Build a zone 2 aerobic base

Log 3–4 hours per week of low-intensity aerobic exercise to develop mitochondrial density and fat-oxidation capacity.

Why it works

Zone 2 exercise — the intensity at which you can hold a conversation but feel genuinely challenged — primarily recruits slow-twitch type I muscle fibers and stimulates mitochondrial biogenesis. The result is a larger, more efficient oxidative base: your muscles can extract more oxygen per minute at any given heart rate, which is the underlying driver of VO2 max. Attia and exercise physiologists argue this base is a prerequisite for high-intensity work to compound on.

How to do it

1. Find your zone 2 heart rate: roughly 60–75% of max HR, or the intensity where you can speak in full sentences but would prefer not to.
2. Choose a low-impact modality (cycling, rowing, brisk walking, easy running) to sustain the duration without injury.
3. Accumulate 3–4 hours per week; sessions of 45–90 minutes are practical targets.
4. Resist going harder — tempo creep pulls you out of zone 2 and away from the mitochondrial stimulus.

Evidence

Zone 2 training reliably improves mitochondrial density and fat oxidation capacity in controlled studies. Its relationship to VO2 max is well established: aerobic base improvements underpin gains at higher intensities. (rct)

Much foundational work is in trained athletes; dose-response in sedentary adults is established but less precisely quantified.

Sources

• Holloszy & Coyle (1984), adaptations of skeletal muscle to endurance exercise, Journal of Applied Physiology

Common mistake

Spending zone 2 sessions in a "moderate" gray zone that is too hard for the mitochondrial stimulus but too easy for interval adaptation — often called "junk miles."