Heat acclimation for endurance athletes
Heat acclimation for endurance athletes is one of the few training adaptations that costs nothing and gets ignored anyway. Two weeks of deliberate heat exposure changes your plasma volume and your sweat response, and lowers your core temperature at a given workload. Most athletes find out the hard way, mid race, in a hall with no air conditioning.
August is when this bites in the UK, because indoor race venues and gyms hold heat far better than the weather outside suggests.
What heat acclimation actually does to you
The 2015 consensus statement on training and competing in the heat, published in the British Journal of Sports Medicine by Racinais and an international expert panel, lays out the adaptations clearly. Repeated heat exposure produces:
- Plasma volume expansion. More blood volume means better stroke volume and lower cardiovascular strain at the same power output.
- Earlier sweating onset. You start cooling sooner in the session rather than waiting until you're already overheating.
- Higher sweat rate. More evaporative cooling per minute.
- Lower sweat sodium concentration. Your sweat glands reabsorb more sodium, so you keep more of it.
- Reduced resting and exercising core temperature. More headroom before you hit the ceiling.
That last one matters most for functional fitness racing. Performance in the heat drops off because core temperature climbs toward a critical limit, and once you're near it, the brain reduces the drive to the working muscles. Push the starting point down and stretch the runway.
The consensus panel puts meaningful adaptation at 1 to 2 weeks of daily exposure, with the largest changes arriving in the first 5 to 7 days. That's a fast return on a small investment.
The protocol that works
Raise your core temperature and keep it elevated for 60 to 90 minutes, on most days, for 10 to 14 days. That's the whole requirement.
Three practical routes:
Train in the heat. Move a session outdoors at the warmest part of the day, or into a gym with the doors shut. Drop the intensity. You're chasing thermal load, and trying to hold normal paces in hot conditions is how athletes dig themselves into a hole.
Overdress. Long sleeves and leggings on an easy run do most of the job for free. Ugly, effective.
Post-session sauna or hot bath. 20 to 30 minutes at around 80 to 90 degrees Celsius immediately after training, or a hot bath at 40 degrees for 20 to 30 minutes. The research here is smaller than for exercise heat training, but it's the option most people will actually stick to.
Whichever route you take, the first few sessions will feel disproportionately hard. That's the point. If it feels normal, the stimulus isn't there.
What it does to your hydration needs
Here's the trade-off. Heat acclimation raises your sweat rate, which means you lose more fluid per hour than you did before, even though you're coping better.
Your sweat gets more dilute in sodium, so the total sodium loss doesn't rise as sharply as the fluid loss. But the fluid demand goes up, and it goes up during a two week block when you're already under thermal stress every day.
Which is why testing matters. If you don't know your baseline, work through how to calculate your sweat rate before you start the block, then retest at the end. Watching the number climb over 14 days is the clearest evidence the adaptation is happening.
Practically: weigh yourself each morning during a heat block. A drop of more than 2% from your normal baseline, repeated across days, means you're accumulating a fluid deficit and the block is doing more harm than good.
Timing it around a race
Adaptations start decaying within days of the last exposure. The consensus recommendations suggest you lose a meaningful chunk within 1 to 2 weeks of stopping, with plasma volume gains going first.
So the sequencing is:
- Start the block 2 to 3 weeks out from the race.
- Run 10 to 14 consecutive days of exposure.
- Finish the main block 3 to 7 days out, then keep 1 or 2 light top-up sessions in race week to hold the adaptation without adding fatigue.
- Don't do heat work in the final 48 hours.
And don't stack a heat block on top of your heaviest training week. Heat exposure is a training load. Treat it like one, and pull volume or intensity elsewhere to make room. Athletes who bolt it on top of a peak block usually end up sick or flat.
Who should skip it
If your race is genuinely cool, the case is weaker. There's ongoing interest in heat training as a general endurance stimulus through plasma volume expansion, and some evidence supports a small benefit in temperate conditions, but the effect is nowhere near as reliable as it is for hot races.
Skip it if you're returning from illness or deep in a fatigue hole. The upside doesn't justify the risk.
Handle the hydration side
A better sweat response only helps if you're replacing what it costs you. Interval's Electrolyte+ covers the sodium and calcium side of that, and our Hyrox hydration strategy guide covers the timing.
Two weeks of being uncomfortable on purpose. It's the cheapest performance gain available to most athletes, and most of them still skip it.