A 30-day Zone 2 experiment. Modest numbers — VO2 Max nudging from 45.6 to 46.1, resting heart rate dropping two beats. Not dramatic. But that's not what Zone 2 is about, and anyone selling you dramatic results in 30 days is selling something else.
The real claim here is subtler: consistent low-intensity aerobic work builds a more efficient cardiovascular engine. You run faster at the same heart rate. You breathe easier during everyday activity. The base becomes stronger, which raises the ceiling for everything above it. That's the actual promise of Zone 2 — and it's one the research consistently supports.
Here's what caught my attention. In our sauna research — specifically an article where physicians were debating overrated versus underrated health tools — one panel made a point worth sitting with: sauna produces cardiovascular stress that mirrors Zone 2 training. Heart rate elevates to the same range. Blood vessels dilate. The body works hard to regulate temperature through increased circulation and sweat.
For Contrast Collective's guests, this is not a footnote. This is a protocol. A 20-minute sauna session isn't just recovery. It's cardiovascular training without joint loading, without cortisol spikes from high-intensity effort. And Rhonda Patrick's work reinforces this — she's noted that intermittent fasting combined with ketosis meaningfully improves her own Zone 2 performance, pointing to metabolic flexibility as the underlying mechanism.
There's broad consensus that Zone 2 builds mitochondrial density and improves fat oxidation. The disagreement is around dose and exclusivity. Purists like Iñigo San Millán argue most people should spend 80% of training time in Zone 2. Others, including Huberman's more integrated approach, see Zone 2 as foundational but not complete — you still need the upper-end anaerobic work to maintain fast-twitch capacity as you age.
The speaker here arrives at the same conclusion instinctively: "I don't want to just strictly do it on slow low heart rate training." That's correct. Zone 2 is the floor you build from, not the ceiling you aim at.
150 minutes per week. Four sessions. Upper arm heart rate monitor — not wrist. Podcasts over pump-up music. Hydrate before, not during. And if you're time-constrained, know that your sauna session is doing genuine Zone 2 cardiovascular work alongside everything else it's delivering.
The most undervalued insight from this experiment isn't the VO2 Max improvement. It's the observation that Zone 2 left the participant with energy rather than depletion. That's mitochondrial efficiency expressing itself in real life — not on a treadmill screen, but in the hours after. More capable cells. A quieter cardiovascular system. The feeling of having more in reserve.
That's what a strong aerobic base actually feels like. Not performance in the moment. Capacity in everything else.