Buzz
No one enjoys feeling sluggish and drenched in sweat, so when the sun is set to scorch, it’s tempting to retreat to an air-conditioned gym. However, the human body can indeed adapt to training in hot conditions. After a few weeks, the heat will feel like the new norm—and studies suggest you might even experience a slight performance improvement when the cooler weather returns.
Exercising in the heat can be risky, so I trust you're aware of the basic precautions when running in high temperatures. Some key advice includes: drink to your thirst (or slightly more), and stop to get help if you start feeling symptoms of heat illness like nausea, dizziness, or weakness. While it's beneficial to build your ability to run in the heat, don't push it too far—seek shelter if it's hotter than your body can manage, and be mindful of smog and ozone levels (which tend to rise on hot days) if you live in a city.
Why does working out in the heat feel so uncomfortable?
Running is tough, and heat is tough, so naturally, running in the heat is a tough combination. But it’s not just about those two factors—it actually feels worse than you might expect when you add them together.
Your brain processes effort differently in hot conditions, which makes you feel slower even before you actually overheat. A study published in the European Journal of Physiology showed that cyclists in a 95-degree lab were slower than when they did the same time trial at 59 degrees. This makes sense, but here’s the odd part: they didn’t overheat and then slow down; they were slower from the start. It seems our brains slow us down proactively on hot days to conserve energy.
As a workout progresses, our bodies naturally heat up. Another study, published in the Journal of Applied Physiology, found that when participants were asked to cycle until exhaustion, they all hit a wall when their core temperatures reached 104 degrees, regardless of the temperature they started in. The athletes who took longer to reach this point were the ones wearing a cooling jacket. You can replicate this effect in your workouts by drinking ice-cold drinks and pouring water over your head. The longer you keep your body cool, the longer you can sustain a tough effort.
It’s the heat and the humidity
However, cooling your body down isn’t the full answer. Pouring a cup of ice water over your head or swallowing one can give you temporary relief, but water-cooling jackets aren’t exactly practical for regular use outside of labs. So let’s explore what actually happens in the real world.
Your body attempts to cool itself by sweating. As sweat evaporates from your skin, it draws heat away with it. However, in humid conditions, sweat doesn’t evaporate as easily because the air is already saturated with moisture. So when we discuss ‘heat,’ it’s actually more about ‘perceived heat,’ which is a mix of heat and humidity. The heat index chart below illustrates this relationship:
You’ll run slower in both the heat and humidity. While charts like this one may predict how much your race time will slow down, the reality is that the impact of heat on your running depends on your body’s adaptation to heat and your body size.
That’s right—not your fitness, but your actual physical size. Larger individuals have more muscle, fat, or both. Muscle generates heat, while fat acts as an insulator. Conversely, smaller people produce less heat but have more skin to release it—thanks to the surface area to volume ratio. This is why shorter runners often perform better in races on hot days.
Some believe that being more fit helps you handle the heat better, but that’s only partially true: the fitter you are, the more body heat you generate, simply because you’re so efficient at working hard. So, aside from changing your body shape (which, let’s face it, isn’t a quick fix), how can you improve your ability to exercise in the heat? The answer is simple: Spend more time training in hot conditions.
Why starting a heat adaptation program is essential
Exercising in hot weather makes you better at handling the heat—and it makes you better overall.
Imagine you do all your workouts outdoors this summer while your equally fit twin trains on a treadmill in a climate-controlled gym. Who do you think will cross the finish line first in a 5K on a sweltering August weekend? That’s right, you will.
Even if the weather is unusually cool on that August day, your heat training will still give you the edge over your twin. The key benefit of heat training is that it increases your blood volume, which helps direct blood to your skin for cooling, while still maintaining enough for your muscles. This effect has been compared to a mild, perfectly legal form of blood doping. While scientists are still debating exactly how it works and whether it always occurs with heat adaptation, the evidence is strong enough that I believe we should all aim to benefit from these adaptations, as long as we do so safely.
Here’s the unfortunate truth: adapting to heat requires effort. It’s not enough to stay in air conditioning all summer, with only the occasional outdoor workout. A study published in the European Journal of Applied Physiology found that those who didn’t intentionally train in the heat saw no improvement in heat tolerance by fall compared to spring. To gain the benefits of heat training, you have to put in the work.
How to train your body to handle exercising in the heat
Option 1: Continue your regular training, but don’t avoid the heat, for two weeks
In scientific research, heat acclimation protocols for athletes typically involve 7 to 10 consecutive days of heat exposure, with 60 to 90 minutes of exercise each day. A simpler approach, which can be done by teams or individuals, is to simply train normally in the heat for about two weeks.
Start slow. Keep in mind that your body will try to convince you that you’re exhausted and need to slow down. Safety guidelines for workers provide an insightful benchmark. On their first day in the heat, the Occupational Safety and Health Administration (OSHA) recommends giving workers only 20% of their usual workload. Within a week, they should gradually increase to 100%.
Option 2: Spend two hours each day in the heat, whether you’re exercising or not
This US Army training protocol offers a clear guide for adapting to the heat: Spend at least two hours in the heat every day, including some form of cardiovascular exercise (like running, cycling, or anything that elevates your heart rate). If you can’t handle two hours without feeling symptoms like dizziness or nausea, do what you can and treat the two-hour goal as something to strive for.
You should notice significant improvements in heat tolerance after about two weeks of spending two hours per day in the heat, although some results may appear after just a few days.
Option 3: Use the sauna after each workout
To combine the above methods without relying on the weather, consider using a sauna or hot bath after your workout. If your gym has a sauna, this is a great opportunity to incorporate it into your heat adaptation training. The time you spend in the sauna will contribute to your heat tolerance, and it can be done even if your workout was in more moderate temperatures. Start with shorter sauna sessions (15 minutes) and gradually work up to 30 minutes or longer.
How to maintain your heat adaptations even when the temperature drops
To maintain your ability to handle heat, you need to keep exposing yourself to it. A few days off won’t do much harm, but skipping a whole week can cause you to lose your heat tolerance. The process is rapid: in just three weeks, the Army estimates you’ll lose around 75% of your heat adaptations.
To continue your heat training in cooler weather, you could try wearing long sleeves and tights, much like elite runner Kara Goucher did while preparing for a world championship race in the humid heat of Osaka. (She won a bronze medal, becoming the first American ever to do so). She also spent several weeks in Osaka ahead of the race; traveling to acclimatize to the heat might be a feasible option if you’re a dedicated athlete with vacation time to spare.
Alternatively, you can defy typical cooling strategies and run at the hottest part of the day on dry, sun-baked asphalt with no shade. You could also return to the sauna protocol after your cooler-weather runs. Whatever you decide, be sure to stay safe and embrace your newly acquired superpowers.
