Much Ado About Zone 2: What I Learned Reading the Papers Instead of the Podcasts

Six in the morning in Villanova, one of Cagliari’s old quarters, where I’ve lived since March. The streets around home go up before they go anywhere else, and a couple of minutes into the first climb my Apple Watch buzzed: 144 bpm, the ceiling I’d set for easy runs.

I knew the script. Zone 2 is where the mitochondria get built. Go above it and the session is wasted. Walk the hill, keep the heart rate down, trust the process. I had absorbed it from podcasts, from people quoting the podcasts, and from the comments under both. So on that climb I could walk, because someone with a microphone says this intensity is magic, or keep running and accept that I was doing it wrong.

I kept running. The run topped out at 161, one beat under the threshold heart rate I’d estimated in May. Then I did what I do when an engineering claim smells off: I went to the primary sources. This is what I found, plus my own numbers, which turned out messier than the story I wanted to tell with them.

Where the rule comes from

The popular Zone 2 has a clear parent: Peter Attia’s Outlive (2023). In the book, Zone 2 is the base of training for longevity: blood lactate “ideally between 1.7 and 2.0 millimoles”, a pace where “you’re able to talk in full sentences, but just barely”, and about three hours a week, or four 45-minute sessions, as the minimum for most people. Attia credits that dose to conversations with physiologists, Iñigo San-Millán first among them, not to a trial.

San-Millán supplies the science. In his 2018 paper with George Brooks, 22 professional cyclists cleared lactate and burned fat far better than moderately active men or men with metabolic syndrome. But it’s a snapshot, not a training study, the groups also differ by age (27, 40, 55) and body fat (10%, 14%, 44%), and the word “zone” never appears. It shows that pros are metabolically different from sedentary people, not that Zone 2 made them that way.

The third piece is Stephen Seiler’s work on how elite endurance athletes distribute intensity. Across sports he kept finding the same shape: about 80% of sessions easy, about 20% hard. That became “80/20”, then “polarized training”, and by the time it reached my earbuds, “do mostly Zone 2”.

Two details rarely survive the trip. The athletes were nationally and internationally competitive, “training 10 to 13 times per week” (Seiler 2010), typically 15 to 30 hours (Seiler and Tønnessen 2009). And 80/20 counts sessions, not minutes. In Seiler and Kjerland’s 2006 study of junior cross-country skiers, the same month of training splits 75/8/17 by session goal and 91/6/3 by heart-rate time in zone.

Then there’s vocabulary. Seiler’s analyses use three zones anchored on two thresholds: Zone 1 below the first lactate threshold (LT1), Zone 2 between LT1 and LT2, Zone 3 above LT2. The popular Zone 2 comes from five-zone models, and a 2025 expert panel that includes Seiler places it “just below LT1 or VT1” (Sitko et al.). So Attia’s Zone 2 is roughly the top of Seiler’s Zone 1, while Seiler’s Zone 2 is the threshold band that polarized training tells you to keep small.

The denominator problem

Eighty percent is a ratio, and a ratio means nothing without its denominator. Mine first: about three hours a week of running. With the spin bike, the volley and now the gym, my total load is higher, so there are two separate questions: how to split the running hours, and how much load I can absorb overall. The table is about the first. The podcast version reads 80/20 as time, so take it at its word:

Ten easy hours a week are a stimulus in their own right, and the elite rations the hard share because recovery is the bottleneck: nobody absorbs ten hard sessions a week. 80/20 answers their constraint. Count by the clock instead, with the 91/6/3 split from Seiler and Kjerland’s skiers, and it gets stranger: a twelve-hour week holds about twenty minutes above LT2, not far from my Friday hills. What I can’t match is the other nine-plus hours. Turning my hard minutes into easy ones doesn’t buy them; it only spends the one stimulus I can afford. And counted the way Seiler counted, by sessions, a recreational runner with one hard run out of four is already close to 80/20. It’s reading the ratio as minutes that turns it into “do mostly Zone 2”.

