A Little Grime Can Boost Kids’ Health. But What Kind?

 KATHLEEN DAVIS: Hey, it’s Kathleen Davis, and you’re listening to Science Friday. Today on the show, is grime good for kids? We asked you, our listeners, to tell us the grossest thing your kids have done.

[RECORDING BEEPS]

CHERYL: Hi, my name is Cheryl. I’m calling from Bozeman. My son, I one time caught him out at the barn. He was about three years old. And he was eating fresh llama poop, putting them in one pellet at a time, like they were raisins. And his mouth was covered with this green-black goop.

KATHLEEN DAVIS: Cheryl, thank you so much for selling your kid out for the story. You may have heard that a little dirt is good for kids. It helps them build up their immune systems, and it sets them on a path to future health. But what type of filth does the trick? Is a little llama poop good for the gut?

Here to answer those pressing questions and dig into the latest science into the benefits of exposing your kids to the outdoors are my guests, Dr. Jack Gilbert, Microbiologist and Professor at the Scripps Institution of Oceanography and in the Department of Pediatrics in UC San Diego School of Medicine, and Dr. Amber Fyfe-Johnson, Associate Professor and Pediatric Epidemiologist at the Institute for Research and Education to Advance Community Health at Washington State University based in Spokane. Thank you both so much for being here.

JACK GILBERT: My pleasure. It’s exciting to be here.

AMBER FYFE-JOHNSON: Thank you so much. What a treat.

KATHLEEN DAVIS: OK, before we really jump in, I want to ask you two, what are your thoughts on Cheryl’s call? Are you as horrified as I was when I first listened to that?

JACK GILBERT: I’ll just start by saying filth. I don’t know, man. We probably shouldn’t be exposing the kids to filth, but that sounds really bad. And I wouldn’t necessarily want my kids eating llama poop purely because their breath would smell really bad. But on the whole, what doesn’t kill them might make them stronger?

KATHLEEN DAVIS: What about you, Amber?

AMBER FYFE-JOHNSON: I have to second Jack’s opinion here. I’m all for dirt and microbes. Poop might be taking a little far, llama poop in particular.

KATHLEEN DAVIS: OK, so Jack, when we’re talking about letting kids play in the dirt, what kind of benefits to dirt are there? How do these different parts of the dirt fit into the equation?

JACK GILBERT: Really, it’s all about immune stimulation. I mean, what we’re dealing with are ape creatures, ourselves, who evolved in a world that’s seething with microbes, microbial stimulants, all the little bacterial cells, viral cells, fungi floating around. And they’re stimulating our immune systems. So all of our ancestors got this monstrously large microbial exposure. And we’re talking about trying to recapture that microbial exposure for children because their immune systems are expecting to see it.

AMBER FYFE-JOHNSON: I think that it’s important to remember that the childhood immune system development is fundamentally foundational across the health of the lifespan. It’s not just immediate. And so, from birth through adolescence, the immune system really is learning how to defend against infections while also avoiding harmful overreactions. And early disruptions in this process, such as limited microbial exposure or things like chronic inflammation, can really bias an immune system and immune system responses towards allergy or autoimmunity or chronic disease in childhood and in adulthood.

And the gut microbiome plays a central role. About 70% of immune cells interact with the gut. So, for example, children with reduced gut microbial diversity in early life have higher rates of asthma and allergic disease later in childhood.

Childhood is this really critical intervention window because immune system development is really highly plastic in early life. And intervening later, when we’re adults, is far less effective than supporting immune development early on. So what happens in early life does not stay just in early life. It programs immune function for health across the lifespan.

KATHLEEN DAVIS: What are the environmental exposures that make the biggest difference for kids?

JACK GILBERT: We found that, really, it’s exposure to animals and dirt. So soil, I should say, for my soil scientist friends, is key. The soil is a very rich microbial world.

And that, along with the microbes and allergens that are associated with furry pets and furry animals, can play a foundational role in supporting immune development. In fact, children that grow up physically interacting with a dog have almost a 15% reduction in the likelihood of developing asthma. So these are significant trajectories.

KATHLEEN DAVIS: So what about an indoor cat, for example? I mean, that cat’s not necessarily going outside and rolling around in the dirt itself. I mean, is it beneficial for kids to have that exposure?

JACK GILBERT: No, weirdly, cats are not immunostimulatory in the same way as dogs. And we think that dogs are actually better than cats. And I’m a dog person, so I’m horribly biased towards this. But yeah, I mean, I get this question all the time. And cats, no, people that grow up physically interacting with the cat don’t seem to have the same kind of beneficial immune exposure as those that physically interact with a dog.

KATHLEEN DAVIS: Wow, cat slander, I guess. OK, how does playing outside improve kids immune systems? How physically do you expose your body to these different allergens? I mean, do you have to eat llama poop to get the effect? Or can you just kind of roll around in the dirt here?

