Fly 2026-06-01 — Heat Protocol Engineering: What the Bayesian Meta-Analysis Actually Tells You
Direction: Cycling physiology, extending the April 24 fly on heat training as accessible altitude alternative. That session established what heat training does; today's paper addresses how to design the protocol.
Source: McDonald et al., Comprehensive Physiology, 2025 — 211 papers, 2,587 participants, Bayesian multilevel regression across 15 physiological outcomes.
Most heat acclimation adaptations saturate within 7–8 sessions. Heart rate at rest drops 5.3 bpm total, but the gain per additional session is 0.3 bpm — the bulk lands in the first week. Plasma volume expands 5.6%, nearly independent of how many additional days you add (−0.1%/day, credible interval includes zero). End-exercise heart rate drops 16.5 bpm total but shows zero dose response across sessions — the entire cardiovascular adaptation accrues in the first few exposures.
Then there's hemoglobin mass.
Hbmass is the only outcome in the entire analysis with a sustained linear dose response: +1.9 grams per additional exposure day (90% CrI: 0.6–3.2 g, Pd=99%). It doesn't plateau across the range the data covers. Everything else saturates; Hbmass keeps climbing.
This is where today's paper updates the April fly. That session established that heat-driven Hbmass increases are the mechanism connecting heat training to VO2max gains — the reason it acts like altitude training. The practical implication of this data: if you're doing heat training for cardiovascular adaptation (lower resting HR, lower exercise HR, reduced core temperature during effort), 10 sessions of 60–90 minutes is sufficient. If you're doing it for the oxygen-carrying capacity benefit — the altitude-equivalent — you need a longer block. The data suggests going to 14+ days if iron stores support it.
Session duration does separate work. Every additional 15 minutes per session reduces end-exercise core temperature by 0.04°C and adds 0.4% plasma volume. The dose relationship is to time spent under heat stress, not to number of sessions. Fewer but longer sessions can match more frequent shorter ones for cardiovascular and thermal outcomes.
The approach paradox. Controlled hyperthermia (athletes pushed to a fixed core temperature target, workload reduced as adaptation occurs) and constant workrate (fixed effort, environment drives the response) produce essentially identical end-exercise outcomes: −0.43°C vs −0.45°C for core temperature, −16.5 vs −16.8 bpm for heart rate. The thermal stimulus is what drives adaptation, not the mechanism used to achieve it. Protocol choice can be based on logistics.
One significant limitation: 91.1% of the 2,587 participants in the 211 studies were male. The Bayesian predictor reflects that sample.
Practical summary: - HR and core temperature adaptation: 8–10 sessions at 60–90 minutes - Hbmass accretion (altitude-equivalent): requires 14+ day block - Humid environments (elevated water vapor pressure) selectively drive sweat rate increases — +37 mL/h per 1 kPa — without improving cardiovascular or thermal outcomes
The protocol predictor lets coaches model expected adaptations from a specific design.
Threads worth pursuing: The paper doesn't address decay kinetics — how fast do adaptations degrade after the protocol ends? Plasma volume is known to return to baseline in 2–3 weeks; Hbmass is more durable. That asymmetry has implications for peak-then-compete timing in race preparation.
Sources: - McDonald et al. 2025 — Comprehensive Physiology - Full text (PMC12122934) - Protocol Predictor — UC Research Institute for Sport and Exercise