Dr. Finlay is a microbiologist specializing in bacterial infections and the Peter Wall Distinguished Professor at the University of British Columbia. Dr. Arrieta is an assistant professor in the Department of Physiology and Pharmacology at the University of Calgary. This essay is adapted from their new book, “Let Them Eat Dirt: Saving Your Child From an Oversanitized World,” published by Algonquin Books of Chapel Hill.
Our friend Julia moved to a small free-range pig and poultry farm when her first child, Jedd, was a preschooler. When her second baby was born, she would strap him on her back every morning so that she could go to the chicken coop to pick up eggs. Jedd would chase and ride the chickens—and sometimes taste their feed and touch the fresh eggs. A couple of times, she even caught him chewing on something he had picked up from the ground.
At first, all of this caused Julia to freak out. But once she realized that Jedd wasn’t getting sick from these encounters with the chickens, she relaxed a bit. Her second child, Jacob, soon followed suit and never hesitated to get dirty on the farm. She once found him knee-deep in a cesspool of pig waste. Her early worries that her children were going to contract diseases from all this messiness dissipated, and she was pleased to see that they remained healthy.
Was Julia being an irresponsible parent—or might we all have something to learn from her example?
For most of the past century, we have considered microbes bad news, and for good reason: They cause disease, pandemics and death. Most human communities have experienced the benefits of medical advances like antibiotics, vaccines and sterilization, which have radically reduced the number and severity of infections that we suffer throughout life. Dying from a microbial infection is now a very rare event in the Western world, and, in the U.S., lifespans have increased by some 30 years since 1915—in large part because of success against infectious diseases.
Unfortunately, this progress has come with a price, as news reports have been telling us for some years now. Our anti-microbe mission has been accompanied, in industrialized countries, by an explosion in the prevalence of chronic noninfectious diseases and disorders. Diabetes, allergies, asthma, inflammatory bowel diseases, autoimmune diseases, autism, obesity and certain types of cancer are at an all-time high. The incidence of some of these disorders is doubling every 10 years, and they are starting to appear sooner in life, often in childhood.
All of these diseases have a genetic component, but their alarming growth cannot be explained by genetics alone. Recent studies find a direct link between the presence and absence of certain bacteria and all of the chronic diseases mentioned above. It turns out that the microbes within us are much more than quiet residents; they are an inherent part of our physiology, and altering them leads to disease.
Our own 2015 study (published in the journal Science Translational Medicine) found, for example, that 3-month-olds who had four particular microbes in their feces were much less likely to get asthma later in life. When those four microbes were introduced into mice, they protected against experimentally induced asthma, showing for the first time that alterations in gut microbes can drive the development of the disease. Lab experiments also have found that obese mice lose weight when they get a transfer of gut microbes from lean mice (and the reverse holds true as well, with lean mice growing fat after a transfer from obese mice).
The practical upshot of all this research is clear: Our health depends to a large degree on maintaining a robust and diverse community of microorganisms in our bodies—and establishing good gut-health as children is especially important.
During the first few months of life, the microbe community in our bodies is considerably less established and stable than later in life. Any drastic changes to it have a much higher chance of permanently altering our microbiota (as specialists call this world of tiny organisms within us) and our long-term health.
From the moment we are born, we begin getting colonized by bacteria, which kick-start a series of fundamental biological processes, including the development of our immune system. Before birth, the lining of our gut is full of immature immune cells. When bacteria move in, the immune cells react to them, changing and multiplying. They even move to other parts of the body to train other cells with the information they have acquired from these intruders. If deprived of this interaction, the immune system remains sloppy and immature, unable to fight off diseases properly.
Never before in human history have babies and children grown up so cleanly.
Scientists haven’t figured out exactly how microbes do this at the molecular level, but we do know that most bacteria will teach these immune cells to tolerate them, whereas some bacteria—the pathogens that cause diseases—prompt strong resistance. The result is to make the intestine a relatively controlled and harmonious place.
Another fundamental function of microbes is to aid in the regulation of our metabolism. Like other animals, humans obtain energy from food that is digested and absorbed in the intestines. Besides helping us digest certain foods that the intestines can’t handle on their own, bacteria produce compounds that help to define how we use or store energy in our bodies. New research also shows that our microbiota plays an important role in neurological development and even in the health of our blood vessels.
Such discoveries have led scientists to call our microbiota a “new organ,” perhaps the last human organ to be discovered by modern medicine. Most of this knowledge is still relatively new and many pieces of the puzzle remain unsolved, but protecting the initial developmental stages of our microbiota clearly has a significant impact on our health.
Inflammatory diseases (such as asthma, allergies and inflammatory bowel disease) and metabolic diseases (such as obesity and diabetes) are characterized by alterations in our immune system and our metabolic regulation. Knowing what we do now about the role of the microbiota, it is not surprising that these diseases are being diagnosed in more children. They are, to a great extent, a consequence of relatively recent changes in our lifestyle—modern diet, oversanitization, excessive use of antibiotics—that have altered the specific microbes that affect our metabolism early on. We urgently need to find ways to modify our behavior so that our microbes can function properly.
Never before in human history have babies and children grown up so cleanly, and our diets have lost many of the elements most crucial to the health of our guts. We have become very bad hosts to our microbes.