JACK GILBERT: I did a study once, propositioned the NIH– I think that’s the right phrase– to ask them if I could have a traveling zoo or traveling farm that I would take around and rub babies faces into the sides of cows so that they’d get that proper immune stimulation. Mostly, it’s through physical contact and breathing stuff in. So if you plunge your hands into the dirt, your immune system is actually being stimulated by all of those microbial antigenic signals in the soil.

And if you rub a pet with your hands or you let it lick your face, you’re getting that kind of microbial exposure. And then, whatever you breathe into your lungs is stimulating the antigenic receptors that are all throughout your respiratory tract and your mouth and your skin. So these are primary ways. It’s physical interaction and respiratory interaction with the world. That’s really important.

KATHLEEN DAVIS: I mean, just by touching something is fascinating to me. So in theory, could you rub your hands in a bunch of dirt and then come inside and wash your hands and still get an effect?

JACK GILBERT: Absolutely, and we recommend that 100% There’s no problem with cleaning your hands. It can help to reduce the spread of infectious disease and reduce the chances that you’re going to put something into your gastrointestinal tract, which might make you sick. So we absolutely say that, yeah, you will get the immune benefit from interacting with the world. But, hey, wash your hands afterwards before you eat.

KATHLEEN DAVIS: There’s a study that came out recently in Nature that showed even just one month of preschool improved the diversity of kids’ microbiomes. I want to ask you about the mechanisms of this, at risk of maybe getting a little grossed out. But what changes can come to kids’ microbiomes just by interacting with other kids?

JACK GILBERT: Hey, we’re social creatures. We have evolved all these wonderful mechanisms to interact with each other– kissing and hugging and eating each other’s boogers and licking each other’s–

KATHLEEN DAVIS: Maybe just you. I don’t about me.

JACK GILBERT: But I think, to me, that’s the key piece here is that kids that can socially interact with their peers will get a greater significant exposure to the world, to the microbial world, to the antigenic world, that will shape their immune system and shape their microbial diversity. In fact, we worked with baboons in the Amboseli National Park, with collaborators from across the US and Africa, to identify how social interactions between these baboon groups drove changes in their microbial structure. And baboons that more socially interact with each other share beneficial microbes that actually help their health. So we think that these kids, when they get that, a year of preschool exposure, are getting much more rigorous social interaction, which is shaping their microbial world, their immune systems, and microbial diversity, potentially for benefit.

KATHLEEN DAVIS: OK, I want to ask you both, in the last decade, there’s been a lot of scientific evidence that backs this idea up of dirtier kids, for lack of a better term, of being healthier kids. But have we seen the culture changing? I mean, is it acceptable to have a kid rolling around in the dirt? Maybe Amber, you want to go first?

AMBER FYFE-JOHNSON: Yeah, I think that there has been certainly an acceleration in interest in outdoor preschools. So the estimated number of outdoor preschools in the United States has skyrocketed. So around in 2010, there were like 25. And in 2020, there were almost 600. And this is across the United States. So to me, that means there’s really been sort of a value shift.

So many of these outdoor preschools, fundamentally, historically, have come from European countries, where being outside is, from a value system perspective, believed to be healthy. And so what we’re seeing is a bit of a lag, but a sharp acceleration in the United States. So to me, that means that the people are embracing this mentality. And these preschools are not isolated simply to Washington. They’re really distributed to Minnesota, the Carolinas, et cetera. So it’s really a national distribution. And that, to me, is really exciting.

KATHLEEN DAVIS: Mm-hmm, I mean, I can imagine some listeners may be skeptical of outdoor preschool, just as it could be easy to push it off as kind of like a hippie, crunchy, granola thing. But, I mean, you’re scientists. Is it that far out of an idea.

AMBER FYFE-JOHNSON: It’s absolutely not. I often talk about how we have this culture that believes that kids, for some reason, thrive in indoor spaces, in spaces that have four walls. But yet, many of us know, from our own experience as adults, that taking a quick walk outside or sitting on the beach, there’s an effect there.

So I would also come back to COVID a little bit here, that COVID kind of pushed the margins in terms of schools and preschools thinking about what was acceptable and what could be the norm. And a lot of schools experimented with doing educational opportunities outside and found really incredible benefits, not just for the kids, but for their teachers as well. And so I think COVID actually was a lesson and sort of pushed us in the right direction in terms of thinking about how education has, for quite frankly, decades, happened outside more.

There was more recess time where kids just were allowed to be outside more. The pendulum shifted a little bit to being more indoor. And now we’re starting to shift back to being more outdoor. And to see that change is really exciting.