For my running, my working hypothesis is that stimulus limits me more than recovery does. It’s a hypothesis, not a fact: with volley and the gym on top, at my age, it’s the first thing I’ll have to keep checking. If it holds, copying the ratio anyway is a three-person startup adopting Google’s microservices architecture because that’s how Google does it: the pattern is real, the conditions that made it right aren’t there. Seiler raised this himself in 2009, asking whether the pattern suits “recreational athletes training 4-5 times and 6-10 hours per week” and noting that “there are almost no published data addressing this question”. In the same passage he reported unpublished findings favouring polarized training in recreational runners. In Muñoz et al. (2014), a trial in recreational runners, the main between-group comparison (10 km improvements of 5.0% versus 3.5%) was not statistically significant.

Attia now says much the same. In “A better way to think about Zone 2”, co-signed with two colleagues on his site on 29 August, he writes that “Zone 2 isn’t magic” and that with two hours a week “I might skip a dedicated Zone 2 training altogether”, and admits the confusion is “something I’ve probably contributed to”. The version living in my head, and on my wrist at 144 bpm, was still the 2023 one.

What the evidence says

I stole this post’s title from a 2025 narrative review by Storoschuk, Moran-MacDonald, Gibala and Gurd, which went looking for the evidence behind the popular claims. The conclusion is blunt: “current evidence does not support Zone 2 training as the optimal intensity for improving mitochondrial or fatty acid oxidative capacity,” and “prioritizing higher exercise intensities (> Zone 2) is critical to maximize cardiometabolic health benefits, particularly in the context of lower training volumes.” Two caveats: it’s a narrative review, not a systematic one, and among the studies it lists in untrained people, eight found no difference between intensities for fitness.

Inglis et al. (2024) is a concrete example. Young, mostly sedentary adults cycled three times a week for six weeks in one of five intensity groups, from moderate (just below LT1, roughly popular Zone 2) to all-out sprints, with most groups matched for total work. Every group above moderate improved VO2max significantly more than a non-exercising control, gaining 3.3 to 6.2 mL/kg/min. The moderate group gained 1.8, not significantly more than the control’s 0.1, despite being the only group to log the guideline 150 minutes a week. The heavy and interval groups did about 120, 90 and 60 to 72 minutes of work a week; the sprint group built up to six 30-second sprints per session, three times a week, which is 9 minutes of sprinting but, with a 10-minute warm-up and 4.5-minute rests, about 35 minutes per session. The higher-intensity protocols produced larger VO2max gains in this study; the time savings were smaller once recovery and warm-up were counted. It did beat the control on peak power.

Matomäki et al. (2024) is the cleanest volume-versus-intensity test I found. Untrained adults cycled for ten weeks: one group 6.7 hours a week at about 63% of max heart rate, the other 1.6 hours a week of intervals. In a quarter of the time, the interval group gained far more VO2max (+0.32 versus +0.02 L/min), more power at the second threshold and more peak power. The low-intensity group matched it on power at LT1, and a companion paper found durability improved equally in both. The title picks its words: volume “cannot entirely substitute for the lack of intensity”. And 63% of max heart rate sits at or below the bottom of Attia’s Zone 2, so a Zone 2 defender can fairly say this tested something easier.

Older work points the same way, if less dramatically: Milanović et al.’s 2015 meta-analysis puts intervals ahead of continuous training for VO2max by a “possibly small” 1.2 mL/kg/min.

None of this makes Zone 2 useless. Granata et al. (2018), pooling dozens of training studies, found that volume tracks gains in mitochondrial content, so easy hours buy something. For health outcomes, the epidemiology says any moderate-to-vigorous activity beats none (Kraus et al. 2019, a paper that comes back later). The argument is about fitness per hour: with three hours a week, spending all of them below LT1 gives up the stimulus that most reliably moves VO2max.

Why the question changes at 50

Age changes what limits you. Capelli, Ferretti, di Prampero and Tam (2025) re-analysed published data on mostly moderately active men, splitting the limit on VO2max into a central share (the heart delivering oxygen) and a peripheral share (the muscle extracting and using it). The peripheral share grows with age: about 23% under 30, around 30% from the mid-forties to the mid-sixties (arithmetic on their table), and 44% in the single data point they have for 85-year-olds. The heart stays the bigger limiter, and the authors say so.

