Is Zone 2 training worth it?

For runners, yes. Most of an endurance athlete's training is easy running, at every level of the sport. The strongest published critique of Zone 2, a 2025 peer-reviewed review, declines to argue against that structure. What it contests is the claim that made Zone 2 famous: that it's the optimal intensity for improving mitochondria and fat burning, as advice to the general public.

Was Zone 2 training debunked?

No. The challenge comes from a review published by Storoschuk and colleagues in Sports Medicine in 2025. What it argues is narrower than debunked: that Zone 2 isn't the optimal intensity for two specific adaptations. It doesn't argue that easy training fails.

The claim it examined is the one that built Zone 2's reputation. In a 2019 podcast interview, the physiologist San Millán described Zone 2 as the intensity where fat burning and lactate clearance improve most. He credited the mitochondria for both. Weekly minute targets for Zone 2 became a staple of podcast training advice.

The review defines Zone 2 by blood lactate, as easy work below the first lactate threshold, and assesses that claim against the training literature. Its conclusion, in its own words: "current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity."

The two halves of the claim fared differently. On fat burning, the review agrees Zone 2 improves fat-oxidation capacity, and finds harder training improves it too. What it disputes is superiority. It says it's aware of no convincing evidence that Zone 2 improves fat oxidation more than higher intensities do.

On mitochondria it's more negative: "the majority of the available evidence argues against the ability of Zone 2 training to increase mitochondrial capacity." The same review acknowledges that Zone 2 exercise activates the signaling pathways that start mitochondrial adaptations.

Does the Zone 2 critique apply to runners?

Not to trained runners. The 2025 review that contests Zone 2's optimality assesses it for the general population, and it defines that population at the outset: "non-endurance-trained individuals who are insufficiently active or meeting physical activity guidelines." A runner who trains most weeks of the year sits outside that definition.

The review is just as explicit about what it is not arguing. "We do not question the health benefits of physical activity, which includes that done at low intensity or characteristic of Zone 2 exercise."

On athletes, it declines to rule at all. It examines one proposed mechanism for why elite athletes' high volume of easy work might pay off. There it writes that a lack of data does not disprove the contention.

The review asks a question about the general public and answers it. What it claims about people who train seriously is close to nothing, by its own design.

Can you train like an elite athlete on 150 minutes a week?

No, and that mismatch is the core argument of the 2025 review that contests Zone 2's optimality. Zone 2 advice for the general public borrowed the training habits of elite endurance athletes. Elites run high volumes of easy and hard work alike, and the review notes their totals are often more than 20 hours a week. Public-health guidelines ask for about 150 minutes of moderate activity a week.

At an elite volume, a mostly-easy week still contains hours of hard training. Inside about 150 minutes, making most of it easy squeezes the hard work out. That trade is the review's warning. Its conclusion argues for prioritizing intensities above Zone 2, "particularly in the context of lower training volumes."

The zone's label hides a second mismatch. What Zone 2 demands in absolute terms scales with fitness, so the same zone names far harder work in a trained athlete than in someone starting out.

The review's conclusion and a runner's training week describe different volumes, which is how both stand at once.

What did San Millán say about the Zone 2 review?

He rejected its framing in an essay published on his own Substack in March 2026. The essay is not peer-reviewed.

San Millán is the physiologist whose description of Zone 2 carried much of the popular claim the review contests. His counterargument is about evidence types. The review, he argues, rests on short-term training studies in recreationally active people at constrained training volumes. In his view it does not engage the decades of applied work with elite athletes and clinical populations behind Zone 2 prescription.

He also calls the debate itself false. Of Zone 2 and high-intensity training he writes that "both are necessary and neither is sufficient." Every serious practitioner, he holds, already trains that way.

What he does not claim is that the review's data are wrong.

As of August 30, 2026, the essay remains the only reply of any kind: no correction and no peer-reviewed response from anyone has appeared.

Do runners still need Zone 2 training?

Yes. The case for running most of your week easy never rested on claims about mitochondria or fat burning. It rests on how endurance athletes train, and on what happens when runners train that way.

Observational studies of endurance athletes across sports find the large majority of their sessions done at low intensity. The published disagreement about that record concerns the hard minority, and how to classify it. The researchers who dispute the "polarized" label recomputed the same athletes' training by time and found the easy share even larger. They state they are not arguing for threshold-heavy training instead. Neither side of that exchange disputes the easy majority.

The intervention evidence agrees. One heart-rate-controlled trial of sub-elite runners held the hard fraction equal across groups. The group that ran more of its remaining volume easy improved a simulated race more. Among recreational marathoners, the faster runners keep a larger share of their training low-intensity.

What easy volume buys is not part of the dispute. It's the training you can do a lot of. It fills the week without costing the next hard session, and it keeps easy days from sliding into unproductive medium ones. It keeps the running consistent, which is what keeps injury risk down.

In one study of runners training only a few hours a week, a mix with more moderate running matched mostly-easy training on every result.

Heart rate holds up as the way to control those easy days. The published critiques of heart-rate prescription argue over which anchor should set the range, not over heart rate as a control signal. In one trial of recreational runners, heart-rate-guided easy sessions stayed in the intended range for more of the session than pace-guided ones.

For the training itself, here's the range, the feel, and the fit with harder work:

What is Zone 2 training? Zone 2 is easy aerobic running at 60–70% of heart rate reserve, an effort you can hold for hours. It's how you build an aerobic base: a lot of easy miles, week after week.

Should you swap Zone 2 for HIIT?

No one in the published dispute says you should. The strongest evidence supports both.

The largest pooled analysis of endurance-training studies concluded that its findings "provide evidence that supports both sides of the debate, underscoring the importance of both training intensity and volume." What predicted adaptation best was training load, intensity times volume. Higher intensities can compensate for lower volumes, and the reverse. Mitochondrial content, the analysis found, rose to a similar extent whether the training was continuous endurance work or intervals. Intensity buys adaptation per hour; volume keeps adaptation moving past the first weeks.

The 2025 review that contests Zone 2's optimality argues the intensity side hardest. Its strongest claim against easy-only training is that cardiorespiratory fitness increases only with intensities above Zone 2, in trained athletes as well as healthy active people. The version of Zone 2 training it argues against directly is a week with no hard work in it.

In one randomized trial in well-trained athletes, the group combining a large easy majority with hard intervals improved more than groups emphasizing a single intensity.

San Millán's reply to the review says coexistence outright: "both are necessary and neither is sufficient." The review's own warning is aimed at people replacing hard exercise with Zone 2, not at easy volume trained alongside it. On coexistence, the review and its most prominent critic agree.