to 70% of your max heart rate. You're breathing harder than normal, but you can still hold a conversation. Breathing is somewhat labored, but you can totally speak with your bestie. For most people over 40, that means brisk walking, light cycling, easy swimming, maybe a slow jog. Not a stroll with your geriatric labradoodle, and not a race. These sessions typically last 30 to 60 minutes, and the research consensus, the number that you hear from practically everybody, is a minimum of 150 minutes a week, ideally spread out across four or five sessions, rather than two long slogs. Now, two people in the comments of that post said something that I want to address directly before I go any further. They couldn't find zone 2, and it was not because they were not trying. For them, walking felt too easy, but jogging pushed them into too hard an effort. There was no middle ground for their cardiovascular system, just this gap, right, where zone 2 should have been. So know that that gap is real, and it isn't that you're doing something wrong or your body's whack. It usually means that your aerobic base is still developing, and your cardiovascular system has not yet built the efficiency to sustain zone 2 effort comfortably, so it ends up pushing itself into zone 3. That zone 2 feels elusive because the fitness that makes it accessible isn't fully developed, right? If you're training outdoors without equipment, the most practical tool that you will have is actually your breath. If you can speak in full sentences without pausing, you're likely in zone 1 or maybe low zone 2. If you can speak, but only in short phrases and, you know, it doesn't feel like a casual conversation anymore, you're probably where you want to be or just past it. If speech feels genuinely difficult, you've definitely moved past zone 2. So use that as your gauge rather than pace or even perceived effort alone, as both of those can be unreliable until that zone becomes familiar. Even heart rate monitors can deceive us here because, one, if it's just a wrist-based monitor- It may not be accurately reflecting your actual heart rate, and your genuine max heart rate may not fit that standard equation of two twenty minus your age. That equation becomes inaccurate for many people once they begin training. As an endurance runner, I can tell you my calculated maximum heart rate is what I can sustain racing a ten K, which means it's not my true maximum. My true maximum heart rate is what would be calculated for somebody ten years younger than I actually am. So know that that two twenty minus your age standard that we've been given isn't necessarily accurate for you based upon your existing state of condition. Anyway, if you cannot truly find a zone two pace on foot that lands in that sweet spot, if walking is too easy, running is too hard, try cycling, swimming, or even an incline walk on varied terrain. Go to the hills, right? This gives you more graduation to work with. Zone two exists, it's just that the access point for you may not be jogging. Now, high-intensity interval training is something else entirely. In research, it is defined as up to four rounds of up to four minutes duration at eighty-five to ninety percent of your maximum heart rate, followed by three minutes of active recovery around fifty percent of your max heart rate. So your total workout time is gonna be under half an hour, including your warm-up and your cool down. That's it. You are not supposed to be able to hold a conversation during work intervals. At a high-intensity interval, you should be hating your life and desperate for that interval to end If you're doing it right. If you can speak, you're not doing HIIT workouts. You're doing something in the middle, and we'll come back to what that middle actually means, in a moment. And then there is a third category that has been partitioned out of HIIT training that is worth speaking to, and that is SIT or sprint interval training. So these are even shorter maximal efforts. Think twenty to thirty seconds all out with an even longer recovery period. There are some unique properties in sprint training that are specific for women that I'll come back to further on in this episode. So the question that everyone wants answered is what should I be doing? And the answer, infuriatingly but correctly, is that this depends entirely upon your goals and what stage of life you are in. Firstly, there's something you need to know about the research underpinning most everything you have ever been told about exercise. The majority of it was not conducted on women in perimenopause or post-menopausal women or men navigating testosterone decline. Most of it wasn't even done on women. Most of the exercise research we have has been from studies conducted on college students, predominantly young men who participated in exchange for a few extra dollars or not infrequently free pizza. This is not an exaggeration. Exercise physiology built its foundational literature on nineteen to twenty-two-year-old men with unencumbered hormonal profiles, no children, questionable sleep habits, and a dietary situation that could best be described as chaotic but calorically adequate. That is your human