The muscle side is where the training literature has good news. Mølmen, Almquist and Skattebo’s 2025 meta-regression of aerobic training, from continuous work to sprints (353 studies for mitochondria, 131 for capillaries), found mitochondrial gains “not influenced by age”, capillary growth in people over 55 “comparable” to younger ones, and concluded that “the ability to adapt to exercise training is maintained throughout life.” The catch is that absolute VO2max gains shrink: 4.8 mL/kg/min under 35, 2.9 between 35 and 55, 2.2 over 55.

That’s the cardio half. The other half is strength, and it’s the second cost of filling a short week with easy miles: an hour of Zone 2 is an hour not spent lifting. I’m only now starting, with a personal trainer. I lost seven kilos this year, and a scale can’t tell me how many were muscle. Lahav et al. (2026) followed 304 dieters at an Israeli nutrition clinic with DXA scans: among men, those doing resistance training lost the most fat and were the only ones to gain fat-free mass, for similar total weight loss (among women, resistance training won on lean mass but not on fat). It’s a retrospective cohort in which people picked their own exercise, so it’s a signal rather than a verdict.

N=1: my numbers

Yesterday I had my annual sports-medicine check, exactly a year after the previous one, with the same ramp test on a cycle ergometer: 40 W more every minute, stopped when my heart rate reaches 150 bpm. It isn’t a maximal test (I could have kept going for quite a while), so the number below is the power I produce at 150 bpm, not my peak.

Power at a fixed heart rate is a crude marker: heart rate trails a ramp this steep, and it moves with sleep, heat and coffee. But it’s the same crude marker both years. The 2026 test also came five days after I donated blood, which lowers hemoglobin (by about 8% at day three in Ziegler et al. 2015), so each beat carried less oxygen; if anything, the second number is a little low. The line I care about most is the last one: fifteen points of systolic pressure in a year.

Now the caveat this section exists for. In September 2025 I was living at my house by the sea. I moved to Cagliari in March and only then started training properly; from June to August a back problem stopped me completely, and I started again at the beginning of September. Add seven kilos lost, and a plan (the one below) that has been in place only since mid-September. Too many things changed at once for an N of 1 to untangle. These numbers say I’m fitter. They don’t say why.

The number that sent me digging was a Polarization Index. intervals.icu gave one of my easy hilly runs in mid-September a PI of 0.27, and my first reading was that I’d been doing threshold training by accident. Treff et al. (2019), who defined the index, disagree: anything at or below 2.00 just means “not polarized”, and values between 0 and 2.00 “must not be interpreted” any further. The index describes weeks of training, not one run, and my zones have never been validated. As evidence it’s worth close to nothing, but it got me to open the trace.

Two easy runs from home, on 16 and 18 September: 40 minutes each, average heart rate 134-136, max 161. Two days later I moved the long easy run to the Poetto, Cagliari’s long, flat beachfront: 55 minutes, average 136, max 150. The terrain differed, the run was longer and the 20th was hotter, so this is no controlled comparison. What the runs share is the intention and the average heart rate. The reading I can defend is narrow: at the same average, the flat removed the peaks. On the hills, my easy runs came with spikes near threshold that I hadn’t planned and wasn’t counting.

What I changed

The reading didn’t push me toward more Zone 2. It pushed me toward being deliberate: easy days that are actually easy, hard work concentrated where I choose to put it, and strength on the calendar. Here’s the week. No study above tested anything like it: it’s my reading of the evidence, not its conclusion. Heart-rate caps are based on that estimated threshold of 162:

On top of that: some weekend volley matches, an hour’s walk most days, and in summer, swimming in the sea.

The caps are looser than a strict Zone 2 plan for a reason no paper gave me. To keep my heart rate inside Zone 2 I’d have to run between 6:30 and 7:00 per kilometre, and at that pace my running mechanics are bad enough that my knees and back pay for it. So I’d rather run between 6:00 and 6:30, even when that takes me out of Zone 2. On a bike that problem doesn’t exist, so Thursday’s spin session is where Zone 2 by the book fits best. A heart-rate zone is a target for the heart; the rest of the body gets a vote too.

