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Dr. Samantha Youmeen (8:37)

Have you ever done that Casual head scratch, then turn to the side to inconspicuously try to check if your armpits smell. I just did it last night. I hate to break it to you, but it was probably obvious what you were trying to do. And that's okay because smelling a little spicy from time to time is a very common experience. And contrary to popular belief, it doesn't necessarily mean you have bad hygiene. There's a whole host of fascinating microbiology happening on our skin. To learn more about it, we have with us Dr. Chris Callewart, who goes by the amusing alias of Dr. Armpit. Dr. Caluaert is a microbiologist at the University of Ghent in Belgium and a science communicator. Welcome to the show, Chris. Or do I call you Dr. Armpit?

Dr. Chris Callewart (9:19)

Absolutely. Thank you for having me.

Dr. Samantha Youmeen (9:22)

We're really excited to chat with you about this. Our whole team was fascinated because there's, there's so much more science than you might expect under the armpits. So for starters, let's talk about what causes body odor and how is it different than just sweat?

Dr. Chris Callewart (9:36)

There's two reasons for body odor. One is sweat and the second is bacteria. So you need, you need both the body odor and in the underarm. We have all types of sweat and we have apocrine sweat there. We don't have that on our forehead or on our scalp or on the upper back. We only have them in the underarms and also in the groin area. And apocrine sweat. You can actually compare it a bit with earwax. It's that sticky yellow liquid, ish, gluey liquid that is actually destined to have a bit of a sense and not necessarily bad, but. And then it boils down to which bacteria we have. If we have the good microbiome, we don't smell in the underarms, but if we have the bad bacteria, we can have a specific musty, sour, oniony, pungent, strong, even cannabis like odor in the underarms. It really boils down to which bacteria we have in the underarms.

Dr. Samantha Youmeen (10:31)

And how much do we know about the different combinations of microbes that cause different smells? Like is there a recipe, a certain mix of microbes gives this scent versus another?

Dr. Chris Callewart (10:41)

Yes, more or less. What we have have on average 100 or 200 different species in the underarm and each underarm, so it's a wild diversity of bacteria present there. It's really dependent on which are the dominant strains in the underarm, whether your underarm has a specific odor.

Dr. Samantha Youmeen (10:57)

I'm curious if there are evolutionary or even cultural factors that shape our perceptions of different body smells and which ones we tend to be drawn to or not. And how has deodorant played a role in that?

Dr. Chris Callewart (11:10)

Let's take a step back. The thing is, also in the last 100 years, our lifestyle has changed a lot. In fact, deodorant as we know it is has only started since the second World War. Before that, you know, our grandparents didn't have any deodorants and they were just fine. They didn't have any body odor at all. They just used water and soap basically. Right? And now basically 70% of the population has a deodorant. So is using deodorant on a daily basis. 95% has a deodorant. So basically everybody has it. Right? So in the last hundred years, things have changed a lot. Our diet has changed a lot, our cosmetic uses has changed a lot. Hygienic practices change a lot. So that has, in my opinion, led to more people having more body odor. And one another example is also that we usually have body odor in the underarms and not in the groin area or not so much. And that's the reason, because we don't apply deodorants in the groin area or not normally we just use it in underarms and deodorants. Antiperspirants, they have they cause a vast shift in microbiome? We've done several studies on that. If you apply antiperspirants in the underarm, it's a short term solution, you kill bacteria and less bacteria is less older. But once there's, there's a leave on product so there's the stuff remaining and then bacteria adapt to that new environment. And instead of having the good bacteria, we have the smelly bacteria that come back the fastest. And as such we select a bit towards more smelly situation. In terms of culture differences, scent is something very complex. It's really, it really boils down to what is happening in our brain. For some person, scent can be good, another person can be totally awful. It's really associated in our brain and it's actually located in the same area as memories. So scents can retrieve memories really well. In terms of cultural differences, I can also note in the east, in Korea, Japan and Southeast Asia, it's less common to have body odor because they have a difference in gene. For instance, 100% of the Koreans have mutation over of the human gene which their underarms not to smell at all. And it's actually also coupled to their earwax. So you can check if you have that mutation or not. And if your ear wax is not liquid ish, if it's white, dry and flaky, you have that mutation. And that leads to a different kind of apocrine sweat in the underarm, which basically leads the bacteria cannot convert it into smelly compounds. It can have a smell.

