"Zone 2 exercise intensity is the best at stimulating mitochondrial function and fat oxidation." That line comes from a 2022 podcast episode by Inigo San-Millan, who works as exercise physiologist to Tour de France winner Tadej Pogacar's team. Peter Attia and Bill Gifford's bestseller "Outlive" pushed the idea further, crediting Zone 2 with "preventing chronic disease by improving the health and efficiency of your mitochondria." Both lines are quoted verbatim in a peer-reviewed paper published last year, not because the authors agreed, but because they wanted to see the receipts. Neither man's camp was contacted for this piece; what follows checks their already-published claims against already-published data, not new reporting on either.
They did not find many. Kristi Storoschuk, Andres Moran-MacDonald, Martin Gibala, and Brendon Gurd, at Queen's University and McMaster University in Canada, published "Much Ado About Zone 2" in Sports Medicine in June 2025. It is a narrative review, not a systematic one, and the authors admit as much: their own methods section says they could not find enough trials that explicitly prescribed "Zone 2" as it is commonly defined, so alongside targeted database searches they pulled in studies "shared through social media" and leaned on physiological stand-ins, blood lactate, heart rate, percent of VO2max (the standard measure of aerobic fitness capacity), to decide what counted as evidence. That is worth holding the review to, given how strictly it holds everyone else. With that caveat attached, the verdict is still blunt: the claim that low-intensity training is uniquely optimal for building mitochondrial capacity does not survive contact with the studies that actually tested it. Where matched comparisons exist, harder training usually wins, or at worst ties.
Zone 2 is itself a slippery target, which the review admits. Popular usage defines it as exercise below the first lactate threshold, roughly 1.7 to 2.0 millimoles of lactate per liter of blood, low enough to hold a conversation. But the review's own lab data show that threshold landing anywhere from 23 to 57 percent of peak power output across three test subjects, and its upper edge ranges from about 24 to 80 percent of VO2max depending on fitness. In absolute terms, Zone 2 for an untrained person might be a brisk walk; for an elite cyclist it can mean 300 watts, roughly ten times resting metabolic rate. Two people can both be "doing Zone 2" while doing entirely different things to their bodies.
That gap is the seam the popular narrative runs through. The pitch points at elite endurance athletes, who log huge low-intensity volumes and also have exceptional mitochondrial capacity. That reads to me as correlation sold as causation, and the review lays out two reasons of its own not to make the leap: those same athletes also do plenty of high-intensity work, so crediting the slow miles alone is not warranted, and they are often training more than 20 hours a week, a volume with nothing to do with the 150 minutes public health guidelines ask of everyone else. You cannot borrow a training philosophy from someone doing eight times your weekly dose.
The matched-load evidence bears this out. Five months of training that was 86 percent Zone 2, seven days a week, failed to raise two enzyme markers of mitochondrial capacity in elite endurance athletes in one 1999 study; high-intensity training in the same population moved one of those markers. A 42-day cross-country skiing expedition at roughly 60 percent of max heart rate, essentially Zone 2 stretched over six hours a day, reduced mitochondrial respiration, even though peak leg and lung oxygen uptake held steady throughout, which the study's own authors read as the body trimming mitochondrial capacity that had exceeded what its actual oxygen demand required, not proof the training simply failed. Four weeks of Zone 2 training in recreationally active men produced no change at all. A 2018 meta-analysis pooling 56 training studies found exercise below 60 percent of maximum work rate, an intensity the review says typically sits at or above what most non-athletes call Zone 2, is not expected to improve mitochondrial content or respiratory capacity, while intensities above 90 percent and sprint intervals were the most effective options measured.
Fat-burning capacity gets a kinder verdict, but not the one being sold. Zone 2 does appear to raise fat oxidation, mainly in people who started out sedentary or overweight. But two recent meta-analyses comparing high-intensity intervals against moderate continuous training found either no difference in fat-oxidation gains, or a modest edge for the harder option in people carrying more weight. The review's own conclusion: there is no convincing evidence that Zone 2 "elicits unique benefits for [fat oxidative] capacity that cannot be achieved by" harder exercise.
The sharpest section concerns cardiorespiratory fitness, the VO2max number every watch now estimates and one of the most consistently validated predictors of cardiovascular disease and all-cause mortality in the literature. Head-to-head trials comparing Zone 2 and higher intensities for fitness gains split three ways: no difference in some, a clear edge for higher intensity in others, and among healthy active people and trained athletes specifically, gains that showed up only in the higher-intensity groups. Even the American College of Sports Medicine's own guidelines, which recommend at least 150 minutes a week of moderate activity, acknowledge that volume alone may not raise cardiorespiratory fitness, and suggest adding intensity or duration when fitness is the actual goal.
None of this is an argument against walking, easy runs, or a Sunday spin you can talk through. The authors say directly that they are not questioning the value of low-intensity movement. What they are contesting is the claim that Zone 2 is the singularly optimal intensity for mitochondrial or metabolic health, a claim used to tell people with an hour or two of free time a week to avoid harder efforts in favor of gentler ones. Their own words: "members of the general public who replace [higher intensity exercise] with Zone 2 exercise may risk minimizing the benefits of exercise on long-term health."
One disclosure worth pricing in: Martin Gibala, one of the review's own co-authors, discloses he is an advisor to and holds equity in Longevity League, a company that also sells exercise-related services. That does not undo the data, most of it decades old and independently repeated, but it is a reminder that the fitness-longevity space has more than one commercial narrative competing for the same anxious hour of your week. The evidence itself does not care whose name is on the zone. It just says that if your training time is limited, spend more of it working harder, not gentler.
