Zone 2 cardio

• Build an aerobic base: more mitochondria, more capillaries and a heart that pumps more blood per beat.
• Improve the muscles' ability to burn fat and clear lactate at a given workload (metabolic flexibility).
• Raise cardiorespiratory fitness, a strong predictor of long-term survival.
• Lower blood pressure and improve blood sugar and blood lipids.
• Add a large volume of training that is easy to recover from, so it can sit alongside strength training and hard intervals.

Exercise physiologists split intensity into domains by what happens in the blood and breath. As effort rises, lactate stays near baseline at first, then starts to climb at the first lactate threshold (also called the aerobic threshold or, measured by breath, the first ventilatory threshold). Higher still, it rises steeply at the second threshold. Popular "zone 2" sits just under the first threshold.

In 2025 a panel of 14 sport scientists and professional coaches agreed on exactly that definition: zone 2 is best done at intensities immediately below the first lactate or ventilatory threshold, as continuous, variable or interval-style sessions [20]. The same panel noted there was no clear consensus before, which is why apps, coaches and podcasts often mean slightly different things by it.

A source of confusion is that zone numbering differs between systems:

So when Stephen Seiler's research reports that athletes do little "zone 2", it means the hard-ish middle zone, not the easy zone that podcasts call zone 2 [18].

The lactate figure most people quote is about 2 mmol/L. In junior cross-country skiers, 71% of sampled sessions stayed at or under 2.0 mmol/L lactate [18], and Seiler's review describes the low-intensity work of elite athletes as about 2 mmol/L [5].

Heart rate zones need a maximum heart rate, and the classic 220 minus age formula is a poor fit. A meta-analysis of 351 studies with 18,712 people found that 208 minus 0.7 times age predicts maximum heart rate better, and that 220 minus age underestimates it in older adults [21]. Norway's HUNT study measured 3,320 healthy adults and found 211 minus 0.64 times age, with a standard error of 10.8 beats per minute [22]. That error means roughly a third of people sit more than 10 beats per minute away from any age-based prediction.

The second problem is bigger. Even with a measured maximum, the first threshold does not sit at a fixed percentage. In a lab study of 46 women and 54 men, the lactate threshold occurred between 45% and 74% of VO2max and between 60% and 90% of maximum heart rate [1]. A review of intensity methods concluded that fixed percentages of maximum heart rate or VO2max have little merit for placing people in the right domain, while the first lactate, gas exchange and ventilatory thresholds work better [23].

What this means in practice:

• A watch's zone 2 can be too easy for a fit person and too hard for a beginner.
• Fitter people tend to have their first threshold at a higher share of their maximum.
• Heart rate drifts upward over a long session at the same pace, especially in heat.
• Beta blockers and some other drugs change heart rate, so zones do not apply.

The cheapest check is the talk test. In a series of experiments, the last stage at which people could still speak comfortably tracked the ventilatory threshold, even when that threshold was moved by training or blood donation [24]. If you can say a full sentence but would rather not sing, you are close.

Mitochondria are the parts of muscle cells that burn fat and also clear lactate, a fuel made when glucose is broken down. San-Millán and George Brooks tested professional endurance athletes, moderately active people and people with metabolic syndrome. The athletes burned more fat and had lower lactate at the same workloads, and in every group fat burning fell as lactate rose (r = -0.76 across all data) [9]. They proposed that lactate and fat oxidation during exercise give an indirect read of mitochondrial function and "metabolic flexibility".

The intensity of peak fat burning also lines up with the first lactate rise. In 33 trained athletes, maximal fat oxidation happened at 63% of VO2max, close to where lactate first rose above baseline at 61% [17]. That overlap is why zone 2 is often described as the "fat max" zone.

San-Millán argues more broadly that mitochondrial dysfunction runs through type 2 diabetes, heart disease, metabolic syndrome and other common diseases, and that physical inactivity is one of its main drivers [25]. The basic biology is old and solid: in 1967, John Holloszy showed that running training increased mitochondrial oxygen use and respiratory enzymes in rat muscle [12].

