“Run slow to run fast” goes the Zone 2 rallying cry that has taken the fitness internet by storm. Over the last three years, this once-obscure sports-science concept has become gospel for many recreational athletes. Running influencers and health podcasters frequently talk about the benefits of low-intensity exercise. Many runners are slowing down their “easy” runs. And TikTokers are promoting incline walking (like the 12-3-30 workout) as the key to fat loss, longevity, and hormonal balance. Keeping your heart rate low—about 60% to 70% of your max—seems to have become a near-universal goal for the health-conscious.
Zone 2, in case you somehow missed the wave, refers to low-intensity cardio where you keep your heart rate just below your aerobic threshold (the point where your body shifts from primarily burning fat to using carbohydrates for fuel.) While doing it, it’s easy enough to hold a conversation, but hard enough to feel like you’re exercising. And, as an exercise category, it has been praised for building mitochondria, improving fat metabolism, and keeping cortisol levels in check (more on all of that later.)
But the backlash has officially arrived. A 2025 literature review published in Sports Medicine has stirred up debate about whether steady-state, low-intensity cardio is actually the most effective form of exercise—especially for time-crunched, non-endurance athletes. Social media has responded, of course, with videos like “Stop doing zone 2” and “Zone 2 training will only get you worse.”
As with most online fitness trends, the truth is more nuanced. Here’s what you actually need to know.
The rise of zone 2
Throughout the 2010s, at the start of the boutique fitness boom, companies and trainers marketed brutal workouts as the only type of workouts that mattered. People stacked CrossFit WODs and OrangeTheory classes five days a week, pushing their body to the point of vomiting or passing out every time they worked out (often cheered on by bootcamp-sargent coaches who lean in to the whole “push until your drop” mentality.) Unsurprisingly, that approach wasn’t sustainable for most people, especially beginners. (Between 2000 and 2019, ER visits related to rhabdomyolysis—a dangerous form of muscle breakdown—rose more than tenfold. Though we have no proof that that was due to high-intensity workout classes, it’s a notable statistic.) And yet, the collective belief that a “good workout” means feeling completely annihilated became so ingrained in American society that many of us still have a hard time taking it easy at the gym.
So when zone 2 cardio gained traction around 2021 after recently disgraced longevity bro Peter Attia did a deep dive on the topic, it felt like a much needed relief. He wasn’t the first health influencer to advocate for it. Exercise physiologists, like Stephen Seiler, had been researching low-intensity endurance work for decades. It first got popular in the cycling world around 2010, then trickled down to marathon runners, then the masses.
“For two and half decades, we've been trying to tell people, ‘Hey, don't go hard every day. You cannot do it. Elite athletes don't do it,’” Seiler says. He’s best known for advocating for “polarized training,” which promotes a split of roughly 80% low-intensity (zone 2 or lower) and 20% high-intensity work. For elite athletes logging tens of hours a week, this split is especially helpful for recovery while still remaining active each day.
But once the concept hit the wellness world, people started treating zone 2 like a miracle drug: What was once a necessary training tool for elite athletes’ exercising became commonplace for everybody, no matter their training load. Influencers cited it as key to longevity, because it could build mitochondrial density (which is important because mitochondria produce energy) and improve metabolic efficiency better than other forms of exercise. Health and wellness podcaster Andrew Huberman even claimed that everyone should get at least 150 minutes a week of zone 2 because it could improve every other aspect of training, like resistance training and speedwork.
Health creators took it even further, warning that high-intensity exercise could spike cortisol levels. While cortisol does rise temporarily during intense exercise, there’s little evidence to suggest that it leads to chronic elevation or hormonal disruption in healthy individuals. When done in moderation, it may actually help to regulate stress hormones. (Alas, nuance doesn’t trend well.) Meanwhile, an expanding market of wearables—from Garmin to the Apple Watch—made it easier than ever to track heart rate zones, even if they weren’t always accurate. Everywhere you looked, people were doing zone 2.