Dr. Samantha Youmeen (13:24)

I was curious if there's like something we've learned culturally, like this is a good smell, this is a smell that we want to be close to or not, but perhaps like we're still in an era of change. So it's not clear yet what that might be.

Dr. Chris Callewart (13:39)

Well, we tend to cover up odors, right? But in the animal kingdom, odors are super important. It's like the primary source of communication. Like bacteria and little one cell organisms, they would just signal through secretions of molecules. We just tried to cover it up so we don't smell it anymore. But we haven't lost it, it's still there. If you sniff, your underarm is a bit sour, or there's some oniony smell, there's. It can actually also be even be a cue for something going on in the body. So basically if we are sick, we can smell differently. If you have some sort of inflammatory disorders, you can smell differently. Usually for us humans we have a short nose and for dogs it's more Easy to pick up differences in scents because they have a bigger nose and their olfactory ability is up to 100,000 times better than ours. So we can train dogs for smelling cancer, for smelling Covid, for smelling explosives and drugs and everything. But a wide variety of diseases are linked to differences in underarm odor or odor or breath odor or body odor in general. So actually, body odor can be an early warning signal for a disease, can also be altered by medication. Many cases, if there's diabetes, people smell more sweet as a result or more musty as a result. So there's actually many cues that can be linked to health diseases as well. Age as well. So with puberty, you know, hormones peak, and then actually over time, it's the levels of apocrine sweats and odors will decrease until your age of 80 or 90. Then you're again at the same level as childhood. So there's many influences throughout life that impact our scent in general.

Dr. Samantha Youmeen (15:26)

I want to follow up on something you mentioned, how, for example, dogs can scent different things. They can smell different things on humans. I'm curious if there are. There's some combination of microbes or some certain scents that influence human animal interactions in general. Or I saw you say something about mosquitoes and them having perhaps preferred being attracted to certain smells.

Dr. Chris Callewart (15:50)

It really boils down to the skin microbiome. There's other influences too, but the skin microbiome has a really big impact on attraction of mosquitoes. We're doing a big project, a big European project on that, on trying to change the skin microbiome in order to have less mosquito attraction or at least have less of virus transfer through to mosquitoes. Because it's an upcoming problem that tropical virus and tropical mosquitoes are coming our way and causing more problems.

Dr. Samantha Youmeen (16:18)

I know you've done some studies on how different skincare products affect the skin microbiome, comparing like facial lotion, moisturizer, foot powders, and then of course, deodorant. Can you tell us more about how those everyday skin products might be changing our skin bacteria and ultimately influencing our scent in ways we may not realize?

Dr. Chris Callewart (16:36)

Yeah, the biggest impact is really antimicrobial ingredients, sunscreen, skin moisturizer did not have such a big impact on our skin microbiome. Of course, on our metabolome, we saw that those lipids were added and we could see them for. Even if you don't use that product anymore, you could see them for up to three or four weeks afterwards. It's really a product. These molecules can really attach to the skin For a long, long time. But for an antimicrobial foot powder and definitely also for antiperspirants, these contain antimicrobial ingredients and it's leave on product so it stays on the skin. And that can have a prolonged impact. What we always saw is that the skin microbiome adapts to that new environment. So you kill all bacteria. So in the morning you don't have any bacteria in theory, but throughout the day you sweat, warm, moist and nutritious, is rich. So bacteria will come back. It's inevitable. And it will not be the same bacteria that will come back. It will be the most stress tolerance, most even pathogenic bacteria that will come back the fastest. Not the friendly, not smelly bacteria. But usually it's a smelly bacteria that come back the fastest. So it creates a little bit of a vicious circle where we apply products and we need to apply more in order to maintain that. And that's also a bit what many people have said to me. They come to me, Chris, we, I have tried everything, nothing's working anymore. They tried one product, went on to another, then that works for, for a couple of months, didn't work anymore. And then he applied another product, works for a while, stops working, need to change again. And at some point they're just at the end of the process and they have completely messed up under our microbiome, completely out of whack, usually containing multi resistant bacteria that are just constantly causing bad smells, not just on the skin, but also inside the skin. So it's a situation that is hard to get rid of in a natural, in a normal way.

