Smithsonian.com

A rigorous experiment lent truth to the idea that hot drinks can help the body stay cool. Photo via Wikimedia Commons/Patrick George

Here in Washington, we finally got a slight break from what is shaping up to be one of the hottest summers in recent memory for pretty much the whole country. As we pondered the fact that this sort of weather could well become the norm in future decades due to climate change, we also remembered a counterintuitive cooling technique that many of us had heard of but doubted. In many countries around the world, conventional wisdom says that you can cool down on a hot day by drinking a hot beverage.

We got in touch with Ollie Jay, a researcher at University of Ottawa’s School of Human Kinetics—and an expert in all things sweat-related—to ask a pressing question: is this claim for real? His Thermal Ergonomics Lab, it turned out, had published a study on this topic just a few months ago.

Their answer, in short: Yes, a hot drink can cool you down, but only in specific circumstances. “If you drink a hot drink, it does result in a lower amount of heat stored inside your body, provided the additional sweat that’s produced when you drink the hot drink can evaporate,” Jay says.

How does this work? “What we found is that when you ingest a hot drink, you actually have a disproportionate increase in the amount that you sweat,” Jay says. “Yes, the hot drink is hotter than your body temperature, so you are adding heat to the body, but the amount that you increase your sweating by—if that can all evaporate—more than compensates for the the added heat to the body from the fluid.”

The increased rate of perspiration is the key. Although sweat may seem like a nuisance, the body perspires for a very good reason. When sweat evaporates from the skin, energy is absorbed into the air as part of the reaction, thereby cooling the body. A larger amount of sweat means more cooling, which more than counteracts the small amount of heat contained in a hot beverage relative to the entire body.

The caveat, though, is that all that extra sweat produced as a result of the hot drink actually has to evaporate for it to have a cooling effect. “On a very hot and humid day, if you’re wearing a lot of clothing, or if you’re having so much sweat that it starts to drip on the ground and doesn’t evaporate from the skin’s surface, then drinking a hot drink is a bad thing,” Jay says. “The hot drink still does add a little heat to the body, so if the sweat’s not going to assist in evaporation, go for a cold drink.”

Jay’s team got to the bottom of the “hot drink” tip by rigorously testing the idea on cyclists in a lab. Each cyclist was equipped with skin temperature sensors and a mouthpiece measuring the amount of oxygen consumed and carbon dioxide produced, which indicated the amount of heat produced by the body’s metabolism. The researchers also carefully tracked the air temperature and humidity, among other factors. The data yielded an overall picture of how much heat each cyclist produced and how much each released to the environment, and those drinking hot water (roughly 122 degrees F) stored less heat in their bodies than the others.

The researchers are still unsure why hot drinks lead the body to produce more sweat, but they have an idea. “It’s commonly thought that the hot drinks raise your core temperature, but we found that that isn’t the case,” Jay says. “What we think is that it’s the thermosensors that line the throat and mouth that elicit the additional sweating response.” He notes that additional research is needed to pinpoint the exact location of these sensors.

To be clear, the tip only works in very specific circumstances: a hot, dry day, where you’re not wearing so much clothing that your sweat is prevented from easily evaporating. In other words, if you’re in a humid locale—for example, anywhere on the East Coast—don’t try drinking hot water. But on a hot day in the desert, a cup of hot tea might actually be the trick to help cool you down.