Down in the Dirt: Say Hello To Your “Old Friends”



Children now spend less time outside than the average prisoner. Adults spend 90-95% of their time indoors in their homes, work places, or cars. Living lives increasingly separated from the natural world comes with a cost, especially for kids, writes Paul Bogart in his new book The Ground Beneath Us. In an interview with National Geographic, Bogart says children need to be exposed to the microbes in the soil to build up their defenses against diseases that may attack them later:

“Kids these days are not being exposed to dirt because they’re not allowed to play outside. Their parents think dirt is dirty. But both the newest science and the oldest traditions tell us the same thing, which is that the ground is alive. The ground gives us life.”

The parents Bogart is talking about subscribe to the theory that dirt = microbes = germs (pathogens). This gave rise to the so-called hygiene hypothesis, the idea that early-life exposure to dirt/germs produces antibodies that protects us from disease – a sort of do-it-yourself vaccination program.

But there was a problem: Since the 1950s, in developed countries only, rates of multiple sclerosis, Crohn’s disease, type 1 diabetes, and asthma soared by 300% or more, and there were also spikes in hay fever and food allergies. And because scientists felt that our exposure to pathogens remained the same, they re-examined the hygiene hypothesis and discovered a problem: Yes, dirt = microbes; but no, microbes do not = germs. That’s because the vast majority of them are either necessary, helpful, or neutral. And by limiting our exposure to them we deprive ourselves of their health benefits.

A seminal article in the Proceedings of the National Academy of Sciences, titled “Cleaning up the Hygiene Hypothesis,” underscores Bogart’s view of the life-giving force of “dirt,” and argues we should replace the hygiene hypothesis with what they call the “old friends” hypothesis:

Today, epidemiological, experimental, and molecular evidence support a different hypothesis: Early exposure to a diverse range of ‘friendly’ microbes—not infectious pathogens—is necessary to train the human immune system to react appropriately to stimuli.

‘We realized human beings coevolved with a whole host of organisms, and it was far more likely what was going on was that we were being deprived of organisms on which we are dependent’ … early and regular exposure to harmless microorganisms—“old friends” present throughout human evolution and recognized by the human immune system—train the immune system to react appropriately to threats. It’s not that children … aren’t subject to enough infections when they are young, but that their exposure to the microbial world is far more circumscribed than it once was.

Young children continue amassing microbiota in every contact with family members, while playing outside in dirt, getting licked by dogs, and sharing toys with friends. The developing immune system takes cues from all of these encounters.

PNAS emphasizes that sequestering children away from the natural environment is one of several factors that reduces our exposure to necessary microbes. Others include the overuse of antibiotics (they kill good & bad microbes); the rise of caesarian sections (deprives the child of microbes found only in the birth canal); and the increased use of sanitizers that view microbes as signs of dirt to be destroyed.

PNAS also says that relaxing hygiene standards would not reverse this trend of rising rates of MS, Crohn’s, etc., but only serve to increase the risks of infectious disease. For example, they say, one can teach children to wash their hands after handling raw chicken but also encourage them to play outside in the dirt: “If your child has been out in the garden and comes in with slightly grubby hands, I, personally, would let them come in and munch a sandwich without washing,” says Graham Rook, an emeritus professor of medical microbiology at University College London.


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