#462 Cardio Faceoff: HIIT vs Zone 2 - Which One Is Superior with Exercise Scientist Kristi Storoschuk

Kristi Storoschuk (0:00)

It really isn't about, oh, zone two versus high-intensity interval training. Like when it comes to help, the amount of physical activity that you need is actually so small to reduce your risk of disease and mortality. Just going from nothing to something drops your risk of mortality by a little bit.

Siim Land (0:19)

Kristi, welcome to the show.

Kristi Storoschuk (0:20)

Hi, Siim. Thank you for having me.

Siim Land (0:23)

Yeah, you had a very interesting paper you published just recently that I found online. And yeah, I wanted to have you on the show to discuss about the details of that paper. But maybe you can also give a short background about yourself. How did you come to writing the paper about the zone 2 cardio?

Kristi Storoschuk (0:43)

Yeah. So I'm a PhD candidate in the muscle physiology lab at Queen's University. I'm nearing the end. I can see the light at the end of the tunnel. My research has a couple of different focuses on fasted versus fed training. I just finished up a training study. And then the idea of just fuel substrate used during exercise and how that influences adaptations, which gets to zone 2 and the idea of burning fat to become a better fat burner, which parallels the fasted exercise research and interest in that as well, which, yeah, like we'll get into the zone 2 paper. But the real motivation, I guess, behind writing that paper, which for the listeners addresses the claims made around zone 2 exercise as this optimal intensity for improving our capacity to burn fat by promoting mitochondrial adaptations and thereby supporting better metabolic health is because those two physiological features of the body are associated and linked to our overall health. So if we're exercising in a way that supports those, then we should be supporting our metabolic health. But I guess the paper really addresses, well, does zone 2 induce those adaptations? Really, that's the question we were asked. Because online, you can literally do a quick Google search, put in zone 2 and mitochondria, and there will just be page after page of blogs being like, zone 2 increases mitochondrial content, zone 2 supports mitochondrial health. Like, there's just a lot of that.

Siim Land (2:27)

In my opinion, there's almost like two camps of people. There's people who are just, you only need to do hit cardio, that zone 2 is a waste of time. And the other group is that you need to do only zone 2 or mostly zone 2 So there's rarely like this middle ground where you do both, which I think would be the optimal scenario, but it tends to be in the two categories, the two groups of people.

Kristi Storoschuk (2:51)

Yeah. And I think the conversation is just a bit too polarized, which complicates it because it really isn't about, oh, zone 2 versus high intensity interval training. And that's not where I land at all. It really was like, do I even need zone 2? Do we need to be doing zone 2? Does it have some sort of unique inherent benefits that you don't get from going in to higher intensities? And I mean, I'll spoil the spoiler. No, like I'm not under the impression. I didn't find any evidence to suggest that we should be doing or prioritizing zone 2 over higher intensities because you can just get the benefits of on mitochondrial adaptations, our ability to burn fat, those adaptations that support better fat oxidative capacity are supported with high intensity exercise or higher intensity exercise.

I want to be deliberate with my language in saying that it's not hit, it's not high intensity interval training that I'm comparing to zone 2 It's the volume accumulated in higher intensity, so zone 3 plus. And I think that gets a lot of confusion online is that we think of all steady state exercise as zone 2 when it's not. It's like most of our studies that use continuous training. So if you read any papers where you're trying to figure out like, oh, hit versus modern intensity continuous training, that moderate intensity continuous training abbreviated to MIKT, it often falls above zone 2 So when we're looking at like, oh, hit versus MIKT, it's really hit or sit sprint interval training. So these high super, the intervals, the intensities that require us to do in small bursts because they're just unsustainable for longer durations. If we're comparing the interval training to continuous training, it's often, okay, we're comparing zone 3, maybe even 4 continuous training to zone 5 plus, and just those intense training. So I think part of the confusion is that all continuous training is being grouped as zone 2 When zone 2 has a very clear definition, it's demarcated by our lactate threshold or our gas exchange threshold, so we can use ventilatory, our ventilation to kind of indicate it's linked to our lactate threshold. But online, lactate threshold is commonly the demarcation for what is considered zone 2 So zone 2, I guess we should define it right from the get-go, is the zone just below our lactate threshold. So as intensity increases, as if we're sitting on a bike, and every few minutes the intensity increases, and we're measuring our blood lactate, we should, blood lactate will stay at a pretty low, maybe just above our resting level, maybe like literally 0.1, 0.2 above our resting level for a couple stages. And then as intensity increases, we'll see this gradual incline, and then we'll hit a certain threshold, which is considered our first lactate threshold, where we jump about 1 to 1.5 above our resting level, millimolars. And there's different definitions for lactate threshold within the literature, but in our lab, we consider the 2 millimolar mark as lactate threshold. So zone 2 is typically defined as just below. So within like the 1.7 to 2 millimolar range would be we're able to engage in continuous exercise for probably up to like 3 hours, 3, 4 hours without our lactate budging. It's a very low intensity. But if we go above that and we start increasing intensity, then there's more of an exponential increase until we hit our second lactate threshold, which then just skyrockets if you increase your duration above our lactate threshold too.