“Current Evidence Does NOT Support Zone 2 Training”

The standard advice from endurance coaches is that meaningful zone 2 gains require 1–1.5 hours at least four times per week — an impractical volume for most. Stanfield’s rationale is that if you can only exercise a few hours weekly, every minute spent at zone 2 crowds out time that could be spent at higher intensities, which deliver a greater return per minute for mitochondrial function and especially for VO2 max. The better your baseline fitness, the more this holds true because a trained body needs a stronger stimulus to adapt. The protocol therefore recommends meeting the 75-minute vigorous target first, including power training to combine aerobic and neuromuscular benefits, and only layering in zone 2 as a bonus activity.

The analysis challenges the elite-athlete inference: those athletes also do substantial high-intensity work, making it unclear which intensity drives their adaptations, and their 20+ hours/week training is irrelevant for people struggling to fit in 2 hours. Looking directly at the claims, the authors examined mitochondrial signaling pathways. They found that zone 2 intensity provides minimal stress to cellular energy systems — one study detected significant energetic strain only after 2 hours of continuous exercise, meaning that for time-limited non-elites, zone 2 is unlikely to trigger mitochondrial adaptations. A meta‑analysis of outcomes showed that for non-endurance-trained individuals, zone 2 intensity did not improve mitochondrial function, whereas higher-intensity training (e.g., HIIT) consistently stimulated it. For fatty acid oxidation, only one study measured fat oxidation rates after repeated, verified zone 2 training, showing an increase after a full year, but overall studies that compare intensities yield conflicting results, with some favoring low intensity, some favoring high intensity, and a recent meta-analysis finding similar improvements across intensities. Thus, zone 2 cannot be claimed to be superior for fat oxidation. The authors conclude that for time‑constrained non‑elites, higher intensities above zone 2 are the more effective strategy.

Stanfield points to a study of heart disease patients followed for 8 years: those with the best VO2 max had an 84% lower mortality risk than those with the worst. He notes that the link between VO2 max and all-cause mortality or heart disease is much stronger than the link with mitochondrial health. Because higher-intensity exercise is more effective at improving VO2 max than zone 2, the limited time that most people have for exercise should be allocated to high-intensity work to maximize this powerful health determinant. This practical reframing moves the debate from ‘which intensity builds more mitochondria’ to ‘which intensity most reduces death risk,’ and the data favor intensities above zone 2.