lab rat, and that is whom most of the baseline recommendations were designed around, All of this means that especially if you are a man in your 20s or 30s, pre-andropause, your hormones are functioning more or less normally, you are actually the demographic that exercise research was built around. Congratulations. You are the intended user for all baseline exercise prescription Now more recently, more recently being within the last 10 years, there has been more and more research done equally on men and women. So clinical trials done where the college kids were, you know, half female and half male. So if you are a premenopausal woman who again have a normally functioning hormone profile The current existing generic exercise standards will more or less work pretty well for you also. But even if you are under 40, research did not take into account your life, because the 22-year-old in the lab was not waking up at 5:00 AM with a toddler, skipping breakfast to make daycare drop-off, running on six interrupted hours of sleep, eating at her desk, and then attempting a high-intensity interval training session at 6:00 on a body that had been running on cortisol since approximately 7:00 AM. The research protocol assumed a recovered, adequately fed, reasonably rested human. And if this is not you, and for a significant portion of people in their 20s and 30s it is not, then the prescription needs to flex, as they say. For folks in their 20s and 30s, the general principle is this: move consistently in ways that feel good and sustainable. Treat your own body's signals as data The generic recommendations for physical activity do apply to you, but do not lock yourself into any rigid protocol at the expense of signals your body is giving you. These signals are not indicative of weakness and do not tell yourself that they are the basis for your excuses. Treat them as information. If you're exhausted going into a workout, that exhaustion is telling you something. If you skip a session or even a week of workouts because life has genuinely overwhelmed your recovery capacity, you don't lose meaningful fitness. Strava is lying to you. When your Garmin watch tells you you are deconditioned because you skipped 10 days of workouts, tell Garmin to go to hell. The research on detraining is actually very reassuring on this point. Fitness is built over months, and it is not undone by days or even a couple of weeks of reduced training. What does erode fitness is the cycle of overtraining on a depleted system, getting injured or burned out, stopping completely, and then starting over months later. And I see that pattern all the time, those on-again, off-again exercise warriors that create some big lofty goal like a mud run or a half marathon or a CrossFit competition, and they beat their body to pieces to try and accomplish that one goal and then don't do anything for a year afterwards, and then get frustrated and start another goal again, right? Focus on what you can do consistently over what makes you look and feel like a badass. Remember, your ego is rarely your best workout buddy. Ultimately, your sleep, your stress load, your nutritional baseline, and your life are all variables that no blanket recommendation will be able to take into account. You have to listen to your body. It is running the most sophisticated monitoring system available, and it is trying to communicate with you constantly. You just have to listen. The goal at this stage is build the habit and the base. Everything else past that is refinement to a specific goal in mind, and that needs more nuance than I'm gonna cover here today. Now, when we get to perimenopause, roughly age 40 to 50 or so, but this can start as early as 35 for some women, we have this window of perimenopause And perimenopause is not menopause. The distinction is very, very important. Research is now only fully acknowledging the difference between perimenopause and menopause, which is stupid, but, um, I'm not gonna go down that rant 'cause it's not worth my time, nor yours. Perimenopause refers to the transition years when estrogen and progesterone begin their somewhat erratic descent into what we then call menopause. Cycles become irregular. Women tend to get more moody. It sort of feels like PMS is showing up whenever the hell it wants to, and leaves whenever the hell it wants to. Hot flashes can show up. Our bodies start to change in ways mom never told us about, right? Estrogen does not disappear during this phase, rather it fluctuates wildly. Sometimes it'll even spike up higher than it ever did when you were in your 20s before dropping suddenly, and then repeat that pattern 64 days later for no discernible reason. This hormonal volatility, rather than the eventual low plateau that we have post-menopause, is what makes perimenopause its own glorious category. Research published late in 2024 from the University of British Columbia specifically studied high intensity interval training's effect on perimenopausal women, and it found that it may be beneficial for cognitive function as well as myelin content. And myelin is that protective sheath around the nerve fibers that can begin degrading during the transition from