Why Your "Optimal" Zone 2 Training Might Be Holding You Back

A new, 167-source review finds the internet's favorite cardio zone isn't "optimal." Here's what is.

Zone 2 exercise—low-intensity cardio—has boomed over the last few years.

Proponents claim that it’s optimal for health due to boosting mitochondria and helping your body burn more fat. And many of us now dedicate hours a week to this specific intensity, believing it’s the best use of our limited training time.

But a new review of the research finds no evidence that Zone 2 is the best for those positive health adaptations for most people. In fact, exercising harder than Zone 2 is most beneficial for the average person.

The findings are subversive. I wanted to understand exactly where things may have gone wrong, and what it actually means for our workouts.

To find out, I spoke with the review’s corresponding author, Brendon Gurd, to hear what the team found and how it can change our thinking around exercise.

Here’s today’s roadmap:

• The Hype vs. The Lab: We’ll explore why a top exercise physiologist saw the Zone 2 boom and his first reaction was, “That doesn’t make any sense.”

• What the Science Says: A breakdown of the new 167-source review and its conclusion about mitochondria, fat-burning, and “optimal” intensity.

• The Great Disconnect: Why the internet (and many experts) got Zone 2 wrong —and who it’s actually very beneficial for (hint: it’s probably not you).

• The Actionable Takeaway: How to rethink your cardio to get the most benefit for the time you spend.

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The post

Brendon Gurd has been studying exercise physiology for two decades. He’s a professor at Queen’s University in Ontario who spends most of his time in his lab studying muscle physiology—and he isn’t very online.

Recently, though, he was thrust into internet-based exercise claims that didn’t add up.

“One day, my grad student, Kristi Storoschuk, came into my office,” Gurd told me. “Kristi is very tuned into health social media accounts and podcasts, and she started asking me about Zone 2.”

Zone 2, at its most basic level, is cardio exercise that’s harder than a leisurely stroll but still easy enough that you can have a conversation while doing it¹. For most people, it’s a fast walk, or a relaxed run or bike ride.

Storoschuk had seen online content saying that Zone 2 was the best form of endurance exercise, bordering on magical. The claim: Zone 2 is optimal for improving mitochondrial and fatty acid oxidative capacity.

• Mitochondria are parts of your cells that convert food and oxygen into energy. The more mitochondria you have, the fitter you are and the better able you are to perform activities of daily life. You’re also less likely to get metabolic diseases, heart disease, etc.

• Being able to oxidize more fatty acids means your body will burn more fat during exercise. That’s linked to better performance and health.

Some people online even said that exercising harder than Zone 2 effectively cancelled out those benefits. The advice: Don’t exercise too hard. And to ensure you don’t, track your heart rate or even run a pinprick of blood through a device called a lactate meter as you exercise.

“My initial reaction was like, well, that doesn’t make any sense based on everything I know about physiology, muscle metabolism, and what I’ve spent my career studying about how mitochondria adapt to exercise,” Gurd told me. “Everything I understood was that Zone 2 is probably the worst intensity, not the best intensity [for building mitochondria].”

But he didn’t dismiss the idea. “I try not to hold my opinions too tightly, because I can be wrong. So I said, let’s go see what we can find. And that led us down the rabbit hole of figuring out what the evidence for Zone 2 is.”

The rise of Zone 2

Twenty years ago, “Zone 2” was a phrase uttered only by elite endurance athletes. These types of people were often training 20+ hours a week, and they spent about 80% of that time in Zone 2, with the remaining 20% at higher intensities.

Then Iñigo San Millán, a professor at the University of Colorado School of Medicine who’s also trained Tour de France-winning cyclists, began appearing on podcasts. San Millán believes that Zone 2 enhances mitochondria the most and trains your body to utilize both fat and carbs as fuel most effectively.

His ideas rippled across large podcasts and social media platforms.

In turn, Zone 2 boomed. Google searches are up between 250 and 300 percent year over year, and the term “Zone 2” now outranks “HIIT workouts.” Major outlets, including NPR, Wall Street Journal, and New York Times, have all covered Zone 2. We’ve also covered Zone 2 here on Two Percent.

And then, of course, there are classic everyday markers that indicate critical mass, like your accountant or dentist telling you she’s doing Zone 2.

What the review found

Gurd, Storoschuk, and two other researchers searched academic databases looking for studies that matched common Zone 2 definitions.

Overall, their review included 167 sources. They titled the paper: “Much Ado About Zone 2.”²

The big takeaway: “We conclude that current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity,” the researchers wrote.

