Few variables shape a summer race as decisively as the weather, and as the season’s biggest events unfold in rising temperatures, heat acclimatisation has moved from a niche concern to a central part of elite and amateur preparation alike. From the exposed canyons of mountain ultras to mid-morning city 10Ks, the body’s ability to cope with heat can swing finishing times by minutes and, in extreme cases, determine whether a runner finishes at all. Understanding how that adaptation works — and how to build it safely — is among the most practical edges available to anyone racing in summer.
The physiology is well established. When core temperature rises, the body diverts blood to the skin to shed heat, which leaves less available to working muscles and forces the heart to work harder at any given pace. Repeated, controlled heat exposure prompts a series of adaptations: runners begin to sweat earlier and more efficiently, lose fewer electrolytes in that sweat, expand their plasma volume and lower their resting and working core temperature. The net effect is that an acclimatised runner can hold a faster pace at a lower physiological cost when the mercury climbs.
Most of these adaptations develop over a relatively short window. Research and coaching practice broadly agree that meaningful heat acclimatisation can be achieved in roughly ten to fourteen days of consistent exposure, with the largest gains arriving in the first week. Typical protocols involve training or sitting in hot conditions for around 60 to 90 minutes a day, whether through outdoor sessions timed for the warmest part of the day, treadmill running in warm rooms, or post-run sauna use. The key is consistency and a gradual build, rather than a single punishing session that does more harm than good.
Acclimatisation is not a substitute for race-day strategy. Even a well-prepared runner needs to start conservatively in the heat, since pacing errors are magnified when thermoregulation is already under strain. Practical cooling measures — ice in caps and bandanas, dousing the head and neck at aid stations, and pre-cooling before the start — can meaningfully blunt the rise in core temperature. Hydration matters too, but so does balance: drinking to thirst with attention to sodium intake helps avoid both dehydration and the dangerous over-drinking that can lead to hyponatremia.
The takeaway for summer racers is that heat is a trainable variable, not simply bad luck. Athletes targeting hot events — or travelling from cool climates to warm ones — benefit from building a dedicated acclimatisation block into the final fortnight of their preparation, then pairing that physiological readiness with disciplined pacing and active cooling on the day. Done well, the combination can turn a feared forecast into a manageable challenge, and in the closing stages of a hot race it is often the best-prepared, not simply the fittest, who come out ahead.