premenopause to post-menopause. Myelin loss is one of the less discussed cognitive risks of perimenopause, and the fact that high intensity interval training appears to help preserve it is pretty significant High intensity interval training or HIIT increases something called BDNF or brain derived neurotropic factor, and that often gets labeled as a kind of growth hormone for your brain. It supports your ability to learn, to adapt, and regulate mood. It's one of the reasons why people often feel sharper and more even-tempered when they're training consistently. And critically, research suggests that these effects were more pronounced in perimenopausal women than post-menopausal women. So if you're perimenopausal, you are still in a window where the brain benefits of HIIT workouts are at their most potent, and so this is the time to use it and use it intentionally. So what does it mean to use it intentionally in practice? Well, most research shows that two to three weekly sessions of up to four minute intervals may be the most effective for improving VO2 max, which is your body's maximum capacity to use oxygen during exercise, and the single strongest predictor of longevity in research. HIIT also improves cardiovascular fitness with no real benefit being seen if you increase the frequency beyond three sessions a week, so more is not better. For perimenopausal women, the current evidence points to two dedicated HIIT sessions a week as the sweet spot for us. It's enough to generate the neurological and cardiovascular stimulus that th- we're looking for without tipping over into cortisol overload, which can become a real risk as estrogen levels fluctuate more erratically, for lack of a better word. Now, what those sessions look like matter also. You know that burning sensation in your legs when you're doing really intense cardio, you know that feeling that makes you want to stop? That is due to lactate. And for a long time, lactate had a bad reputation as a waste product. I remember learning that in undergraduate school a very, very long time ago. Lactate was considered a waste product, something that your body produces when it's working really hard. But since then, science has evolved and now recognizes that lactate is actually a fuel source for the body, and it's one that your brain can use directly. Research is showing that doing genuinely high-intensity work for at least three days a week consistently over six weeks increases the amount of lactate that your brain can produce and use for energy. And this matters because the brain's ability to fuel itself efficiently is one of the biggest factors in whether or not Alzheimer's disease takes hold. It's not just that you're getting fitter; you're actually keeping your brain fed in a way that may slow or prevent the progression to dementia. You're providing your brain with an alternative energy source and effectively offering it a buffet of choices to gather energy from. Now, there's a critical caveat for this phase. If you are training at high intensity all the time, then you train your body to burn immediate energy stores and keep your body in a hyper-intense state that doesn't permit it to be able to effectively convert fat into energy. And I see this clinically when folks come in and they're, say, endurance runners who are consistently running at higher intensities, or they are very addicted to their HIIT workouts at the gym, and they're struggling with weight loss. They cannot figure out why they are not losing weight when they're exercising intensely five or six days a week. It is because they have trained their body to effectively burn glycogen and glucose and their cortisol levels are consistently elevated over too long a period of time. And when cortisol is consistently elevated, it can inhibit estrogen as well as progesterone production and throw everything off of balance. It also keeps our glucose levels elevated post-exercise for longer, and this also inhibits the fat-burning potential. Perimenopause is already a state of hormonal instability, so to throw unmanaged cortisol load on top of that is like pouring accelerant onto a fire that's already threatening to go out of control In real life, this looks like someone who goes out for a workout and then they come home and they find themselves wiped out instead of energized, too exhausted to make a meal or do anything else for the rest of the day. They'll nap and lay about as though they raced a marathon when in reality they went to a 45-minute class at the gym. This is someone who is working at too high an intensity for what their body can handle. Okay? Now, the perimenopause recommendation, generally speaking, looks like this. One or two HIIT sessions a week. This is genuinely high intensity, where you are breathless, you can't speak during those high intensity intervals. You are feeling that burn. You are hating life. That is a HIIT workout. Okay? You push yourself that hard and then you recover, and then you push yourself that hard again. It is limited to less than 30 minutes a workout session. If you can go beyond 30 minutes, it's no longer a HIIT workout. You're working in an endurance zone at that point in time. High-intensity