Too hot outside for a workout? A hot bath can actually help.

A man exercises in a park on a smoggy morning in New Delhi, India, November 9, 2017. REUTERS/Saumya Khandelwal

When springtime temperatures spike and your usual outdoor ride, run and walk suddenly feels sizzling and punishing, it may be time to take – a hot bath.

Acclimating to hot weather is a good idea for anyone who wants to reduce risk of heat illness and feel more comfortable exercising outside or just moving around. But getting our bodies used to the heat can be time consuming and uncomfortable, often requiring weeks of slowly ramping up the time we spend working out outside, in nature’s blast furnace.

But there’s an alternative. Recent research suggests we may be able to rapidly acclimate to the heat by lounging in a hot bathtub or sauna for at least half an hour a day, preferably after a workout.

This method, repeated for multiple days in a row, seems to offer a relatively simple and accessible way to start preparing for hot weather. Studies show it’s appropriate for both robust athletes and many people who are more out of shape or older.

The idea, said Edward Cole, a doctoral candidate at Hull University in England, who studies exercise and heat, is to make heat acclimation “available to more people.”

But there is a catch. Pleasant as it may sound, sitting for prolonged periods in a torrid tub can be surprisingly hard.

– – –

Why you need to acclimate to the weather

Hot weather, especially when it shows up suddenly, makes outdoor activities draining and even dangerous. As we move around, our bodies generate internal heat, which we must shed to keep our core temperature stable. Otherwise, we risk heat exhaustion, illness or even stroke.

To dissipate this heat, our hearts pump warmed blood away from our core and up to the skin. We also sweat. But when the air is warm and humid, these processes barely keep up. Our hearts labor to move more blood, sweat pools on our skin, each step feels laborious, and we grow hotter and hotter.

Acclimation helps. If we slowly get used to exercising in the heat, we add blood volume, reducing strain on our hearts. We also start perspiring earlier and more profusely and should feel less flattened by soaring temperatures.

But this type of acclimation is not easy. In athletes, 10 or more hot workouts of gradually increasing duration and intensity during the hottest part of the day are usually needed, a regimen that is unlikely to be attractive or even achievable for many of us.

Enter hot water.

– – –

Why hot baths are like exercise

“Using passive heat acclimation methods like hot-water immersion certainly removes barriers” to acclimating, said Andrew Greenfield, who studied exercise and acclimation while a graduate student at California Baptist University in Riverside, Calif.

It turns out that slipping into hot water is, in some ways, indistinguishable from exercise, as far as our bodies are concerned. It raises our core temperature, heart rate and sweating.

So maybe, some scientists have speculated, it also can stimulate heat acclimation.

A pioneering 2015 study put the idea to the test, with 17 healthy, active men running on treadmills at an easy pace in a normal-temperature room for 40 minutes and then sitting up to their necks in water warmed either to a coolish 93 or steamy 104 degrees for up to another 40 minutes.

After six days of this, the men who had marinated in the hot water showed many of the hallmarks of acclimation. While exercising in an overheated lab, they started sweating earlier and reported feeling less hot than the other men. They also ran farther and faster in a 5-kilometer time trial.

– – –

How anyone can acclimate with hot water

Since then, researchers have lightly parboiled other volunteers in a variety of experiments, including older people, ages 68 and up, who, in a 2021 study, either exercised in hot conditions for an hour or rode a bike slowly for 30 minutes and then soaked in hot water for another 30 minutes. After five days of these routines, they all felt less hot and moved around more quickly and easily in the heat.

Even young men who only soaked in hot water for 40 minutes, without exercising first, showed signs of heat acclimation after three days in a 2021 study.

But whether hot tubbing is the best – or safest – way to acclimate remains in question.

“Heat is much more intense in hot water versus hot air at equivalent temperatures,” said Greenfield, who led the 2021 study of young men. Most of them could barely tolerate remaining in the water for the full 40 minutes, he said.

Michael Zurawlew agrees. Now a postdoctoral research officer at Liverpool John Moores University in England, he led the 2015 study that helped ignite interest in what is often called passive acclimation and has conducted many related studies since. In his groups’ experiments, the volunteers rarely were able to stay long in the 104-degree water at first, he said.

But their tolerance “gradually increased so that by the sixth day, they could complete the full 40-minute bath,” he said, and be considered, at that point, heat acclimated.

– – –

Start with a 20-minute bath

Want to prepare for your own imminent heat wave or upcoming hot-weather race with a long soak? Zurawlew recommends starting slowly. Fill up your tub with water heated to about 104 degrees, then “complete a 20-minute bath” on your first try, he said, and gradually add to the amount of time you gently stew over the coming days.

“If you start to feel dizzy, lightheaded or nauseous at any time, you should carefully remove yourself from the bath,” he said.

Immerse as much of your body as tolerable. In most acclimation experiments, volunteers sat covered to the neck, although sometimes only to the waist or with just their legs dangling in the hot water. But less skin in the water usually demands more time in the tub, Zurawlew pointed out, to heat up your insides and prompt your body to adapt.

That’s also why he and many other researchers recommend exercising first. It gets your body appropriately hot and bothered, even before you soak.

The good news is that acclimation starts quickly, usually within three days, Zurawlew said, although you’ll be better acclimated after about six or more days of fervid exertion and soaking.

You can tell you’re acclimated when the same hot water you could barely withstand two days ago feels bearable today, he said.

But even then, remain cautious during hard workouts in the high heat. Drink water; seek shade if you feel ill, and head out in the morning, if possible, when ambient temperatures tend to be lowest.




Please enter your comment!
Please enter your name here