Running in the heat: how to acclimatise and keep performing

Every summer the same thing happens. A pace that felt comfortable in spring suddenly leaves you gasping, your heart rate is ten beats higher for no obvious reason, and the last few kilometres turn into a survival shuffle. You have not lost fitness overnight. You are simply running in the heat, and heat changes the physiology of endurance in ways that are large, measurable and, importantly, trainable.

This article explains why heat hurts performance, what happens to your body over ten to fourteen days of heat acclimatisation, and how to pace, cool and hydrate so that hot weather running becomes a strength rather than a liability.

Why heat wrecks endurance

When you run, your muscles generate a large amount of heat. To shed it, blood is diverted to the skin so that warmth can radiate away and sweat can evaporate. In a hot environment that cooling job competes directly with the job of delivering oxygen to your working muscles. Périard, Eijsvogels and Daanen (2021) describe the result in detail: as core and skin temperatures climb, more blood pools in the skin, the heart has to beat faster to maintain output, and aerobic capacity falls. The classic signature is “cardiovascular drift”, where heart rate creeps upward even though your pace is steady.

The performance cost is real. Heat raises core temperature toward a critical limit, increases perceived effort, and accelerates fatigue, which is why marathon and middle distance times are consistently slower in hot conditions. Guy et al. (2015) note that the impairment is greatest in events lasting long enough for body temperature to climb and dehydration to accumulate, precisely the territory of distance running.

What heat acclimatisation actually changes

Here is the encouraging part. The body adapts to heat remarkably quickly. Périard, Racinais and Sawka (2015) catalogue the core adaptations that develop with repeated exercise in the heat, and they are substantial:

• Plasma volume expansion.Blood volume increases within the first few days, improving the heart’s filling and stroke volume so it does not have to race as hard.
• Earlier and greater sweating. You begin to sweat at a lower core temperature and produce more sweat overall, cooling yourself more effectively.
• A lower core temperature and heart rate. For the same workload, both resting and exercising body temperature and heart rate fall, reducing physiological strain.
• Better fluid balance. Sweat becomes more dilute as the body conserves sodium, and overall fluid regulation improves.

Tyler et al. (2016), in a meta analysis of heat adaptation studies, confirmed that these changes translate into measurable benefits: lower core and skin temperatures, a reduced heart rate, higher sweat rate, and improved exercise performance and capacity in the heat. In other words, the discomfort of those first hot sessions is the price of a genuine, well documented upgrade.

The surprising bonus: faster in cool conditions too

One of the most striking findings in this field is that training in the heat can improve performance even when you later race in the cold. Lorenzo et al. (2010) took trained cyclists through a ten day heat acclimation programme and tested them in both hot and cool laboratory conditions. As expected, performance in the heat improved. But maximal oxygen uptake and time trial performance in cool conditions improved as well, by several percent.

The likely mechanism is plasma volume expansion. A larger blood volume increases stroke volume and cardiac output, which benefits aerobic performance regardless of temperature. This is why some coaches now treat a heat block as a low cost alternative to altitude training. The effect is most reliable in athletes who are already well trained, so it is a sharpening tool rather than a substitute for base fitness.

How to acclimatise: the protocols

The principle is simple: expose yourself to exercise in the heat, repeatedly, for long enough to raise core temperature, over roughly ten to fourteen days. Racinais et al. (2015), in a consensus statement on training and competing in the heat, recommend exactly this pattern of repeated heat exposures across one to two weeks as the single most effective intervention for reducing strain and protecting performance.

Two warnings. First, the adaptation fades. The benefits decay over a couple of weeks once heat exposure stops, so time your block to finish close to a hot race rather than months before it. Second, the early sessions are the hardest; ease into intensity and let your body catch up. If you are tapering into a hot goal race, fold the heat work in carefully so it does not undo your freshness. Our guide on how to taper for a race covers how to balance stimulus and recovery in the final weeks.

Pacing and cooling on the day

Even fully acclimatised, you will run slower in the heat at the same effort. The single most important tactic is to pace by feel, not by your cool weather splits. Because heat pushes heart rate up at any given speed, chasing your normal pace forces you above your intended effort and blows up your back half. Start conservatively and let the clock be slower than usual.

Cooling helps, and it works best when applied early and to the right places. Cooling before the start, using cold drinks, an ice slurry, or cold towels, lowers your starting core temperature and buys you time before you hit the critical limit. During the run, pouring water over your head and neck, running in the shade where possible, and dousing the skin all aid evaporative cooling. Périard, Eijsvogels and Daanen (2021) group these cooling strategies, before and during the run, among the most effective mitigation tools alongside acclimatisation itself.

The hydration link

Heat and hydration are inseparable, because sweating is your main cooling mechanism. Lose too much fluid and your blood volume falls, your heart rate climbs further, and your ability to dissipate heat declines, a vicious circle. For most runners, drinking to thirst keeps losses within a safe range. On long, hot efforts with very high sweat rates, a planned intake helps, with the goal of limiting body mass loss to around two percent rather than replacing every drop.

Sweat also carries sodium, so on long hot sessions a drink with electrolytes is usually better than plain water. Fuelling matters too: heat does not remove the need for carbohydrate on long efforts, and getting it in can be harder when your gut is under thermal stress. We cover the numbers in detail in our guides on hydration for runners and how many carbs per hour to take while running.

Heat illness: know the warning signs

Acclimatisation reduces the risk of serious heat illness, but it does not eliminate it. Heat exhaustion and the far more dangerous heat stroke can affect anyone who pushes too hard in hot conditions. The signals to respect are dizziness, nausea, a throbbing headache, clumsy or uncoordinated movement, confusion, and, ominously, a sudden stop in sweating despite the heat. Confusion, disorientation or collapse with hot skin is a medical emergency: stop immediately, get into shade, cool the body aggressively with cold water or ice, and seek help. No training session or finish line is worth ignoring these signs.

Frequently asked questions

Related reading: hydration for runners: how much to drink and when.