interval is a high intensity, and therefore it limits your capacity to go for long periods of time. You also wanna be doing three to four zone two sessions, aiming for about 30 to 45 minutes, building up towards that 150 minutes a week that is recommended. Now, note that zone two is not filler. It does its own valuable work. As with the younger folks, listen to your body's cues and let it guide you. If you are perimenopausal and you're struggling with insomnia, maybe pull back on the HIIT sessions and do it once a week or once every two weeks instead of twice a week until your sleep gets back on track and you're more rested. This is going to make it more sustainable, it's going to reduce the wear and tear on your body, and it will also reduce the risk of throwing your hormones out of whack even more and making things worse. As estrogen level declines through the years, our insulin sensitivity drops, and this makes zone two a very protective activity from a heart health and metabolic standpoint. Zone two helps keep us metabolically flexible, able to burn fat as fuel as well as glucose for fuel. Zone two improves mitochondrial function, health, and density, and it maximizes the capacity for our mitochondria to produce energy. In high school, we're taught mitochondria are the little energy factors of the cell, but here they become the cellular infrastructure that everything else depends on. You are building your energy grid when you are working in zone two. One more thing that I tell my perimenopausal clients, or honestly anyone who's carrying a great deal of stress, After a high-intensity interval session or a sprint training session, build in a 20 or 30-minute window of a slow walk for recovery before you transition out of your workout to life. This is not like a cool-down lap around the block. I want a genuine unhurried walk like you're taking a stroll with that geriatric Labradoodle. What I see happen far too often is someone finishing a hard session, rushing to the shower, and then rushing on to work, either skimping on breakfast by grabbing a slice of toast with peanut butter or a muffin and a latte or maybe nothing at all, And then midday you wonder why you're wired and exhausted. That pattern, when repeated consistently, is where women get into real trouble. The workout itself is not a problem so much as the absence of a landing. The cortisol post-workout needs somewhere to go, and bringing in a slow walk as an intentional cool down and a very specific part of that overall workout gives the cortisol load a proper exit for your body to kind of discharge it, right? To process it and eliminate it so that your central nervous system remains more balanced over time So let's talk post-menopause. Menopause itself is one day. It's the 12-month anniversary of your last menstrual cycle. And then at 12 months and one day, you're post-menopausal, and there you remain for the rest of your life. Estrogen is not fluctuating anymore. It's kind of settled into this chronic low level, and that new normal changes everything about how your body responds to exercise stress. The cortisol picture itself changes significantly. Zone 2's biggest benefit is not actually burning fat for fuel. It's reducing cortisol levels by moving at a lower level, reducing our circulating blood sugar and creating a base to build greater levels of fitness. Post-menopause, with estrogen no longer buffering the stress response, the cortisol argument for zone two does become more compelling. High-intensity workouts, especially when done frequently, can keep cortisol levels elevated. And for post-menopausal women who are under chronic stress, zone two's lower cortisol demand can be really, really supportive for overall health and wellness. And there's also something to be said for changes in recovery capacity, as well as joint tissue quality that warrants a more conservative approach to high-intensity interval training than we might see in younger demographics. So I'm gonna circle back and push against the this-not-that framing that keeps circulating on social media. With regards to exercise, the conversation has kind of been set up as a choice. Zone two or high-intensity intervals, pick a side, defend it, bold it for emphasis. That framing of this versus that, this is the correct way, that way is gonna harm you, it's that argument itself that is doing real harm because the reality is neither side is wrong about the benefits that they're describing, but Because they are pitting two tools against each other, it often causes people to abandon one entirely. Now, post-menopause, the argument about cortisol dysregulation supporting a predominantly zone two workout style is legitimately compelling, and I'm gonna get to that, but it does not mean that it's time to throw out high intensity interval training altogether. You just use it differently, and HIIT workouts are still incredibly valuable, I would say essential, as part of a fitness regimen for post-menopausal women or men over 50 Now on heart health specifically, the research is moving in a direction that many clinicians didn't quite expect. In studies of older adults, high-intensity interval training produced