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

Owen Jeffries, a sports physiologist who studies heat adaptation, competed in an Ironman in 2022 when temperatures reached 95°F.

• 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. 
• Here are five tips for exercising in heat inspired by his Ironman training. 

Owen Jeffries, a sports physiologist from Newcastle University who specialized in heat adaptation, recently finished an Ironman, one of the most demanding sports events in the world, during a heat wave.

Temperatures during the race in Austria, which he finished in 12h and 24 minutes, reached 95°F. The weather was so hot that athletes were banned from using wear wetsuits for the swimming portion in a glacial lake. 

He shared with Insider 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.

You should avoid strenuous exercise in a sudden heat wave because the body needs time to adapt to the heat. 

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.

To prepare for the heat, Jeffries trained for 60 to 90 minutes over ten days in his lab's extreme environmental chamber that simulated conditions where he'd be running.

Precise measurements showed his body started adapting four or five days into the training and was fully adapted to the heat within ten days.

Even after ten days, 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

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

Dehydration is bad for the body. As blood volume reduces, the heart has to pump harder to keep it flowing. In hot weather, that happens more quickly as the body sweats more to keep itself cool.

However, it is also possible to drink too much, especially when you feel thirsty. This can also cause a condition called hyponatremia, when nutrients like sodium or potassium are too diluted in the blood. This can be life-threatening in excess. 

To optimize his performance, Jeffries used instruments in his lab to assess exactly how much sweat he loses in an hour of exercise. He used that figure to calculate how much fluid he should take over his fluid intake during his 12-hour-long Ironman. 

A low-tech way to do the same at home is to weigh yourself before and after about an hour of exercise.

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

Start off by drinking 500 ml of water 45 minutes before the exercise. This figure will be accurate if you don't drink water or go to the bathroom and remove any sweat-soaked clothing. 

Not all liquids are created equal

If you're going to exercise for longer than 90 minutes, it is recommended that you replenish your sodium levels, said Jeffries.

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

As you sweat, you don't just 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. Early research suggests that sodium citrate could be more gentle on the gut, said Jeffries. 

"General recommendations during exercise are around 1000 mg/hour," said Jeffries. He highlighted that when not engaging in heavy exercise, general diet recommendations are not to exceed 1500 mg of sodium a day.

Help your body by keeping your core temperature low 

Ironman has mist fountains or cold water buckets to help athletes keep their body temperature down, but dowsing yourself and your clothing with water has a similar effect, said Jeffries.

Ice slushies or cooling vests can also help, but they are "not practical in a hot environment," Jeffries said. 

Jeffries also pointed to 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, which is made worse by heat, Jeffries said.

To avoid this, you can train your stomach little by little, by eating and drinking while you exercise, said Jeffries. He trained specifically with food he knew would be available at the Ironman aid stands to avoid any surprises.

If you're aware of your body's needs in heat, you can keep it 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.

"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.  

"All these different signs suggested to me that I had hydrated optimally."