Dr. Samantha Youmeen (18:27)

I was chatting with our producer Theresa Carey and we were talking about the cultural belief among some backpackers and hikers that not using deodorant and showering less frequently can help reduce body odor over time. Especially if you're out there for many, many days. Because the skin microbiome can adjust to that less frequent washing. So is that true and how does the composition of sweat change if you're now altering your hygiene? Just kind of counterintuitive to what most people would expect to do.

Dr. Chris Callewart (18:57)

I've heard many cases where it was very successful and actually many people have tried it during COVID then they didn't have to go to the office, they could stay at home and then they could apply instead of applying every, every morning deodorants, they can just stop using it, go for a detox or apply something else. And usually with a detox you stop all underarm cosmetics in the first week or first Two weeks will be hard. You smell more and it's unpleasant. You're not used to it, but after a while the microbiome adapts. Basically you have 100 different species living in the underarm and some of those, you know, gram negative ones are really became quite dominant. But if you leave it up to the microbiome, they really fight each other. They will secrete molecules that kill each other. Not themselves, but not other bacteria. And so it's really bacterial warfare among each other. And it's usually some dominant species can become dominant again and will kill all the bacteria. And hopefully that's a good bacteria that will then be on the winning hands and will basically create a microbiome that is stable that will ward off any musty, sour, pungent small odors and you can free of body odor. Not a guarantee for recipe for success. Sometimes it's also one of those smelly bacteria becomes dominant.

Dr. Samantha Youmeen (20:14)

See I love to experiment with things, especially when it comes to skincare. So I'm very tempted to try out this experiment. But at the same time I do a sport where I'm like sweating very heavily for like two to three hours a day. So it feels like that's an experiment that wouldn't go well for me. Is, is that related? Like does it matter how much you're sweating on a given day to, to whether this type of approach would work for you?

Dr. Chris Callewart (20:39)

It's in fact not related to the wet sweat and the smelly sweat is, is not, not necessarily related to each other. You can have hyperhidrosis, you can have wet underarm or underarm stains which is not smelly at all. So it's sometimes can be smelly but in theory it's irrespective of each other. There, there, there's no direct influence on really boils down to not how much you smith, but how much which bacteria we have there. And they're quite stable over time. You can influence it by changing your underarm cosmetic, your deodorant or antiperspirant. You can influence it by hormone stress have a big impact. Also diet has a big impact. You can steer a bit based on consuming meat, vegetables, caffeine, coffee, sugary drinks, fast food. All these things can really have an impact on older generation under arm as.

Dr. Samantha Youmeen (21:27)

As well Anecdotally it seems like nervous sweat has a different smell than just like regular sports sweat. I don't know if that's a, if there's a truth to that or like.

Dr. Chris Callewart (21:37)

And that's because adrenaline that is caused by stress has a direct impact on apocrine sweat glands. And these are actually located in the hair follicle. And they will push out the content of the apocrine sweats. Remember the yellow sticky stuff? They will push it out and inside there's already bacteria living there and converting everything in there. So stress sweats can actually really smell much stronger than ultra sweat. Yeah.

Dr. Samantha Youmeen (22:04)

We've talked on the show before about the microbiome in the gut. That's a very popular topic in research these days and in pop culture or pop science, and how that gut microbiome can impact health in so many ways that we're still learning about. I was reading in one of your papers that the skin and gut microbiome are connected. Can you explain more?

Dr. Chris Callewart (22:25)

Yeah, it's called the skin, the gut skin axis. And it's actually a bidirectional relationship. We've done a deep dive into literature and we saw that that's most, if not all of the common skin disorders have a direct relationship with the gut gut microbiome, the gut, health in general, psoriasis, eczema, acne, they're all related to the gut and even body odor. We found that it is also related to the guts. So you are what you eat, you smell how you eat. They have a direct impact on your health.

Dr. Samantha Youmeen (22:57)

Just to end off here, I wanted to ask, is there anything that you want to share about something people might want to think about changing or experimenting with, or do you have any maybe interesting or surprising things that you do based on your research and how it's impacted the way you deal with your armpits? Because you are Dr. Armpits. I gotta ask, what tips do you have for my pits?