significant improvements in fat mass, in waist circumference, as well as testosterone levels. They were modestly but significantly greater than those who did moderate continuous training, right? And that would be like jogging or running at the same pace for, you know, 30, 45 minutes, an hour. This came as a surprise because it was previously believed that high-intensity interval training would be too brief in its duration to provide any meaningful clinical benefit in cardiovascular or hormone health. But that's not the case. For post-menopausal women, HIIT appears to drive a quality of cardiac remodeling that zone two alone cannot replicate at the same speed. So high-intensity interval training will make changes to the structure of the heart and the efficiency of pumping blood more quickly and more dramatically than zone two. Additionally, VO2 max responds to HIIT training much more dramatically than any other intervention. So remember, VO2 max is the single greatest predictor of longevity. Now, with regards to brain health, the BDNF picture does change post-menopause. Remember that the acute BDNF spike that comes from high-intensity intervals becomes less reliable post-menopause as estrogen levels drop. But that's not like the nail in the coffin or the end of the story. Post-menopausal women will still see meaningful reductions in total body fat as well as visceral fat around internal organs with HIIT training or sprint training. Now, specifically with sprints, the load and impact from sprints and jumping is extremely beneficial for bones. You wouldn't think it, but granny doing sprints on the track is actually a really good thing. Those short maximal efforts hit bone density, visceral fat, and cardiovascular fitness for the post-menopausal woman in one very short, very efficient, but very painful session. It goes without saying, if you're post-menopausal and not a runner, please don't go out and start sprinting today. It-- That is a great way to hurt yourself. Get a coach who is familiar with the post-menopausal population. Work up to that level of intensity on your body, and this can take months, okay? So don't think in, you know, four to six weeks you're gonna be sprinting. Sometimes it takes three to six months to work up to that level, but it's worth it. Like, what else are you gonna be doing with your time? You might as well work up to the sprints. Now, if you've been running through adulthood, and you're noticing that those long endurance sessions that we were all gunning for in the '90s now make you feel like shit, well, it's time to pull it back and incorporate some sprints, girlfriend. Like, give those high school girls something to talk about. Go to the track, run some sprints, and you will start to notice a really significant difference in how you look and feel. You also get bonus points if you put on little terry cloth And maybe some calf-high white socks and a sweatband while you run track So circling back on topic, with regards to body fat specifically, during menopause, the estrogen decline leads to greater insulin resistance. So we are to insulin. And there-- And this often leads to a subsequent increase in fat stored around internal organs, visceral fat. Zone 2 cardio, even though it is beneficial overall for fat burning, does not generate the post-exercise oxygen consumption effect that is way more pronounced in high-intensity workouts. And it's that EPOC, that post-exercise oxygen consumption, that ends up burning more calories and more fat overall post-workout than you get in the individual session itself. So for metabolically dangerous visceral fat, the answer post-menopause is not just do zone 2. Use zone 2 as a foundation, and then add in high-intensity intervals or sprints as a periodic targeted tool. That is what you wanna aim for Work up to that recommended 150 minutes or more a week. Add in resistance training three to four days a week. That is really, really important, ladies, honestly starting in your 30s, if not sooner, all the way through life. I'm gonna talk about that in another episode. And then reduce your high intensity intervals to no more than two sessions a week, 20 minutes max, with full recovery between sessions. You do not need frequent or long sessions to get benefits from high intensity interval training. It actually works against you to push for more. No more than two sessions a week between 10 and 20 minutes can be seriously impactful, like way beneficial for post-menopausal women. Any more than that, you risk injury as well as fighting the cortisol issue rather than working within it and making the most out of it Now men over 45 are having their own hormonal situation, but the timeline is different in ways that do matter when it comes to physical activity. Testosterone decline in men is much more gradual. It's roughly an estimated 1% per year from around age 30. They don't get to experience the relatively abrupt hormonal cliff of menopause that ladies do, which means that the cortisol and stress sensitivity that changes so dramatically for post-menopausal women tends to arrive more slowly in men. So the argument for high intensity interval training holds longer. HIIT training in men