The long run moved to the flat because of the comparison above. Judged by VO2max per hour it looks like my least efficient session, but that’s one yardstick; in Matomäki’s study the low-intensity group improved on low-intensity measures that VO2max misses. I keep it for time on my feet, tendons, and durability (how well your physiology holds up as a long effort drags on), which in Matomäki’s companion study improved as much with low intensity as with intervals. The Wednesday steady has no ceiling on purpose. The middle zone isn’t evil; it just shouldn’t be every run’s default. In recreational runners, the evidence so far doesn’t establish a clear, general advantage for polarized training (see Sources). Next to the evidence above, that reads to me as: you need some intensity, and past that the exact ratio matters less than the podcasts suggest. The Friday hills are that intensity.

Three things I’m keeping

1. At low volume, intensity matters more than the label. With three hours a week, the hard sessions are the ones to protect. Turning them into more Zone 2 trades your most effective minutes for easier ones.
2. The long easy run stays, for durability, tendons and time on my feet, without assuming Zone 2 has mitochondrial benefits no other intensity offers.
3. After fifty, strength isn’t optional. It’s the one item on this list I’m only starting now, and the best bet I’ve found for making sure the weight I lose is fat.

The watch still buzzes on Monday if my recovery run creeps past 140. On Friday, on the hills, there’s no alarm at all.

Postscript: how to read fitness journalism

Halfway through all this, I read a Healthspan article (April 2026) titled “The Real Reason VO₂ Max Declines With Age”. It’s built on the Capelli paper above, and its first half transcribes the numbers correctly. Then it comes apart:

• All 13 links to its reference [3] go to Kraus et al. 2019, the mortality review cited above, which never mentions mitochondria, capillaries or VO2max. Eleven of those sentences carry numbers from the Mølmen meta-analysis.
• It says sarcopenia strips away “type II muscle fibers, the fast-twitch fibers that are particularly rich in mitochondria and oxidative capacity.” That’s backwards, and its own reference, Ørtenblad et al. (2018), says type I fibers “have the highest oxidative capacity.”
• It says that “by late middle age, nearly half of the limitation on VO₂ max is peripheral in origin.” Capelli’s 44% is the single data point at 85; in late middle age it’s about 30%.
• It presents Mølmen’s “endurance training” results as Zone 2 benefits, but that category covers all continuous training below the second threshold, threshold work included. And where it says intervals produced “notably larger” mitochondrial gains, Mølmen found them “similar.”

I’d love to make this a story about bad popular science, but my own outline failed the same test. I drafted it with an AI assistant, and five claims in it were stronger than the data: the PI as proof of accidental threshold training, the flat-versus-hills comparison as “same effort”, Capelli’s muscle as nearly as limiting as the heart at my age, the muscle as “the more trainable part”, which the paper never says, and my ergometer result as “peak power” from a test that stops at 150 bpm. The third is the Healthspan error almost word for word. All five pointed toward a better story.

That goes a step past what I wrote about a viral tweet on ChatGPT and creativity. The errors that survive are the ones that improve the narrative, and a model helping me write a post wants a good post exactly as much as I do. Opening the PDF is the one step where nobody is optimizing for the story.

Methodology note

I wrote this with Claude as a research assistant. It found and read the papers, in full text except for Outlive (quoted through published excerpts), the Milanović meta-analysis (abstract and accepted manuscript), and the blood-donation study (abstract), and every figure in the post was checked against its source. The training data are mine, from an Apple Watch and intervals.icu; the clinical numbers come from my two sports-medicine checks. I’m not a physician or a coach. This is one person’s reading of the literature, not advice. If you’re past fifty and about to start hill repeats, get the medical check first.

