Sports scientist who ran Ironman race in 95°F shares 5 tips for exercising in extreme temperatures

• Owen Jeffries is a sports scientist who specializes in heat adaptation.
• He used his knowledge to run an Ironman in Austria during a heat wave.

Heat is back. Already this year, a heat dome over the Pacific Northwest brought in record-breaking temperatures.

As the thermometer revs up, you might be questioning how you can keep exercising without compromising your safety.

Owen Jeffries, a sports physiologist from Newcastle University who specialized in heat adaptation, finished an Ironman during a heat wave, in 12 hours and 24 minutes. Temperatures during the race reached 95°F.

He shared five science-tested ways he prepared his body for intense exercise in extreme heat.

Give your body a few days to adapt to the heat

Avoid strenuous exercise in a sudden heat wave because the body needs time to adapt to the heat.

To prepare for the heat, Jeffries trained for 60 to 90 minutes over ten days in his lab's extreme environmental chamber. Precise measurements showed his body was fully adapted to the heat within ten days.

At first, the body doesn't sweat as much as it needs to. The heart has to pump harder to bring more blood to the skin, where the evaporation of sweat cools it.

After a few days, the body starts sweating more and the heart rate goes down, Jeffries said.

Even after your body adapts, you should be wary of pushing yourself towards your usual targets, said Jeffries.

"Don't go out and expect to be able to sustain the same amount of physical activity as you would do in a normal climate. Performance in a hot environment is reduced by between 10 and 30%," said Jeffries, citing a 2019 study.

Look out for any signs that you might be experiencing heat illness, such as dry mouth and tongue, thirst, headache, lethargy, dry skin, muscle weakness, dark urine, and dizziness.

In that case, immersing yourself in relatively cold water is the best way to cool down quickly, he said, though be cautious of cold shock.

Weigh yourself to optimize hydration

If you're doing intense exercise, you can aim to optimize your hydration level with a simple trick.

Of course, it's important to stay hydrated so your blood can keep pumping properly. In hot weather, you sweat more to keep yourself cool, and if you get to the point of dehydration, your blood loses water. Then, the heart has to pump harder to keep the blood flowing around the body.

"Dehydration as little as 2% can have an impact on exercise performance," said Jeffries.

However, you can drink too much, especially when you're thirsty because you're exercising. This can also be dangerous if you go into hyponatremia when there is so much liquid that it dilutes nutrients like sodium or potassium to a point where the body can't use them properly.

Jeffries had access to high-tech equipment, to measure exactly how much sweat he would lose during an hour of exercise, but he said there is a cheap and low-tech way to do the same at home: drink 500 ml of water 45 minutes before a workout, then weigh yourself before and after.

"The weight loss will correspond almost directly to sweat loss," he said.

To make sure to get the best measurement, don't drink water or go to the bathroom during the exercise, and remove any sweat-soaked clothing before weighing yourself.

Not all liquids are created equal

If you're going to exercise for longer than 90 minutes, you should think about replenishing your nutrients, said Jeffries.

"Loss of sodium needs to be considered beyond 90 min activity," he said.

As you sweat, you don't only lose water, you also lose minerals.

One sign that you're not giving the body what it needs is that you crave salt after exercise, said Jeffries.

"If you were really craving salt, that means your body needs it. You need to get it whichever way you can," for instance by eating chips, he said.

Drinking liquids that are electrolyte-rich or taking sodium supplements can prevent this from happening.

Some nutrition websites offer sweat tests that can help estimate how much salt you're losing during long-ranging exercise, Jeffries said.

But you have to know what you're doing. Just like you can drink too much water, it's important not to consume too much salt outside of very intense exercise.

Keep your core temperature low with a little planning

If exercising in intense heat, it's important to keep your core temperature low.

You can dowse yourself and your clothing with water to cool yourself, said Jeffries. Ice slushies or cooling vests can also help if you can keep them frozen until you need them. They are "not practical in a hot environment," Jeffries said.

Jeffries also pointed that in the 2022 Tour de France, where the athletes had to perform in 100-degree heat, "some of the professional teams have actually adapted their jerseys with little pockets to insert little ice packs around the body," he said.

If the event is long, train your stomach for the heat

If the event is long, you can't count on a single meal to keep your energy levels high. The downside is that eating while exercising can lead to gastrointestinal distress (stomach ache), which is made worse by heat, Jeffries said.

To avoid this, you can train your stomach by eating and drinking little by little while you exercise, said Jeffries.

Learn how your body reacts to heat to keep in great shape

Jeffries said that monitoring his body's needs in the heat and sticking to a nutrition plan did wonders for his body.

In previous events, he had felt terrible the next day. But after he ran this Ironman, he felt pretty good. He credits careful planning with his recovery.

"Considering how long the day was, I felt pretty recovered the next day, enough to go for a light jog and a swim," said Jeffries.