age 35 to 40 results in a significant increase in the anabolic to catabolic index. So it both increases testosterone and human growth hormone levels while decreasing cortisol. And that is a genuinely different hormonal response than what a post-menopausal woman would experience. Men in their 40s and early 50s can generally tolerate higher frequency of HIIT workouts, two to three sessions a week, without any of the cortisol dysregulation that becomes concerning for women within that same age range. Now, that said, the zone two argument for men over 45 is not weak. The thing is, is that most people end up neither genuinely in zone two or genuinely high intensity. But the principle holds. The majority of aerobic volume should be a true zone two, building the mitochondrial stamina and the cardiovascular base that makes high intensity work more effective Now, the brain health picture for men does parallel the general HIIT BDNF finding without the complication of estrogen being involved. A twenty twenty-four randomized controlled trial found that both medium intensity and high intensity exercise produced similar increases in circulating brain-derived neurotropic factor in older adults. But HIIT produced superior improvements in hippocampal dependent spatial learning. For men over forty-five, HIIT remains a reliable cognitive tool in a way that it becomes less reliable for post-menopausal women, especially if they're not on any hormonal support. Men over forty-five aim for three to four zone two sessions a week, again up to forty-five minutes, building to a hundred and fifty, a hundred and eighty minutes total. You can add in two to three genuine HIIT sessions. Again, that super high intensity. You're breathless. Your muscles are burning. You hate it, right? And then you recover, and then you do it again. Twenty-five minutes or no more than thirty minutes for sure using that four-by-four protocol. As testosterone continues to decline in your fifties and sixties, start shifting that ratio towards more zone two and then pull back on the high intensity interval to one to two sessions a week. If you're fifty or older and you're just starting out, you can start with maybe five to ten minute high intensity interval sessions once or twice a week while working up to those three to four zone two workouts a week. I strongly recommend getting a personal trainer or a fitness coach to walk you through what high intensity intervals look and feel like for a few sessions until you can safely get it under your belt. I'm sorry guys, but as a demographic, you all tend to blow past your body's limitations and get hurt much more frequently than women do. And that's not doing you any favors ever, but it becomes a huge risk the older you get. So get yourself an experienced trainer who is not scrolling Instagram while mindlessly counting your reps for you, and have them show you how to do this wisely. As you find that your soreness and recovery lessens with each high intensity workout, you can incrementally increase your HIIT times to get towards that thirty minute threshold, but don't go balls out on the first go, okay? Now I wanna talk about the forgotten zones in between, mainly zone three There is a widespread claim in fitness circles that zone three, which is also known as tempo training, is a dead zone. It's too hard to build an aerobic base and too easy to generate a genuine high-intensity stimulus, and therefore it's the worst of all worlds. Well, I'm gonna call bullshit on that. Like everything else online, this has become an influencer talking point that oversimplifies the actual science. Training in zones three and four increase the number and efficiency of your mitochondria, and it does improve how well your body handles the demands of harder effort. It also trains your body to process and recycle lactate more effectively. Remember lactate, that burning sensation from hard work that I mentioned earlier? At zones three and four, your body gets better at clearing it quickly and using it as fuel rather than letting it accumulate. So the real world practical result of that is that fatigue is delayed and you can sustain harder efforts for longer. Zone three develops both your aerobic system, the one that runs on oxygen and fat, and your faster burning anaerobic energy system that kicks in when oxygen itself cannot keep up with demand. It does this by increasing the density of micro, like super tiny blood vessels that deliver oxygen to your muscles, as well as improving mitochondrial function and strengthening your cardiovascular output overall. Zone three is the zone where most people run the majority of a marathon. So if you're training for endurance performance of any length, times in zone three and zone four is never wasted. It's necessary At zone four, which is known as threshold training, your body recruits both endurance muscle fibers and power muscle fibers simultaneously. In zone four, your power fibers produce the lactate, but then your endurance fibers consume it as fuel. Which I think is so freaking cool. When you incorporate zone four workouts into your regular training schedule, those power fibers gradually become more aerobically efficient, and