Let Them Eat DirtImprovements in hygiene and the development of antibiotics made huge improvements in health over the 20th century. But B. Brett Finlay, co-director of CIFAR’s Humans & the Microbiome program, argues that our anti-germ ways have gone too far. In his new book Let Them Eat Dirt, co-authored with Marie-Claire Arrieta, he tells parents that a little dirt is good for all of us.

News & Ideas: Let Them Eat Dirt is a provocative title. What message are you trying to get across?

Finlay: The premise of the book is really that we have to ease off on our hyper-hygiene and let kids be kids, and allow them – especially early in life – to encounter the microbes that they normally would encounter, because we know that’s beneficial for them.

N&I: That’s still a new idea to most of us. We tend to try to keep everything as clean as we can.

Finlay: I think it’s still a hangover from the last hundred years, and the major kick we’ve been on to clean up the world. We realized that microbes cause disease, so we introduced sanitation and sewers, we got rid of the mice and the rats, and we started to have cleaner food and water, and then antibiotics came along and vaccines.

In the last hundred years we’ve dropped the mortality rate of kids less than a year of age from close to 30 per cent to down to less than 0.1 per cent, which is just fabulous. Kids don’t die of infectious diseases anymore.

But what we’re now realizing is that microbes play a big role in our development as an organism. So early in life microbes play a key role in shaping and pushing and developing your immune system, and that ties into asthma-type diseases and autoimmune diseases.

Microbes also play a role in brain development. We now know that there are microbial links to autism and depression and anxiety and stress. And even normal body development, bone development, muscle mass development. These are all linked to microbes.

And then finally the main link of course is through the gut in breaking down food and making things that our body needs to grow.

N&I: The suggestion in the book isn’t just that microbes are good for us, but in a way that they’re part of us.

Finlay: We have to think of ourselves as more of an ecosystem. There are 15 times more microbial genes than human genes in and on us, so when you’re talking about our genome, one should be actually thinking about all these microbial genes. We’re a sort of super-organism, the sum of both Homo sapiens as well as the 10 trillion plus microbes living on us.

After all, we evolved with microbes. We know that microbe-free humans and animals won’t live – microbes make essential vitamins, plus all these other processes we talked about.

N&I: This is actually one of the key ideas behind the CIFAR Humans & the Microbiome program.

Finlay: Yeah, it’s exactly this, this realization that we are this super-organism. In the program we’re exploring very radical things, like can microbes shape human behaviour, or if there’s a societal change, what does that do to the microbes that then might affect humans.

And so we have anthropologists in the program and as well developmental biologists. We know that microbes play a big role in our development, and the whole field of development biology has virtually ignored this. And now we’re having this stunning realization that these microbes and all the small molecules they make actually influence normal development.

N&I: Your book is written largely for parents. What’s your advice to them? How do they walk this line between too clean and not clean enough?

Finlay: It’s really a balance. And that is stressed throughout the book. We try to offer helpful hints based on the scientific knowledge we have. You know, like get a dog. It decreases allergies by 20 per cent. Or if a kid spits their soother on the ground, studies showed that if the parent puts it in his or her mouth and back into the kids’ – as opposed to washing it off and then putting it back in the kids’ – those kids that had the parents’ mouths’ microbes, they actually had less asthma and obesity.

So we’ve tried to fill it with helpful hints as best we know, based on the science of what a parent can actually do to expose their kids to get the best microbial exposure they can in the world we live in.

N&I: What about antibiotics?

Finlay: They’re a beautiful wonder-drug for killing bacterial infections in kids, and they’ve saved countless millions of kids’ lives.

But there are two problems. One we’ve all heard about is antibiotic resistance. But another is that we previously thought that for an individual, at worst antibiotics might not work, but they’re harmless.

And now we realize that antibiotics truly carpet-bomb the microbiota and have profound effects on the microbial composition. And there are studies coming left, right and centre now about how early use of antibiotics affects asthma, affects allergies, affects obesity, and even later in life affects dementia and Alzheimer’s and these mental diseases.

So we have to rethink antibiotics. When you need them, use them, but use them wisely.

N&I The book is aimed at a popular audience, and it’s been getting a lot of attention. What’s that been like?

Finlay: As a scientist I’ve spent all my life trying to publish papers and change the world. This is just an alternate way of doing it. And this seems to have a bit of an audience, so that’s pretty cool.

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