Cardio and Zone 2: the evidence behind the trend

Zone 2 became popular through podcasts and elite endurance coaching. The idea is that long, easy sessions below the lactate threshold build the mitochondria and fat-burning capacity of muscle, and that this is the foundation of metabolic health. Part of that is true. Part of it is an extrapolation from elite athletes that the evidence in ordinary adults does not support.

What Zone 2 is

Zone 2 is low-intensity exercise below the first lactate threshold. In practice it is a pace where you can hold a conversation in full sentences but would rather not sing: a brisk walk uphill, an easy cycle, a slow jog for fitter people. Heart rate zones from a watch approximate it, but the formulas vary and individual thresholds differ, so the talk test is as good a guide as most.

Why fitness is worth chasing

Cardiorespiratory fitness, usually measured as VO2 max or METs on a treadmill test, is one of the strongest predictors of how long people live.

The first two are observational: fitter people live longer, but fitness tracks with many other things. The training trials show that fitness can be raised by both continuous and interval training, and the cohort data suggest raising it is worth doing.

What the 2025 review argued

Storoschuk and colleagues, writing in Sports Medicine in 2025, looked specifically at whether Zone 2 is the best intensity for improving mitochondrial capacity and fitness in the general population. Their conclusion: the case for Zone 2 rests largely on observations of elite endurance athletes who train for many hours a week, mostly at low intensity. In people with a few hours a week, the evidence points to higher intensities doing more for mitochondrial capacity and cardiometabolic health. Zone 2 still works; it is just not uniquely effective, and it needs a lot of time.

How much cardio the guidelines ask for

The US Physical Activity Guidelines and the WHO both set 150 to 300 minutes a week of moderate-intensity aerobic activity, or 75 to 150 minutes of vigorous activity, or a mix. Seventy-five vigorous minutes count the same as 150 moderate ones. The guidelines also say that more brings additional benefit, and that the least active gain the most from any increase.

Telling the intensities apart

A MET is the energy cost of sitting quietly; 3 METs is three times that. Fitness changes where a given activity falls: a brisk walk is moderate for most people, vigorous for someone very unfit and light for a trained runner. The talk test adjusts for that automatically.

A sensible mix

• Mostly easy. Two to four sessions of 30 to 60 minutes at a conversational pace. This is where Zone 2 fits: it is easy to recover from, low in injury risk and builds the habit.
• Some hard. One session a week of intervals once you have a base: for example, four to six efforts of three to four minutes at a hard but controlled pace, with equal easy recovery. This is the part that raises VO2 max fastest.
• Walking counts. Brisk walking is moderate intensity for most people. See daily steps.
• Keep the strength work. Cardio does not replace the two days of muscle-strengthening the guidelines ask for. See strength training for beginners.

If you have heart disease, chest pain on exertion, or have been inactive for a long time with risk factors, check with a clinician before starting hard intervals.

Measuring progress at home

Without a lab test, three signals are useful. Resting heart rate, taken the same way each morning, tends to fall as aerobic fitness improves. The pace or power you can hold at the same easy effort tends to rise. And many watches estimate VO2 max from heart rate and pace; the absolute number is an estimate with real error, but the trend over months on the same device is informative. Compare yourself with yourself, not with the chart on the watch.

Cardio and weight loss

Aerobic exercise burns energy and improves fitness and blood sugar, but on its own it does less to protect muscle during weight loss than resistance training. The 2025 GLP-1 nutrition advisory recommends both: at least 150 minutes of moderate aerobic activity a week plus strength training at least three times a week. See exercise to keep muscle during weight loss.

Logging it

Zone 2 is defined by intensity, so heart rate and time matter more than distance. Peptide AI pulls workouts, heart rate and resting heart rate from Apple Health, Oura, WHOOP and Garmin through wearable integrations and puts them next to the training log. Resting heart rate falling over months is one of the simplest signs that the cardio is working.