Zone 2 training vs HIIT — which is better for longevity?

Despite Zone 2's popularity in longevity circles, the actual evidence doesn't back its central claim. A 2025 expert-consensus panel of 14 sport scientists couldn't agree on how to even define "Zone 2," and concluded the mitochondrial and fat-burning adaptations it's marketed for aren't unique to that intensity. HIIT, by contrast, is backed by several randomized controlled trials and meta-analyses showing real fitness and body-composition gains — often matching or beating moderate-intensity training in less time. That doesn't make Zone 2 useless (easy aerobic work has its place), but the evidence gap between "science-backed" and "popular narrative" is unusually wide here, in HIIT's favor.