Everyone knows to "drink more" in the heat. But what actually happens in your body during 10–14 days of heat exposure — and how to deliberately trigger those adaptations before a target race — is far less understood. This guide explains the physiology and how to use it.
Key Takeaways
- Meaningful heat adaptations begin by day 6 and stabilize around day 10–14
- Key adaptations: increased plasma volume, earlier sweat onset, lower heart rate at the same pace, lower sweat sodium concentration
- 1–2% dehydration reduces aerobic performance 5–10% — a 4-hour marathon loses 12–24 minutes
- Deliberate acclimatization protocol: 60–75 min in heat at moderate intensity for 10 consecutive days
What changes in your body with heat exposure
Heat acclimatization produces measurable, progressive physiological changes. After 6 to 14 days of regular heat exposure:
- Increased plasma volume (+10–15%): your blood becomes more dilute and higher volume, improving oxygen transport and thermoregulation
- Reduced heart rate: at the same pace and temperature, your heart beats less after acclimatization
- Earlier and more efficient sweating: you start sweating sooner, with lower sodium concentration (fewer minerals lost per liter of sweat)
- Lower core body temperature: at equivalent effort, your temperature rises more slowly
These adaptations are functional — they genuinely improve performance in hot conditions. And they persist 7+ days after exposure ends.
The deliberate acclimatization protocol
If you have a hot-conditions race in 3 to 4 weeks, you can accelerate this process intentionally. Research-backed protocol:
- 10 consecutive days of training in hot conditions (not necessarily outdoors — a gym without AC works)
- 60 to 75 minutes per session at moderate intensity (RPE 5–7, not all-out)
- Maintain adequate hydration during and after each session
- Start the protocol 2 to 3 weeks before race day to be fully adapted by then
Hydration: the rules that change in heat
Dehydration of 1 to 2% of body weight reduces aerobic performance by 5 to 10%. On a 4-hour marathon, that's 12 to 24 minutes lost. The ACSM confirms that carbohydrate and electrolyte drinks are absorbed up to 30% faster than water alone. For races over an hour in heat, an isotonic drink or water + electrolyte solution is more effective than plain water. Post-session rehydration matters as much as during-session — aim to replace 150% of fluids lost in the 4–6 hours following effort.