Sources

Zone 2 and its origins

• Attia P, Gifford B. Outlive: The Science and Art of Longevity. Harmony; 2023.
• Nelson, Yeater, Attia P. A better way to think about Zone 2. peterattiamd.com, 29 August 2026.
• San-Millán I, Brooks GA. Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and less-fit individuals. Sports Med. 2018;48(2):467-479.
• Seiler S. What is best practice for training intensity and duration distribution in endurance athletes? Int J Sports Physiol Perform. 2010;5(3):276-291.
• Seiler KS, Kjerland GØ. Quantifying training intensity distribution in elite endurance athletes: is there evidence for an “optimal” distribution? Scand J Med Sci Sports. 2006;16(1):49-56.
• Seiler S, Tønnessen E. Intervals, thresholds, and long slow distance: the role of intensity and duration in endurance training. Sportscience. 2009;13:32-53.
• Sitko S, Artetxe X, Bonnevie-Svendsen M, et al. What is “Zone 2 Training”?: Experts’ viewpoint on definition, training methods, and expected adaptations. Int J Sports Physiol Perform. 2025;20(11):1614-1617.

Intensity, volume and adaptation

• 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 Med. 2025;55(7):1611-1624.
• Inglis EC, Iannetta D, Rasica L, et al. Heavy-, severe-, and extreme-, but not moderate-intensity exercise increase V̇O2max and thresholds after 6 wk of training. Med Sci Sports Exerc. 2024;56(7):1307-1316.
• Matomäki P, Heinonen OJ, Nummela A, Kyröläinen H. Endurance training volume cannot entirely substitute for the lack of intensity. PLoS One. 2024;19(7):e0307275.
• Matomäki P, et al. Durability is improved by both low and high intensity endurance training. Front Physiol. 2023;14:1128111.
• Milanović Z, Sporiš G, Weston M. Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials. Sports Med. 2015;45(10):1469-1481.
• Granata C, Jamnick NA, Bishop DJ. Training-induced changes in mitochondrial content and respiratory function in human skeletal muscle. Sports Med. 2018;48(8):1809-1828.
• Kraus WE, Powell KE, Haskell WL, et al. Physical activity, all-cause and cardiovascular mortality, and cardiovascular disease. Med Sci Sports Exerc. 2019;51(6):1270-1281.

Ageing and strength

• Capelli C, Ferretti G, di Prampero PE, Tam E. Cardiovascular and peripheral factors affecting the decay of maximal oxygen uptake across the spectrum of age in humans. Eur J Appl Physiol. 2026;126(3):1635-1643 (published online October 2025).
• 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;55(1):115-144.
• Lahav Y, Yavetz R, Gepner Y. Resistance training as a key strategy for high-quality weight loss in men and women. Front Endocrinol. 2026;16:1725500.

My numbers and the training plan

• Ziegler AK, Grand J, Stangerup I, et al. Time course for the recovery of physical performance, blood hemoglobin, and ferritin content after blood donation. Transfusion. 2015;55(4):898-905.
• Treff G, Winkert K, Sareban M, Steinacker JM, Sperlich B. The Polarization-Index: a simple calculation to distinguish polarized from non-polarized training intensity distributions. Front Physiol. 2019;10:707.
• Muñoz I, Seiler S, Bautista J, España J, Larumbe E, Esteve-Lanao J. Does polarized training improve performance in recreational runners? Int J Sports Physiol Perform. 2014;9(2):265-272.
• Festa L, Tarperi C, Skroce K, La Torre A, Schena F. Effects of different training intensity distribution in recreational runners. Front Sports Act Living. 2020;1:70.
• Rosenblat MA, Watt JA, Arnold JI, et al. Which training intensity distribution intervention will produce the greatest improvements in maximal oxygen uptake and time-trial performance in endurance athletes? A systematic review and network meta-analysis of individual participant data. Sports Med. 2025;55(3):655-673.

The case study

• Tawfik D, Pinzone AG. The Real Reason VO₂ Max Declines With Age. Healthspan, 4 April 2026.
• Ørtenblad N, Nielsen J, Boushel R, Söderlund K, Saltin B, Holmberg HC. The muscle fiber profiles, mitochondrial content, and enzyme activities of the exceptionally well-trained arm and leg muscles of elite cross-country skiers. Front Physiol. 2018;9:1031.