• For building mitochondria, intensities higher than Zone 2 appeared to be significantly more effective. The research all suggested the harder you work, the more mitochondria you’ll get. In fact, Zone 2 may not be hard enough to build mitochondria.

  • Gurd and team wrote, “Given the clear intensity-dependent effect of exercise … members of the general public who replace high-intensity exercise with Zone 2 exercise may risk minimizing the benefits of exercise on long-term health.”

• For fat oxidation, it was a wash. Zone 2 can raise fat-burning capacity, but so can higher intensities. There wasn’t a strong case that Zone 2 is best or that you’d lose benefits if you went harder than Zone 2.

“I definitely don’t want to discourage people from doing Zone 2 exercise,” Gurd told me. “All exercise is good. But I think the claim that Zone 2 is the best and you have to do Zone 2 … if that discourages people from doing higher intensities of exercise, then that could potentially limit the benefits.”

Why the disconnect

I asked Gurd why there’s such strong messaging around Zone 2’s benefits when the research suggests otherwise.

“I think one thing that’s informed this idea is looking at elite athletes who train, say, 16 hours a week in Zone 2 and 4 hours a week at higher intensities,” Gurd said. “So we look at that and think Zone 2 is optimal.”

“But it could be that most of these athletes’ performance benefits are coming from the four hours of higher intensity work. What we think is happening is that elite athletes reach a point where they can’t perform more high-intensity exercise, or they risk burnout and overtraining. They’ve maxed their ability to train intensely, so they do massive volumes of more relaxed Zone 2 to push things further without burning themselves out. And that might give them an extra two or so percent increase, which for a pro athlete is the difference between winning and losing, so it’s worth it.”

But most people don’t exercise 20 hours a week. In fact, 80 percent of the population doesn’t exercise for more than 2.5 hours a week.

He thinks that those of us exercising less than four to five hours a week would probably get more from our time by going harder, because we haven’t yet maxed out the higher intensities.

Is Zone 2 dead?

No. No study or review is ever the final word.

It’s another perspective among many. That said, this review is a perspective supported by 167 sources. Another recent review³ concluded the same: Exercising harder than Zone 2 boosts mitochondria the most.

Gurd also told me the point of the review wasn’t to say that Zone 2 “is bad.”

“Some people have interpreted this paper as us pushing high-intensity interval training, but we’re not,” he said. “Any exercise you can do is good. Whatever you enjoy, that’s what you should do. If the Zone 2 craze has gotten more people to exercise, that’s great.”

“But let’s say you’re trying to maximize the return on your time. If some of the messaging says to only do Zone 2 and that going harder could be causing some harm, that’s problematic. If people are now exercising less intensely because of this messaging, then that’s a downside.”

For an outside perspective, I texted Brady Holmer, who writes the greatPhysiologically Speaking Substack. Brady studied endurance exercise in grad school, wrote the book VO2 Max Essentials, and is himself an elite endurance athlete.

Big takeaways

If you have limited time to exercise, this review tells us that we should make exercise uncomfortable. Gurd offered some useful guidelines:

Train as intensely as you can. Focus on higher intensity exercise until you start to feel like you can’t do more.

I don’t know what that number is—maybe two, three, or four hours a week.

Then, if you feel like you can’t do more higher-intensity exercise because you need to recover, that’s probably the point where you add lower-intensity exercise, like Zone 2.

He continued, “I’m not saying do only high-intensity intervals. I’m saying, don’t just go out and stroll, go for a brisk walk. If you’re going for a bike ride, don’t just do a Zone 2 ride. Push a little bit and work harder on your bike. You’ll get more benefits from the time you spend exercising.”

Of course, when you aren’t “exercising,” be a Two Percenter. Find ways to work more movement into your day. Take a walk as a work break instead of doomscrolling. Take the stairs. Just move more. That may be all the low-intensity exercise you need.

Have fun, don’t die, make exercise uncomfortable again,

-Michael

1. Researchers typically place Zone 2 below your first lactate threshold (LT1). For sedentary people it can be a normal walk; for endurance athletes it can be ~300 watts on the bike. This paper covers common definitions, complications, and guidelines around Zone 2.

2. Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population. Sports Medicine (2025).

3. Mølmen KS, Almquist NW, Skattebo Ø. Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression. Sports Med. 2025 Jan;55(1):115-144. doi: 10.1007/s40279-024-02120-2. Epub 2024 Oct 10. PMID: 39390310; PMCID: PMC11787188.