JACK GILBERT: And every rural farmer that I’ve ever spoken to says, oh, I’ve known this for years, peck of dirt before you die will keep you healthy. I think this is not something which is totally isolated to hippie, granola-eating people. It’s a fundamental precept of what it means to live in a rural environment.

KATHLEEN DAVIS: After the break, we’re going to keep rolling in this pigpen of knowledge. Don’t go away.

[GENTLE MUSIC]

OK, I want to ask about this experiment that you’re both involved in, that compares kids who go to traditional, mostly indoors preschools versus kids who go to an outdoor preschool, like we’ve been talking about. Amber, what have you found so far?

AMBER FYFE-JOHNSON: We’re early in the project. We’re just finishing year 2. So we don’t have results from our current project. But we used preliminary results to get funding to do this larger project.

And I think one of the more exciting things that we found is that we compared preschool-aged children attending an outdoor preschool versus kids attending an indoor preschool. And we found a difference in gut microbial diversity, which was really encouraging, in that while we hypothesize that there’s going to be some changes in this space, we actually did indeed see them in this preliminary data, and also that it is likely related to some cardiovascular outcomes, in particular, childhood obesity and childhood overweight, so that we are hoping, in some way that the project that we’re doing will influence cardiovascular health, both from an adiposity framework, but also cardiovascular health, long-term, into adulthood as well.

KATHLEEN DAVIS: So, Jack, let’s say that you grew up as an indoor kid with extra vigilant and sanitary parents. I mean, can you make up for lost time? Can you diversify your microbe by, say, going hiking every weekend as an adult?

JACK GILBERT: I think you can definitely have an impact later on, especially on your microbiome. You can increase the diversity of foods that you eat. You can increase the amount of time you spend outdoors. And as Amber was suggesting, just being outdoors makes you feel good.

Maybe it’s the fresh air, maybe it’s the sunshine, or maybe it’s all of those things plus microbial stimulation. And we do see that people that spend more time outdoors have lower levels of cortisol, lower levels of systemic inflammation. And we believe that that’s due to this mediating influence of the microbiome on those impacts.

KATHLEEN DAVIS: OK, to wrap up, I want to play a voicemail from a VIP caller.

FLORA: Hey, it’s Flora, long time listener, first time caller. Hello, from Phoenix, Arizona, where I am this week. So I have little kids, and they are constantly doing gross things, like little kids do, including last weekend, we were at a bowling alley. And they were eating french fries off the ground. And when I left that night, I had genuine stress where I was like, are they going to be OK.

And here’s my question for your guests. What kind of gross behavior could my kids do that would actually put them in danger? Where is the line? What behaviors do I actually need to be concerned about when it comes to rolling around in the New Jersey Transit bathroom, for example? OK, tell me. Thank you. Have a good show.

KATHLEEN DAVIS: OK, can you put our dear Flora at ease here? I mean, is the line llama poop?

JACK GILBERT: For me, the line is, and for Flora’s edification, there are dangerous pathogens in your kitchen. Wash your hands and wash the surfaces after you’ve been using raw chicken. Make sure your kids are not putting those potential pathogen concerning things into their body.

And the same is true for certain bathrooms. I mean, it’s extremely unlikely in our modern environments, in the United States, that you will be exposed to enteric pathogens through those systems. But it is possible. So, hey, wash your hands. I wouldn’t pick up the fries if they’d fallen in the toilet.

KATHLEEN DAVIS: [LAUGHS]

Any parting words for parents or caregivers out there who are maybe rethinking a few things after this conversation?

AMBER FYFE-JOHNSON: My summary, which will come as a surprise to nobody, is try to promote outdoor time for yourself and your children. Aim for frequent micro doses. It doesn’t have to be a hike in the mountains. It can be, but think about things that are pragmatic and doable. You can walk around the block.

And I like the concept of exploration versus destination walk. Does your child want to walk far, or do they just want to meander around the block and find something cool in the leaf litter or something like that? And also, try and advocate for green space and outdoor time at schools.

Kids are at schools. They’re getting recesses. This is an important place where all kids can have access to outdoor time and beautiful outdoor spaces.

KATHLEEN DAVIS: Well, thank you both so much for being here. This was really fascinating. Dr. Jack Gilbert, Microbiologist and Professor at Scripps Institution of Oceanography and in the Department of Pediatrics in UC San Diego School of Medicine, and Dr. Amber Fyfe-Johnson, Associate Professor and Pediatric Epidemiologist at the Institute for Research and Education to Advance Community Health at Washington State University, based in Spokane. Thank you both so much for being here.

JACK GILBERT: Thanks, it was a blast.

AMBER FYFE-JOHNSON: Thank you so much.

KATHLEEN DAVIS: This episode was produced by Shoshannah Buxbaum. Thank you for listening. I’m Kathleen Davis. We’ll see you next time.

[GENTLE MUSIC]

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