Regularly practice Zone 2 training, defined as an aerobic intensity that keeps blood lactate below 2 mmol/L.

holds up

The concept of Zone 2 is based on the optimization of mitochondrial function and metabolic efficiency.

noli's read

The concept of Zone 2 is based on the optimization of mitochondrial function and metabolic efficiency. Research, including observational studies and physiological trials on substrate metabolism (e.g., San Millán et al.), supports that this type of training improves the cells' ability to use fat as fuel and increases mitochondrial density. It is well established that this low-intensity training supports cardiovascular health while minimizing systemic fatigue, which allows for a high volume of training over the long term. What is sometimes exaggerated in popular discourse is the idea that this zone is the only path to longevity, when it should ideally be part of a structure that also includes high intensity (Zone 5) for maximal aerobic capacity (VO2 max). The approach is solidly anchored in sports physiology. No scientific evidence contradicts the utility of this practice, although the precise 2 mmol/L threshold may vary slightly from one individual to another depending on their metabolic profile.

the sources

Claimed by Peter Attia. Noli grades the claim, never the person.

Decoded by Noli · July 18, 2026 · the original post

What the graded evidence says

Evidence: C

Effect: endurance performance (VO2max, time trial) polarized vs threshold intensity distribution — polarized modestly favored in a small meta-analysis (only 3-4 RCTs); a larger 2024 review found no clear superiority over other distributions
Measured on: trained endurance athletes (>2 years experience); mechanistic mitochondrial data in adults

The head-to-head evidence for a specific intensity split is limited to a handful of small trials with mixed results, so precise prescriptions like exact Zone 2 percentages shouldn't be treated as settled science.