What this chain of reasoning does not show is that zone 2 is the best way to get those adaptations. The athlete data are cross-sectional: athletes have high fat oxidation after years of mixed training that includes hard intervals, so the comparison cannot isolate zone 2. The next section covers what training trials found.

Fitness. A meta-analysis of 28 controlled trials in 723 healthy adults found continuous endurance training raised VO2max by 4.9 mL/kg/min and interval training by 5.5 mL/kg/min versus no exercise, a small extra 1.2 mL/kg/min for intervals [6]. In a well-known Norwegian trial with total work matched, long slow running at 70% of maximum heart rate did not raise VO2max significantly in 8 weeks, while 4 x 4-minute intervals raised it 7.2% [7]. In 48 already-trained athletes, a 9-week high-volume, low-intensity block gave no further gains, while a polarized block raised VO2peak by 11.7% [8].

Mitochondria. A review of training studies concluded that training volume drives mitochondrial content, while relative intensity drives mitochondrial respiratory function [10]. In a 4-week biopsy study, maximal mitochondrial respiration rose 25% only after sprint intervals; continuous cycling below the lactate threshold did not change it [11]. The 2025 review "Much Ado About Zone 2" concluded that current evidence does not support zone 2 as the optimal intensity for mitochondrial or fat oxidation capacity, and that higher intensities matter more at lower training volumes [4].

Blood pressure. Across 93 randomized trials, endurance training lowered blood pressure by 3.5/2.5 mm Hg overall and 8.3/5.2 mm Hg in people with hypertension [2]. Most of these programs were moderate continuous exercise, much of it in or near zone 2.

Fat loss. In 13 trials in adults with overweight or obesity, moderate continuous training and interval training reduced fat mass and waist size equally; intervals took about 40% less time [16]. Burning a higher share of fat during the session does not translate into more fat lost over weeks.

Survival. Pooled cohorts of 661,137 adults linked meeting one to two times the guideline minimum with 31% lower mortality, levelling off at three to five times the minimum [3]. Accelerometer data show light activity counts too [14]. No trial has compared zone 2 with other intensities for mortality.

For related lab markers, see triglycerides, HbA1c, fasting insulin and hs-CRP.

The public health baseline is clear: all adults should do 150 to 300 minutes of moderate or 75 to 150 minutes of vigorous aerobic activity a week, or a mix, plus muscle strengthening [13]. Zone 2 sessions are moderate-intensity minutes, so they count toward that.

Popular advocates go higher. Andrew Huberman recommends 180 to 200 minutes of zone 2 a week, and Peter Attia and Iñigo San-Millán discuss zone 2 dose and frequency at length (see the people and videos below). Elite endurance athletes train far more, 10 to 13 sessions a week, with about 80% at low intensity [5].

What the evidence supports:

• The first 150 minutes give the largest return. Below the minimum, risk was already 20% lower than doing nothing; one to two times the minimum gave 31%; three to five times gave 39%, with no further gain and no harm beyond that [3].
• Volume matters for mitochondrial content, so more easy hours do add something if you have the time [10].
• With limited time, mix intensities. The 2025 review argued that people training only a few hours a week get more from including higher intensities than from zone 2 alone [4].

A practical template for a healthy adult with 3 to 4 hours a week: two or three zone 2 sessions of 45 to 60 minutes, one interval session such as the Norwegian 4x4 or a short sprint session, and two strength sessions.

The comparison is often framed as a contest, but the research mostly shows complementary tools. Intervals give a little more VO2max per session [6] and much more per minute. Zone 2 gives volume at low stress, and volume drives mitochondrial content [10].

The strongest training pattern in endurance sport combines them. Elite athletes across sports do about 80% of sessions at low intensity and about 20% with periods of hard work near 90% of VO2max [5]. A small meta-analysis found this polarized pattern improved time-trial performance more than threshold-heavy training [19], though only 3 to 4 low-quality trials were available.