One speed doesn’t fit all
It was only a matter of time until the foundation of the zone 2 craze started shaking. Brendon Gurd, one of the co-authors of the Sports Medicine review and a researcher at the Muscle Physiology Lab at Queen’s University in Canada, says that so much of the messaging he encountered around zone 2—that it offers different or superior benefits compared to high-intensity training—struck him as suspect. “We weren’t able to find any evidence that that’s true,” he says. “Based on years of research, we believe muscle adaptation is tied to stress. And there’s more stress with higher intensities of exercise.”
The review, conducted by Gurd and three other colleagues in 2025, focused on studies of the general population—people training 150 minutes or fewer per week—and found that moderate to high-intensity workouts tend to deliver greater improvements in cardiorespiratory fitness and metabolic health. Zone 2 was not found to be superior for improving mitochondrial health or fat oxidative capacity. However, Gurd notes that very few studies have explicitly investigated the impact of zone 2 on mitochondrial density. More research is needed.
Even Seiler acknowledges that zone 2 can backfire when misused. “Recreational athletes often go too hard on their easy days and not hard enough on their hard days,” he says. “That’s one of the big differences between the really good endurance athletes and the rest of us. They can put their ego at the door when they’re doing those easy runs.”
Another point of confusion is fat-burning. Zone 2 training does burn a higher proportion of fat during the workout and has been found to increase fat-oxidation capacity (the body’s ability to break down stored fat), but that doesn’t mean it’s a better strategy for fat loss. You’re not going to lose more weight from burning fats instead of carbs during a workout, according to an analysis in Frontiers in Physiology. Total energy expenditure and diet are more important.
What’s right for you?
There's no doubt that zone 2 training is great for overall health, but there’s not much evidence that zone 2 is the magical cardio fat-loss shortcut that the internet has hailed it to be. “Each zone doesn’t do a very specific thing—that’s a misnomer,” says Seiler. Mitochondrial adaptations and cardiovascular improvements happen across the intensity spectrum. The difference is that the lower the intensity, the longer you need to spend doing it.
Low-intensity work becomes crucial for people with high training loads, like those running a marathon or preparing for Hyrox, to balance recovery and performance. “The intensity zones are mostly good for managing the stress of training and putting it all together,” says Seiler. But zone 2 may not be as important for people who aren’t training hard in the first place. If you’ve only got a few hours a week to exercise, higher-intensity work might get faster benefits, says Gurd. Seiler agrees: “If you’re only training two or three times a week, you can get away with quite a lot. You’re not going to do anything dangerous to your body because you’ve got these rest days in between.”
However, where Seiler and the authors of the recent study seem to differ is where most people should start. Seiler says that physiological adaptations aside, a lot of factors go into forming an exercise habit. Staying consistent is the biggest hurdle for most people, as over 53% of adults do not meet the CDC’s physical activity guidelines. Some people may find it easier to stick with zone 2 because it feels more gentle (and less challenging) than high-intensity workouts, Seiler says.
He recommends keeping it simple at first: Start building the frequency of your workouts, then gradually up the length. Once you’ve developed a solid habit, then you can dial up the intensity. “When we go in all guns blazing with tabata or high-intensity, it kicks us in the butt,” he says. “We get injured, then we gotta start over again. If you’ve spent years getting out of shape, it’s going to take more than four weeks to get in shape.”
Gurd says that the study’s lead author, Kristi Storoschuk, suggests the opposite: Starting with intensity, then layering in low-intensity volume once you’ve maxed out your high-intensity time.
Ultimately, it comes down to personalization. People have jobs, kids, responsibilities. For some, adding high-intensity exercise on top of daily stress may feel like too much. Even cozy cardio—setting some mood lighting and walking on a treadmill while watching Netflix—can improve health markers compared to being sedentary, says Seiler. Others may prefer to use their limited time to push hard and feel accomplished.
When in doubt, just spice it up. A recent study suggests that simply doing a mix of different types of movement—varying the intensity, duration, and activity itself—is one of the best ways to prevent premature death. So, for the majority of us, all this talk about locking into a single zone or meticulously tracking heart rate is overkill. The smartest way to train is the way that’s fun for you.