As a child, you may have been told that playing outside in the dirt is good for you because it strengthens your immune system. But is there any evidence to back this claim?
In short, yes — several studies suggest that early exposure to dirt may reduce kids' risk of developing allergies and autoimmune conditions. In other words, it may guard against conditions in which the immune system unhelpfully reacts to allergens or to the body's tissues.
As a child's immune system develops in the early years of life, the army of protective cells within the body have to learn how to distinguish between the body's own cells and foreign substances that are either harmless or disease-causing, such as bacteria and viruses. It must learn to spot the disease-causing pathogens in order to attack them.
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It turns out that the molecular signals that drive the expansion of this regulatory arm of the immune system mainly come from microbes in the gut, Graham Rook, a professor emeritus of medical microbiology at University College London, told Live Science. This collection of microbes is called the "gut microbiome" and is essential to our health. For instance, some of these microbes help produce vitamins that we need to live, and they help us digest our food.
The first year of life is crucial to the microbiome's development. Babies receive bacteria as they pass through the birth canal, if they're born vaginally, and from milk if they're breastfed. As children grow, they are steadily exposed to microbes from a wide array of sources.
A theory called the "old-friends hypothesis" suggests that, the greater the range of microbes we're exposed to in early childhood, the more diverse our microbiomes will be and thus the better our immune systems will recognize friend from foe. The term "old friends" refers to helpful, or "commensal," microbes that live on and in the body without harming a person's health.
This theory, proposed by Rook in 2003, is similar to the more widely known hygiene hypothesis, which suggests that a lack of early exposure to germs makes people more prone to immune conditions. Multiple studies, for instance, have demonstrated a link between growing up on a farm or in a household with pets and having a lower likelihood of children developing allergies, compared with kids in urban or pet-less environments.
However, the old friends hypothesis stresses the importance of being exposed to commensal microbes in early life, as opposed to infectious pathogens. This idea is backed by research: several studies in Europe suggest early exposure to germs does not guard against the development of allergies. Another critique of the hygiene hypothesis is that it downplays the importance of good hygiene to preventing disease, pushing the idea we've become "too clean," Rook and colleagues argued in a 2016 review.
The old friends hypothesis, on the other hand, could help explain why antibiotic overuse in early life, which can wipe out much of the gut microbiome, and cesarean deliveries, which don't expose newborns to vaginal bacteria, have been tied to an increased risk of allergies.
A trial in Finland looked at whether city kids' immune systems could be bolstered with grass and soil that had been taken from the forest floor. They found that, within a month, children who played in the dirt had a more diverse collection of harmless bacteria on their skin and more immune-regulatory cells and signaling molecules in their blood than those who played on gravel playgrounds. This hints that exposure to bacteria within dirt could help the immune system mature, theoretically reducing the chances of it becoming overactive.
While the microbiome is important, there are many other factors that affect someone's risk of developing allergies, including their genetics, Dr. Robert Wood, a professor of pediatrics at Johns Hopkins Medicine in Baltimore, told Live Science. As a general message, though, children should be encouraged to go outside and play in the dirt, he said.
However, scientists' current understanding of risk factors for immune conditions can't always be translated into practical advice. For instance, if you have a dog, you may have a somewhat lower chance of developing allergies than a pet-less person — but you can't tell somebody to get a dog as a guaranteed way to prevent allergies, Wood said.
Dirt in heavily polluted areas can also be unhealthy for kids, as it could contain harmful contaminants, he noted. This clearly wouldn't be the sort of dirt that you'd want your child to be exposed to. And because dirt can contain potentially harmful chemicals, such as lead, as well as parasites, care should be taken to stop children from inhaling or eating it.
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