Q&A on Zone 2 Exercise with Peter Attia, M.D.

KEYPOINTS

• 01

  Attia defines Zone 2 by lactate, not by heart-rate percentages

  Wearables define zones as percentages of maximum heart rate, so a Garmin's 'Zone 2' (60–70%) doesn't match his. Attia uses blood lactate as a proxy for when your effort exceeds what your mitochondria can handle. Zone 2 is the highest output you can sustain while keeping lactate at or below about 2 mmol/L. He stresses that this is the definition he uses personally and with patients, following Iñigo San Millán.

• 02

  Why it matters: mitochondrial function declines with age

  Attia believes Zone 2 is the most important intensity for training mitochondria. Since mitochondrial deterioration is a hallmark of ageing, anything that delays it should help. The more Zone 2 you do, the better you are at both insulin-dependent and insulin-independent glucose uptake, which makes for very efficient glucose disposal.

• 03

  Dose depends on your starting fitness

  A deconditioned person might manage only 30 minutes three times a week, at an exertion so low it feels like nothing. Elite cyclists can spend 20–24 hours a week in Zone 2. Attia recommends at least three sessions a week (he does four). Iñigo San Millán thinks 45 minutes is about the minimum per session, so three 45-minute blocks beat daily 20-minute ones. Start at 30 minutes if you're new to the activity.

• 04

  Do Zone 2 before strength work, and stay in the zone

  If both are in one session, do Zone 2 first so cardio doesn't eat into your strength goals. Drifting into Zones 3–5 changes the stimulus, which is why Attia rides indoors where he can control his output. He admits riding indoors is boring, but says the control is worth it.

• 05

  Watts per kilogram is the number to track

  Attia thinks everyone should know their Zone 2 power in watts per kilogram, and care about it as much as VO2 max or deadlift strength. People with type 2 diabetes can have a Zone 2 threshold as low as 1 W/kg, while the fittest reach about 4 W/kg and are extremely carbohydrate-sensitive. Holding 3 W/kg for 45 minutes on any machine is very good.

• 06

  His training spans two extremes

  He trains almost only in Zone 2 and Zone 5, occasionally Zone 6, with almost no time in Zones 3 or 4, because he doesn't race. He is optimising for metabolic health. High-intensity interval training (HIIT), he argues, is overrepresented in research because it's easy to study: a four-minute Tabata twice a week is a simple 'pill' to give. That doesn't make it a substitute for Zone 2.

• 07

  Heart-rate formulas are starting points

  MAF (180 minus your age) and 80% of your true maximum heart rate are reasonable starting estimates, but they aren't the same thing. For Attia, 180 minus his age comes out about 8 bpm lower than his measured Zone 2. To use a percentage you need your actual maximum, for example from a carefully done Bruce protocol, the 21-minute cardiac stress test.

• 08

  Zone 2 sits well below FTP

  In power-based training every zone is a percentage of functional threshold power (FTP), which sits in upper Zone 4. So app-based zones built on FTP won't line up with a lactate-defined Zone 2, and you should calibrate with a lactate meter or your perceived effort.

• 09

  Metformin appears to lower Zone 2

  In Attia's experience with himself and patients, metformin consistently lowers Zone 2 thresholds. On and off the drug he sees a 15–20 W difference. Whether that matters is unknown. A study of non-diabetic people led by San Millán at the University of Colorado, delayed by COVID, is meant to look at it.

• 10

  Anecdotes: better deep sleep and post-workout carb tolerance

  He has seen no data on heart rate variability (HRV), but many patients with Oura rings report deep sleep rising from 45–60 minutes to about 90 minutes a night. After Zone 2, Attia can eat a couple of hundred grams of glucose with barely a rise in blood sugar. Both observations are anecdotal.