Zone 2 has become the cardio darling of the last few years. Podcasts pushed it. Longevity influencers built entire brands around it. Somewhere along the way, plodding along at a conversational pace got rebranded as the single most important thing you could do for your metabolic health. And a lot of people — busy, tired, doing their best — started dedicating hours a week to it, trusting that the slow burn was quietly rewiring their mitochondria into something magnificent.

Then along comes a physiology researcher who has actually read the literature, and she's not buying it. Kristi Storoschuk, a Ph.D. candidate at Queen's University who's spent much of her doctorate combing through cardio training studies, thinks the zone 2 gospel is oversold — and that if your workout window is short, you're probably spending it in the wrong heart-rate range.

I want to walk through her argument carefully, because if she's right, a lot of well-meaning people are grinding out hours of low-intensity work for a fraction of the benefit they'd get from something shorter and harder.

So what is zone 2, exactly?

Zone 2 is defined physiologically as the intensity where your blood lactate stays roughly steady — you're clearing it about as fast as you're producing it. In a lab, that's confirmed with a finger-prick lactate meter, and the standard cutoff is usually a reading under 2 mmol/L. Simple enough if you happen to own the gear.

Most people don't. So we rely on proxies: a percentage of max heart rate, or the classic talk test. Storoschuk described the version researchers sometimes use — asking someone to count backward from 30 while pedaling. If they can do it comfortably, they're probably in zone 2. If they're gasping between numbers, they've climbed higher.

Here's where things get slippery. A recent review of how zone 2 is described across sources found definitions ranging anywhere from 30 to 80 percent of VO2 max. Those are wildly different efforts. So when someone tells you they did 45 minutes of zone 2 this morning, there's a decent chance they were doing something else entirely — and no way to know without a meter.

Where the evidence gets thin

The whole pitch for zone 2 rests on two adaptations: better mitochondrial content and function, and improved fat oxidation. Both matter for insulin sensitivity and long-term metabolic health. Nobody's arguing with that goal. The question is whether zone 2 is actually the best way to get there.

When Storoschuk and her team went through the studies, they found something inconvenient for the true believers. Zone 2 can trigger those adaptations — sometimes. Higher intensities trigger them more reliably, and often to a greater degree. As she put it, when you look at higher-intensity work, you see these benefits show up consistently.

Which means the research doesn't support zone 2 as the optimal intensity for building mitochondrial density or fat-burning capacity. It's one path. It's not the best path. And when people are told it's essential and dedicate three hours a week to it, they may be giving up better results they could've gotten from 40 minutes of harder work — or a couple of 20-minute HIIT sessions.

Where did the zone 2 obsession even come from?

Storoschuk traces the enthusiasm back to observational data on elite endurance athletes — the cyclists, cross-country skiers, and marathoners who train 20 to 40 hours a week. Those athletes famously spend around 80 percent of their training time at low intensity, with the remaining 20 percent up high. So the takeaway got flattened into: do what the pros do, mostly zone 2.

Except 20 percent of 30 hours is six hours of high-intensity work per week. That's more hard cardio than most recreational exercisers do in total, ever. The elite polarized model isn't really 80/20 low-to-high — it's a mountain of easy volume built on top of a huge chunk of hard work most of us never do.

There's another wrinkle. The fitter you get, the more your training zones compress. An elite athlete's zone 2 sits much closer to their VO2 max ceiling than a beginner's does, because heart rate has a physiological cap. So their easy pace is objectively a bigger physiological effort than someone else's easy pace. Copying their ratio without their conditioning isn't the same intervention.

Do you actually need to build an aerobic base first?

This is the stickiest claim of all — the idea that you have to grind out months of low-intensity work to establish an aerobic base before you're allowed to do harder cardio. Storoschuk pushes back on this in two ways.

First, nobody has a clean definition of what "aerobic base" even is. It gets used constantly, but pin someone down and the answer gets hand-wavy fast. Second, the actual evidence suggests the opposite of what the base-building crowd claims. You raise your lactate threshold by training above it — not by staying comfortably below it and hoping the ceiling drifts upward on its own.

She pointed to a study that compared groups training in zone 2, zone 3, zone 4, HIIT, and sprint intervals against a sedentary control. Every group that trained above zone 2 improved both VO2 max and lactate threshold. The zone 2 group didn't meaningfully improve on either metric compared to sitting on the couch. Read that sentence again. The intensity that's supposed to build the base didn't move the very ceiling it claims to build.

What to do when your cardio time is limited

Storoschuk's rule of thumb is simple: as your available time goes down, the importance of intensity goes up. If you have all the hours in the world, sure, do lots of easy volume — it's low stress and easy to recover from. If you have three to five hours a week and a full life, the math changes.

She's not trying to prescribe one perfect protocol. She actually thinks the variety of effective options should feel liberating, not stressful. But when pushed for practical guidance, here's roughly how she'd allocate limited time:

• 30 minutes and you love running: just go run. But don't artificially throttle yourself to stay at a low heart rate when your body wants to push harder.
• 20 minutes: one minute hard, one minute easy, repeat. Make each hard minute genuinely vigorous. It's okay if the last couple rounds are ugly.
• 15 minutes, a couple times a week: Tabata-style intervals — 20 seconds all-out, 10 seconds rest, eight rounds. Four minutes of work per session on a bike, rower, or ski erg. The pace should be one you couldn't hold much past 20 seconds.

Frequency matters too. A meta-analysis she referenced found that six sessions a week produced better VO2 max gains than four, which beat two — regardless of intensity or how long the study ran. Little and often beats occasional heroics.

The takeaway isn't "zone 2 is bad"

Storoschuk is careful about this. She isn't out to demonize easy cardio. Zone 2 is better than nothing. Walking is better than sitting. Any movement beats no movement. What she objects to is the idea that zone 2 is required, or that skipping harder work in favor of more slow miles is somehow the sophisticated choice.

Her own training, by the way, is refreshingly unstructured. She checks the CrossFit workout of the day, swaps it out if she doesn't feel like it, sometimes runs for an hour and sometimes runs for 15 minutes. She'll tell you it's probably not optimal — but she's been doing it for over a decade, which means it's sustainable, which means it's probably the best thing for her. That's a useful reframe on its own.

The bigger point: the training habits of athletes with 20 years of conditioning and 30-hour training weeks don't automatically translate to someone squeezing a workout in between meetings. Copy the ratio without the volume and you get the worst of both worlds — a lot of easy work that isn't hard enough to trigger real adaptations, and not enough hard work to move the needle.

If you're the kind of person who thinks about training the way you think about sleep or diet — as one of the levers you're pulling to stay well for the long haul — this reframe matters. Recovery routines, breathwork, sauna sessions, contemplative practice: these all pair beautifully with a smarter cardio approach that respects your actual schedule. For readers curious about broader wellness and retreat offerings that support this kind of holistic health, a curated selection can be explored on our marketplace here.

Spend your cardio minutes above zone 2, in a dose you can recover from and repeat next week. That's the whole prescription.