Why Sweat Is A Human Superpower
Sweat is an "evolutionary marvel," says Sarah Everts, the author of The Joy of Sweat. In her new book, Everts delves into the science of perspiration and how this trait has enabled humans to thrive.
Think sweat is gross?
Turns out human sweat — our body's air conditioning system — is really pretty tame on the "yuck" scale of animal cooling methods.
Dogs drool to stay cool, while vultures will poop on their legs and seals urinate on their feet, she says. When you think about what evolution could have bequeathed us, Everts says, "sweat is arguably a million times better."
In fact, Everts tells NPR's Short Wave, instead of thinking of sweat as gross, think of it as an "evolutionary marvel." She even calls it a human superpower and a highly efficient one at that. "We effectively dispatch water to our skin and, as it evaporates, it whisks heat away from our bodies," she says.
Everts points out in her book that this superpower enables humans to thrive and dominate across the globe. "Sweating allowed us to forage out in the sun without overheating, while our predators were relegated to the shade for survival," she writes, and for us to adapt to many new environments. "Like the (city) pigeon and desert dove, we're capable of surviving almost anywhere."
Short Wave's Rhitu Chatterjee talked to Everts about the incredible science of sweat, which includes facts most people don't know. Like get this: How much you sweat is affected by both nature and nurture. And did you know you can sweat in different colors? Yep, both are true.
Check out Everts' fascinating interview below. Don't have time to read it? No sweat! You can listen to it by clicking on the audio link above.
This interview has been edited for clarity and length:
In your book, you write that to sweat is human. How much someone sweats is sort of a mix of their genetics and also where they've grown up, right?
It's kind of interesting because these glands — they're called eccrine glands — you're born with all the eccrine glands that you'll ever have. But these glands don't become fully active until you are in your toddler years. And so in those very early years of your life, your body is kind of learning about the climate that you're in. And researchers are trying to figure out how much of the environment in which you spent your early years is ultimately going to affect how much you sweat as an adult.
That's the nurture part. But there's also a nature part. Some people and some families are just sweatier than others. And, perhaps you have more sweat glands than average. Most people have between 2 [million] and 5 million.
I got mine counted. I have 3 million! And there's also the flow rate, right. So some people are very efficient with their sweating and some, the floodgates just open right up. And so that's also possibly related to genetics but also possibly related to acclimatization and your body learning on the go.
I mean, athletes do this right. In preparing for the Olympics in a really hot place, they will try to train in very similar conditions so that their bodies learn to cool down efficiently in that kind of environment.
You write in the book that sweat contains so much more than water.
Oh, yeah! So this was the thing that really blew my mind. When you think about sweat, it took me a long time until I was writing this book to be like: Where is it really coming from? It's effectively the liquidy parts of blood, minus the big stuff like red blood cells and platelets and immune cells.
And so if you open up a body, you're very wet inside. Right. You have this thing called interstitial fluid that's keeping all your organs damp and moist. And that liquid is sourced from blood. And when your body gets the cool-down directive, then your sweat glands source sweat from that interstitial fluid. So literally anything that's in your blood that's small can percolate out.
So, evidence of my morning coffee comes out in my sweat. When I have a drink of gin and tonic on a hot day — definitely the alcohol comes out. Evidence of the food we eat, evidence of our health or even how we're exercising.
There's a story that you write about in the book: the South African nurse whose sweat turned red.
Oh, to be this woman. She was a nurse and wore a white uniform and noticed that her sweat was red. And this is a very strange situation, right?
She was really insecure about it and went to a dermatologist. But they could not for the longest time figure out what was going on because she was in her 20s; she was perfectly healthy. They checked her for everything.
One day she comes to the clinic for a follow-up appointment, and one of the doctors notices her fingers are kind of stained like a reddish brown color. It turns out she had had a pre-appointment snack, and it was this corn chip called Spicy Tomato. She was very fond of these chips. She was eating multiple bags a day — so very fond. The dermatologist called it a fetish.
They analyzed her sweat for the same colorants and dyes as the chips and got a match. And so effectively, once she dialed down her predilection for these particular chips, her red sweat disappeared. But other people have turned their sweat all sorts of colors for all sorts of reasons. It's a funny little corner of the medical literature called chromhidrosis: chrome, light colored and hidrosis for sweat.
What about the other components that sort of leak out into sweat? Do they have any role? And what makes sweat stinky?
What's coming out of your sweat pores — the entire medical role for it, is to keep you cool. But in the process, your body also dispatches some proteins that do crowd control for the microbiome of your skin. Right. So helping the helpful bacteria thrive and trying to keep pathogens at bay. But most of the stuff that comes out in this watery sweat called eccrine sweat is just what happens to be flowing around your body.
Of course, there is another kind of sweat: The stuff that makes you really stinky. That's the sweat that comes out in your armpits. It's actually an entirely different kind of sweat gland. It's called the apocrine gland, and it gets active at puberty — as many of us know.
And that one is not like salt water at all. It's kind of like waxy, a little bit similar to earwax, but a lot more minute, and the bacteria living in your armpits eat that sweat and metabolize it and effectively poop out what is the body odor that comes from your armpit.
So body odor changes, depending on the content of your sweat and the mix of bacteria that are metabolizing the sweat?
Exactly. And so we all have a unique body odor print, right? We all have our own smell. And that is effectively the mix of waxy molecules coming out of those apocrine glands, plus the unique-to-you ecosystem of bacteria living in your armpit. And so that combination is what gives you your unique odor print and mine. And allows dogs, for example, to track humans based on having sniffed something that they've worn.
You write that sweat is poorly understood by scientists. What are the things we still don't understand about sweating, and what are researchers investigating?
Some of the really interesting areas are the evolution of sweat glands and how we actually came to have evolved one of the most efficient ways to cool down in the animal kingdom.
We don't even know how many genes are involved in sweating. And a lot of other really fabulous research is still going on. [Researchers are] trying to, for example, do the forensics of science — like monitoring what is coming out when we sweat and monitoring what are the chemical residues in a fingerprint. Because a fingerprint is just a sweat print. And forensic researchers are now able to learn all about your biological identity from the actual chemicals left behind in the fingerprint.
If you can analyze the chemicals of that fingerprint and find out, wow, that person was, you know, drinking alcohol or that person actually has cancer — all sorts of very private information is being left behind in the drips we leave on our yoga mats but also literally on everything that we touch.
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