Dr. Chris Callewart (23:23)

Tips? People don't need to take an example to me, but I don't use any, any shampoo, I don't use any deodorants and facts. It's quite good for me. But what are tips for the pits? I can give you some tips. Let's say eat more vegetables, specifically spinach, kale, anything green that will help the guts, that will help protect the guts inside. Eat less red meat, eat more fish. Omega 3 fatty acids, which is good. No fast foods or very limited amount of fast foods. No fizzy drink, no sodas, no sugary, no fast, refined sugars. Limit the number of cookies, chocolates, just stick to one or a couple. Have less stress. Easier said than done. Of course.

Dr. Samantha Youmeen (24:07)

Don't be stressed.

Dr. Chris Callewart (24:09)

Yeah, exactly. I would say also limit amount of caffeine. That can also lead to more stress and more body odor. Don't wear polyester, but wear cotton. Natural fiber is better than synthetic fibers. People always rate their underarms as worse than it is in reality, so it's usually not as bad as you think it is. Because your underarms are right underneath your nose. You're the first one to smell it, but therefore another person will not necessarily smell it. You will be the first one to notice it, so don't stress about it too much. A bit of odor is perfectly okay.

Dr. Samantha Youmeen (24:44)

Dr. Chris Callowart, aka Dr. Armpit, is a researcher and science communicator. Sharing the science on armpit body odor in relation to the microbiome. I first discovered Chris on threads actually, where you go by Dr. Armpit. That's rmpit. Highly recommended. Follow so many fun things to learn from you and thank you so much for being on our show. Chris.

Dr. Chris Callewart (25:05)

Thank you. Thanks Sam.

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It's been 700 years since the Black Death decimated the globe. Since then, we've learned a lot about what actually happened. We know that that plague is the most deadly documented pandemic in human history. It was spread by fleas. We know that it likely started in Central asia during the 1300s, and that it spread super fast, killing anywhere between 75 to 200 million people across Europe and Asia in two decades, including roughly half the population of Europe at the time. But a new paper published in Communications, Earth and Environment is shedding light on some of the unknowns surrounding the Black Death, namely, the perfect storm of coincidences that brought the plague to Europe in the first place. The Black Death, or bubonic plague, is caused by a bacterium that infects fleas. During the medieval period, the infectious disease was transmitted by the fleas to small mammals like rats, and it eventually spread to humans. Researchers have known that global trade was a likely cause of the plague's transmission from Asia to Europe, but they weren't sure of any specifics until now. The researchers analyzed samples of ice from Antarctica and Greenland, as well as tree rings from Spain that grew during the 1340s. In the ice, they found evidence of high concentrations of sulfur, and the tree rings showed that the temperatures were unnaturally cold during the summers of 1345 and 46, the coldest summers in nearly a century. They combine this new knowledge of a sulfuric cold atmosphere with records of poor harvests, high grain prices, and notes on weather during that time. Using all that information, they suggest that either one massive volcanic eruption or a cluster of eruptions occurred, pushing tons of sulfur high into the Earth's atmosphere. This would have blocked out the sun's heat and caused harvests to fail around the Mediterranean region. Coastal cities in Italy were particularly impacted, and food supplies began to decline. The Italian city states had to resort to emergency international imports to feed their people, and that is where the fleas come in. The fleas that carry the plague are able to survive for long periods without a host by eating grain dust, which allowed them to stay alive during the long journey across the Black Sea from Asia to Europe. The fleas arrived with the grain. They were stored and distributed, and the Black Death devastated the continent. This research actually tells us a lot about the climate's potential role in perpetuating deadly illnesses throughout history. The term quarantine actually came as the result of the Black Death. It's from the Italian word for 40 quaranta, because people learned that there was less disease when the sailors were forced to stay aboard their ships for 40 days before coming ashore. As climate change continues in the aftermath of COVID 19, it's important to look at the relationship between environmental health and human health so that we can be better prepared for next time. For Warner Bros. Discovery Curiosity Weekly is produced by the team at Wheelhouse DNA. The senior producer and editorial correspondent is Teresa Carey. Our producer is Chiara Noni, our audio engineer is Nick Kharisimi and head of Production for Wheelhouse DNA is Cassie Berman. And I'm Dr. Samantha Youen. Thanks for listening.

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