One practical reason to keep zone 2: hard sessions are harder to repeat. People can do many easy hours a week without heavy fatigue, while two or three hard interval sessions is usually the ceiling. For more on the hard end, see Norwegian 4x4 intervals and sprint training.

The debate is about emphasis, not about whether easy exercise works.

The case for. Iñigo San-Millán's lab work links low lactate and high fat oxidation to mitochondrial health [9], and he argues mitochondrial dysfunction underlies many chronic diseases [25]. Elite endurance athletes self-organize toward large volumes of low-intensity training [5]. Physicians such as Peter Attia and podcasters such as Andrew Huberman present zone 2 as a cornerstone of longevity training.

The case against. Kristi Storoschuk, Martin Gibala, Brendon Gurd and colleagues reviewed the evidence in 2025 and concluded the popular message rests largely on observational data from elite athletes, and that it contradicts substantial evidence for higher intensities in improving mitochondrial capacity and cardiometabolic health [4]. Even the expert panel that defined zone 2 said its expected adaptations might not be unique to it and could come from slightly higher or lower intensities [20].

Where they agree. Both sides accept that people who do more aerobic exercise live longer [3], that fitness strongly predicts survival [15], and that hard intervals and easy volume both have a place. The practical disagreement is about people with little time: critics say that group should prioritize some higher-intensity work.

What the evidence does not show

• No trial shows zone 2 extends life more than other intensities.
• No trial shows zone 2 builds mitochondrial function better than intervals in untrained adults.
• No study validates a single universal heart rate band for zone 2.
• Fat burned during a session is not the same as fat lost over time.

Because zone 2 is defined by your physiology, the same activity can be zone 2 for one person and too easy for another. The test is the same: steady breathing that is deeper than at rest, a slightly strained conversation, and a pace you could hold for an hour.

Good options:

• Stationary or road cycling. Easy to hold steady and low impact, which is why most lab studies use it.
• Incline treadmill or hill walking. Lets fit people reach zone 2 without running; adding a weighted pack (rucking) does the same.
• Easy running. Many runners must slow down a lot, sometimes to a jog-walk, to stay under the threshold at first.
• Rowing, elliptical, swimming. Useful when joints need a break.

For most fit adults, flat walking at a normal pace is below zone 2. For people starting from low fitness, older adults or those with heart or lung disease, brisk walking can already reach the first threshold. Device-measured studies show even light activity is linked to lower mortality compared with sitting [14], so walking is valuable whether or not it reaches zone 2.

• Going too hard: drifting into tempo pace on easy days, the most common base-phase mistake.
• Trusting the 220 minus age formula, which underestimates maximum heart rate in older adults, or a watch zone without checking the talk test.
• Judging a session by average heart rate, which can hide long stretches above zone 2.
• Thinking that a higher share of fat burned during exercise equals more fat lost.
• Doing only zone 2 when time is short; with 2 to 3 hours a week, some harder work raises fitness more.
• Chasing a lab number so strictly that sessions become stressful instead of easy and repeatable.

• Chest pain, pressure, fainting, unusual breathlessness or palpitations during exertion: stop and see a clinician before training.
• Known heart disease, heart failure, arrhythmia or uncontrolled high blood pressure: get clinician clearance and, ideally, a supervised exercise test first.
• Diabetes treated with insulin or drugs that can cause low blood sugar: plan glucose checks and snacks with your care team.
• Beta blockers or other drugs that change heart rate: heart rate zones do not apply; use the talk test and perceived effort.
• Pregnancy, recent surgery, or a recent illness with fever: check with your clinician about intensity and volume.

• Overuse injuries such as tendon or shin pain when weekly volume rises too fast.
• Boredom and time cost, which are common reasons people stop.
• Low energy availability if large volumes are added without eating enough.
• Very low acute cardiac risk; vigorous exertion raises the short-term risk of sudden death far more than moderate exercise, and habitual exercisers have the smallest rise [@albert2000].