1:20am

Wed July 11, 2012
The Salt

Cool Down With A Hot Drink? It's Not As Crazy As You Think

Originally published on Wed July 11, 2012 7:47 pm

Hot tea on a hot day? Not for me, thank you. Not my idea of how to cool down.

But I've been doing a series of stories for Morning Edition called Summer Science, where I tackle such subjects as how to roast the perfect marshmallow and what causes that sharp headache some people get when they eat ice cream. For my next installment, Morning Edition Executive Producer Madhulika Sikka asked me to explain why drinking hot tea cools you off on a hot day.

It does?

"Trust me," she said. "I'm Indian, I'm British. A billion Indians can't be wrong. They drink hot tea in hot weather."

So I started to make some calls.

Most scientists I asked for comment demurred, but Peter McNaughton, a neuroscientist at the University of Cambridge, offered an answer.

McNaughton agrees the whole thing is counterintuitive. "Obviously a hot drink makes you hotter and a cold drink makes you colder. So why would you want to get hotter on a hot day?" he asked rhetorically.

Here's how he explains why you might want to do that. Turns out there are nerves in our tongue and mouth that have special molecules in them called receptors. As the name suggests, these receptors receive signals from the world outside the nerve.

There are all sorts of receptors in all sorts of nerves, but the nerves in the tongue have a lot of one particular receptor that responds to heat. It's called the TRPV1 receptor, if anyone wants to know.

So when you eat or drink something hot, these receptors get that heat signal, and that tells the nerve to let the brain know what's going on.

When the brain gets the message "It's hot in here," it turns on the mechanism we have to cool ourselves off: sweating.

Yes, the hot drink makes you hotter ... but it does something else, too.

"The hot drink somehow has an effect on your systemic cooling mechanisms, which exceeds its actual effect in terms of heating your body," says McNaughton.

One other interesting thing. These TRPV1 receptors respond to hot heat, but they also respond to chemicals in chili peppers, which is why chili peppers seem hot. "That's probably why chili peppers are so popular in hot countries because they cause sweating and activate a whole raft of mechanisms which lower the temperature," he says.

I have to say I'm still a little skeptical about hot tea on a hot day. I'd still rather have a tall glass of ice water. What about you?


This story is part of Joe's Big Idea, an NPR experiment exploring how ideas become innovations and inventions. Follow us at Facebook.com/JoesBigIdea. And enter your big idea in our video contest.

Copyright 2013 NPR. To see more, visit http://www.npr.org/.

Transcript

STEVE INSKEEP, HOST:

All this summer, MORNING EDITION has been exploring the science behind some familiar summer activities. It's a series we're calling Summer Science, and today's installment begins at a café near NPR headquarters.

MADHULIKA SIKKA EXECUTIVE PRODUCER, MORNING EDITION: Some Darjeeling tea, please.

UNIDENTIFIED WOMAN: Sure, anything else?

EDITION: Can you make that tea for two?

UNIDENTIFIED WOMAN: Sure.

RENEE MONTAGNE, HOST:

Hey wait, Steve. That sounds like our very own MORNING EDITION executive producer Madhulika Sikka.

INSKEEP: It is. She went there with NPR science correspondent, Joe Palca, to take their tea outside into the sweltering D.C. heat.

EDITION: Tea, the best drink of the day, Joe.

INSKEEP: They were there to discuss an assignment Madhulika had given Joe to answer a question she'd always wondered about.

EDITION: Why does hot tea cool you off on a hot day?

JOE PALCA, BYLINE: Are you sure about that?

EDITION: Trust me, I'm Indian, I'm British. A billion Indians can't be wrong. They drink hot tea in hot weather.

PALCA: Well, I looked into it and I have an answer for you, but can I give you the answer back in the studio? It's boiling out here.

EDITION: Joe, we're drinking hot tea, you should be cool, but it really looks like you can't handle it, so I guess we're gonna have to go back to the studio.

PALCA: You see, it's much nicer in here.

EDITION: But you forgot to bring the tea, Joe.

PALCA: Oh, well. Anyway, I asked Peter McNaughton your question. He's a neuroscientist at the University of Cambridge. He agreed there's something a little counterintuitive about hot tea on a hot day.

PETER MCNAUGHTON: Obviously, a hot drink makes you hotter and a cold drink makes you colder. So why would you want to get hotter on a hot day?

PALCA: Well, here's his explanation. Turns out there are nerves in our tongue and mouth that have special molecules in them called receptors. And as the name suggests, these receptors receive signals from the world outside the nerve. Now, there are all sorts of receptors in all sorts of nerves, but the nerves in the tongue have a lot of one particular receptor called TRPV1.

EDITION: Do I have to remember that?

PALCA: No. It's just a name, don't worry about it. Anyway, do you know what kind of signal the TRPV1 receptor responds to?

EDITION: What signal, Joe?

PALCA: Heat. So when you eat or drink something hot, these receptors get that heat signal, and that tells the nerve to let the brain know what's going on. And when the brain gets the message, wow it's hot in here, it turns on the mechanism we have to cool ourselves off, and you know what that is.

EDITION: Sweat?

PALCA: Precisely. A hot drink makes you sweat, and the sweat cools you off. Yes. The hot drink makes you hotter, but it does something else too.

MCNAUGHTON: The hot drink somehow has an effect on your systemic cooling mechanisms, which exceeds its actual effect in terms of heating your body.

PALCA: And here's one other interesting thing. These TRPV1 receptors respond to hot heat, but they also respond to the chemicals in chili peppers, which is why chili peppers seem hot.

MCNAUGHTON: And that's probably why chili peppers are so popular in hot countries, because they cause sweating and activate a whole raft of mechanisms which lower the temperature.

EDITION: So hot as in hot, spicy food, hot drinks, same thing happening?

PALCA: Apparently so. Anyway, Madhulika, it seems this time you were right.

EDITION: I love it when a reporter says that to me.

MONTAGNE: That is MORNING EDITION's own Madhulika Sikka with NPR science reporter, Joe Palca. In addition to answering our summer science questions, Joe is spending his summer finding out where big ideas in science come from and how they make their way from idea to innovation.

INSKEEP: Turns out a lot of game changing ideas come from young people, and that's why we're launching a video contest to get those good ideas seen and heard.

MONTAGNE: It's a contest for people ages 13 to 25. To enter, make a short video explaining your big idea and sign up at npr.org/jbi, that's for Joe's big idea. You've got until August 12, so start thinking big. Transcript provided by NPR, Copyright NPR.