they start behaving more like endurance fibers. So the lines blur, but they do so in an overall useful direction. The polarized training model, which is an approach associated with the highest endurance performance improvements seen in research, recommend spending the large majority of your training time at a genuinely easy effort, a small but meaningful amount at genuinely hard effort, and very little in the middle. Note that even in this model, the middle does have a deliberate role. The critique is not that moderate intensity is useless. It's that zone three should be a conscious choice, not where you inevitably end up because easy feels too slow and hard feels too hard. Exercise in all zones produce similar adaptations. The difference is that training in particular zones gives your body a better opportunity for particular adaptations. Zone three intervals can replace zone two endurance training when you're in a time crunch. Zone four threshold training provides significant results for improved functional threshold power and depending on your constellation of hormones, it could arguably be done more frequently than, say, a high-intensity interval training. So the question isn't whether zone three is legitimate. It is. The question is whether the moderate intensity exercise that you are doing most of the time is intentional or whether it's the path of least resistance that keeps you uncomfortable enough to feel like you're working but not hard enough to generate specific adaptations that you're actually needing. Chronic unvaried middle intensity training is the problem, not the fact that zone three even exists. If your zone two is genuinely zone two and your HIIT workouts are genuinely high intensity, what happens in between on other days is still productive. If neither anchor is in place, the middle is kind of where fitness plateaus tend to occur. Endurance athletes who spend too much time in the middle, they don't get faster and recreational athletes don't lose the body fat that most of them are looking to lose. The devil lies in those details and well, details don't make for great clickbait, do they? The primary takeaway from all of this is that zone 2 and high intensity interval training are not rivals. They are tools with different functions, and the functions shift depending on who you are and what stage of life you are in, as well as why you're training in the first place. Zone 3 and zone 4 are not our body's enemies either. They are what happens when you're fit enough to train specifically, and for anyone building towards an endurance race, they are mandatory. The research is not saying do more or do less. It's saying do the right thing at the right intensity, at the right frequency for your specific biology. And to anyone else who has ever spent time feeling like they're failing at zone 2 training or any exercise that they haven't been properly taught, I wanna close with something that I said in that original thread that I meant completely. Nobody's nona lived to 102 obsessing over zone 2 heart rate zones. The a priori goal is to move your body in a way that feels good and that you can sustain for years, because whatever that is, it's likely gonna be better than sitting on the damn couch. The science in this episode exists to highlight the benefits of each zone of cardiovascular movement, not to pit one against another so you feel like you have to suffer through something you don't like or feel like you're failing at staying healthy because your heart doesn't cooperate based on external metrics that have been imposed upon it. Zone 2, HIIT, zone 3, spri-sprint training, pick your weapon and use your brains to move your body wisely. And for the love of God, eat breakfast after your workout Thanks again for listening to Blasphemous Nutrition. If you found this episode helpful, send it along to someone who has kept up at night wondering how to master zone two cardio. In the next episode, I am gonna talk about strength training and the things that it does for us that neither zone two nor HIIT intervals can and also why after age 45 it might be the most important thing of all three, much to my chagrin. Be sure to subscribe so you are among the first to catch that episode when it drops. Until next time, my friends, stay curious and keep questioning everything. Any and all information shared here is for educational and entertainment purposes only, and is not to be misconstrued as offering medical advice. Listening to this podcast does not constitute a provider-client relationship. Note, I'm not a doctor nor a nurse, and it is imperative that you utilize your brain and your medical team to make the best decisions for your own health. The use of information on this podcast or materials linked to this podcast are at the user's own risk. No information nor resources provided are intended to be a substitute for professional medical advice, diagnosis, or treatment. Be a smart human and do not disregard or postpone obtaining medical advice for any medical condition you may have. Seek the assistance of your healthcare team for any such conditions and always do so before making any changes to your medical, nutrition, or health plan