How to Heat Acclimate for a Trail Race

Most runners prepare for a hot race by worrying about it. Very few prepare for it the way the evidence says works, which is ten to fourteen days of deliberately getting hot on purpose, starting about two weeks before the start line.

That gap matters more than almost anything else you can do in a short taper. A well-acclimated runner and an unacclimated one can run the same pace in the same heat with a ten-beat difference in heart rate and a noticeably lower core temperature. On a five-hour mountain race in the sun, that difference is the whole outcome.

What actually adapts

Heat acclimation is not toughening up psychologically, though there is some of that too. It is a specific, measurable set of physiological changes.

• Plasma volume expansion. Your blood volume increases by roughly 5-10% within the first week, which means more blood available to carry oxygen to muscle and heat to skin at the same time - the central conflict that heat exercise creates.
• Earlier, heavier sweating. You start sweating at a lower core temperature and produce more of it, which cools you faster before things get uncomfortable.
• Lower sweat sodium concentration. Your body gets better at conserving sodium, which is one reason heat-acclimated athletes generally need less aggressive electrolyte replacement than the same athlete unacclimated - though not zero, and the hydration and sodium guide covers what to do with the difference.
• Lower heart rate and core temperature at a given pace. The most practically useful outcome. The same effort simply costs less.
• Improved perceived comfort. Heat that felt unbearable in week one feels merely unpleasant by week two. This is not just habituation - it tracks the physiological changes above.

The timeline

The athletes who skip heat training almost always tell me the same thing afterward: "it wasn't that much hotter than home, I just felt terrible for no reason." The reason is that their body was doing its first-ever heat adaptation live, during the race, instead of two weeks earlier in a sauna.

Fourteen days captures the large majority of the available adaptation. Extending well past that keeps producing small gains, but for a single goal race, two weeks is the number worth protecting on the calendar.

Two ways to get there

Not everyone has heat to train in. Both routes below work; combine them if you can.

Active heat training has an edge you can't get from a sauna: it lets you rehearse race-specific pacing, fueling, and gear in the conditions you'll actually race in. If you have real heat available, use it, even if it means running at an unglamorous hour.

If you don't, passive heat exposure produces most of the same core adaptations with far less logistical cost. Take the sauna or bath immediately after an easy run rather than on its own - starting already warm and mildly dehydrated shortens the time it takes to reach a meaningfully elevated core temperature, which is what drives the adaptation.

Hydrate deliberately around these sessions. A sauna after a run is a second, separate bout of fluid loss on top of the run itself, and it is easy to under-drink through both.

Fitting it into the last two weeks

The protocol wants to sit late in your build, but not so late that it collides with your running taper.

Finish the heat protocol 3-5 days before the race. The adaptation is durable enough to survive that gap and fresh enough that you arrive still carrying most of it. Stopping much earlier than that - say, ten days out - lets a meaningful fraction decay before you need it. Running it right up to race morning leaves no buffer if a session leaves you unusually fatigued.

Keep heat sessions easy the whole way through, including in the final week. This is heat stress layered onto whatever running taper you're already doing, and the two need to be managed as separate loads, not combined into one.

I had an athlete two summers ago build for a July 50-miler in the hills above a Spanish coastal town - a world away from Lecco's mountain air. Fourteen sessions of twenty-five minutes in a 90°C sauna, taken straight after her easy runs, finishing six days before she flew out. She came back saying the race "felt ten degrees cooler than the forecast." The athlete I got this wrong for the year before did the same protocol but let her heat sessions taper down alongside her running volume, finishing her last one twelve days out instead of six. By race morning enough of the adaptation had faded that she spent the first two hours fighting a heart rate that shouldn't have been that high for the pace. I now keep the two tapers deliberately separate, and it's the reason I built the timeline table above the way I did.

Race-day heat tactics

Acclimation reduces the cost of heat. It does not remove it, and you should still race like the day is hot.

• Pre-cool in the last 15-20 minutes before the start. Ice slurry if you can get it, cold wet towels on the neck and wrists otherwise. This buys you a genuinely lower starting core temperature, and the effect lasts longer into the race than it looks like it should.
• Start slower than feels necessary. Heat raises heart rate and perceived effort at any given pace from the first kilometre, and the runners who blow up in hour three of a hot race are almost always the ones who paced the first hour against a normal-conditions number.
• Raise your fluid and sodium targets toward the upper end of what the hydration guide lays out, and use every water source for external cooling - head, neck, forearms - not just drinking.
• Use shade and terrain deliberately. On an exposed climb in full sun, effort that feels moderate can be running you hotter than a much harder effort in shade. Let perceived heat stress, not just perceived exertion, inform your pace.

Know the difference that matters

Heat exhaustion - heavy sweating, weakness, nausea, headache, still mentally clear - responds to slowing down, cooling, and fluids, and most runners can keep moving or walk it off.

Heat stroke is a medical emergency: confusion, loss of coordination, or hot, dry skin where sweating has stopped altogether. If you or someone near you shows those signs, stop immediately and get medical help. This is not a "push through it" situation under any circumstances.

The mistakes

1. Starting the protocol too close to the race. Fewer than about seven days rarely gets you the sweat-sodium and full plasma-volume adaptations.
2. Stopping it too early because it coincides with the running taper. Manage the two tapers separately - see above.
3. Treating heat sessions as hard training. The heat is the stimulus. Keep the pace or effort easy, especially in active heat training, or you're stacking two loads at once.
4. Ignoring hydration around sauna or hot-bath sessions. They cause real fluid loss on top of whatever you lost on the run itself.
5. Assuming a handful of sessions equals full acclimation. Three saunas the week of the race is better than nothing, but it is not the same protocol, and shouldn't be raced as though it were.
6. Never testing race nutrition and pacing in heat. If you only have sauna access, at minimum do one or two real runs in whatever heat you can find to rehearse the practical side.

If you're building toward a specific hot race, the free training plan generator accounts for terrain but not climate - heat preparation is the part you layer on yourself using the protocol above, or the part I help athletes time correctly in 1:1 coaching. Racing somewhere both hot and high has a second layer to plan for - see the altitude acclimatization guide. And if your training year runs the other direction, cold-weather running covers the opposite problem.