Heat acclimatization: how to train your body for hot races
Runners tend to treat a hot race as something to grit through. But heat tolerance isn't a personality trait — it's a trainable physiological adaptation, and you can build most of it in about two weeks.
When you train in the heat repeatedly, your body remodels itself to cope. The first and most important change is plasma volume expansion: your blood plasma can increase by roughly 5–20%, which gives your heart more to pump and your skin more blood to shed heat. That alone lowers your heart rate at any given effort. The gold-standard protocol gets your core temperature above about 38.5°C for at least 60 minutes a day, repeated for 10–14 consecutive days.
Your sweating changes too. After acclimatization you start sweating earlier, sweat more, and — usefully — lose fewer electrolytes per liter of sweat, because your body learns to reclaim sodium. The net effect is a lower core and skin temperature at the same workload, and a race effort that simply feels more manageable.
Here's the honest part: the first several days feel awful. Paces that are normally easy will spike your heart rate and your perceived effort, because the adaptations haven't landed yet. That's expected, not a sign you're unfit. Ease in — start with shorter, easier sessions in the heat and build — and hydrate aggressively, because your sweat losses are higher before your body learns to conserve.
Two practical notes. First, time it: the adaptations are strongest right after the block and decay over about two to three weeks, so finish your acclimatization in the couple of weeks before a hot goal race rather than months out. Second, you don't need to live somewhere hot — overdressing on easy runs, or finishing a session with a sauna or hot bath, can stack heat stress when the weather won't.
What to do with this: if a hot race is coming, build in 10–14 days of daily heat exposure beforehand, run by effort instead of pace while you adapt, and drink more than feels necessary. The early discomfort is the adaptation forming.
When RunNerd sees you start training in the heat — a move to a hot climate, or the first sweltering week before a summer race — it treats the first 7–14 days as an acclimatization window and softens your prescribed paces by roughly 3–6%. Hitting your normal splits in that window costs more than it's worth while your body is still adapting. As your morning RHR and HRV settle back toward their usual range, the coach reads that as adaptation taking hold and pulls the paces back to baseline.