"Don't get dirty!" used to be a constant family refrain, as parents watched in despair as their children ruined their best clothes.
Whether they were running through fields, climbing trees or hunting tadpoles, it was inevitable that by the end of the day their white clothes would turn brown.
Today, many parents are sorry that their children no longer have the opportunity to feel a little dirt on themselves.
With the rise of urbanism and the appeal of video games and social networks, contact with nature has become much rarer than in the past.
For many, there are simply no more opportunities to get involved.
What is saved in laundry costs could be lost in children's health.
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According to recent research, outdoor dirt is full of friendly microorganisms that can train the immune system and build resistance to a wide range of ailments, including allergies, asthma, and even depression and anxiety.
These findings show that outdoor exercise isn't just beneficial because of the chance to run around freely - but also because certain natural materials, such as soil and mud, contain surprisingly powerful microorganisms whose positive effects on children's health are only beginning to be understood.
Mental renewal
Many of the psychological benefits of outdoor play are well known.
Our brains evolved in natural environments and our perceptual system is particularly suited to wild, outdoor spaces.
This means that natural scenes provide the perfect level of stimulation, which is thought to help the brain recover when it's tired or starts to lose concentration.
Supporting this theory, a 2009 study found that children with attention deficit hyperactivity disorder (ADHD) were able to concentrate better after a twenty-minute walk through the streets of a well-maintained urban neighborhood.
Being around grass and trees seemed to have a positive effect on their minds.
The authors suggested using such "doses of nature" as a safe and affordable way to help children with ADHD, along with other aids.
In addition to these restorative effects, outdoor play can also offer beneficial educational experiences.
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For example, the act of sculpting and kneading materials such as mud or sand can help children develop the interaction between senses and movement, known as sensorimotor development, according to Francesco Vitrano, a child neuropsychiatrist, psychotherapist and lecturer at the University of Palermo in Italy. , who has many years of experience in applying these therapies.
This allows the child to gradually understand his own body signals.
Such activities - outside the home or classroom - can also help children find ways to cope with emotions that may be difficult to explore in other settings.
So-called "sandtray play therapy," which involves using sand and miniature figurines to express thoughts and feelings, is an accepted form of counseling for children who have trouble verbalizing their emotional state.
When it comes to children's physical health, the most obvious benefit of playing outside is sheer activity.
It may be easier for a child to gain strength and endurance in the vast open space, which reduces the risk of obesity, according to a study led by Elizabeth Gershoff, a professor of human development and family sciences at the University of Texas, Austin, in the US.
The latest findings, however, suggest that there may be a number of other benefits to playing in the natural environment - and the secret may lie in the mud rolling itself.
Old friends
New research offers a fresh look at the "hygiene hypothesis", first put forward in the late XNUMXs.
According to this idea, the great reduction in childhood infections during the 20th century had an unwanted effect on people's immune systems, making them overreact to even the slightest stimulation.
The result is thought to have been an increase in asthma, hay fever and food allergies.
Many scientists, however, today dislike the term "hygiene hypothesis," since it seems to have discouraged important habits such as hand washing.
And they abhor the idea that infections, in and of themselves, are good for children.
"It was quite problematic from a public health perspective," says Christopher Lauri, professor of integrative physiology and director of the Behavioral Neuroendocrinology Laboratory at the University of Colorado, Boulder, US.
Instead, non-infectious organisms are now thought to be the key - not the ones that actually make children sick.
These "old friends" have been present throughout most of evolutionary history.
They are mostly harmless and train the immune system to moderate activity, instead of overreacting to any potential intruder.
More importantly, our bodies encounter these old friends every time we spend time in nature.
With increased urbanization and reduced outdoor play, many children now lack that exposure - meaning their immune systems are more sensitive to any threat and more likely to go into overactivity.
Various studies support this idea.
People who grow up on farms are generally less prone to developing asthma, allergies, or auto-immune disorders like Crohn's disease—thanks, it seems, to their exposure to a much wider range of organisms during childhood in a rural environment that promoted more efficient regulation of the immune system.
Much of the healthy stimulation provided by these bacilli is thought to come through the digestive system - today it is common knowledge that the friendly microbes in the gut can improve our health in a variety of ways.
But they can work on and through the skin, according to Michele Antonelli, a doctor in Reggio Emilia, Italy, who has researched the ways mud therapies affect health.
The outer layer of our body hosts a variety of microbes, he says, and people with disorders such as atopic dermatitis (a common form of eczema) and psoriasis appear to have a depleted community of organisms.
Microbial diversity appears to be linked even to conditions such as arthritis.
"These microorganisms can play a big role in many serious chronic diseases," he says.
Healthy body, healthy mind
Most surprising of all, friendly bacteria from nature can even influence the body's response to stress.
When we feel vulnerable and threatened, the immune system starts to increase our bodily inflammations.
Since inflammation is one of the first defenses against infection, this response evolved as a way to prepare the body for potential physical injury from the threat we face—but it's less useful for the kinds of stresses people face today.
Incredibly, people who spent most of their childhood in rural environments tend to show a more muted response to stressful events such as public speaking, with reduced expression of inflammatory molecules such as interleukin 6, compared to those who grew up in cities.
This remained the case even when the scientists controlled for other factors - such as their socioeconomic status.
This could have serious consequences for long-term health, as chronic inflammation in the body can contribute to a wide range of conditions.
For example, it appears to increase the risk of depression.
"People who grew up in cities are kind of 'walking time bombs' when it comes to their inflammation," says Lauri, who co-authored the study.
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"Dramatic Consequences"
And while the results supporting the "old friends hypothesis" continue to pour in, some researchers have begun to identify the specific organisms that might be responsible for the positives and the ways in which those changes occur.
Lauri is particularly interested in Mycobacterium vaccae, which can often be found in soil.
When mice are exposed to M. vaccae, they show increased activity of regulatory T cells—which, as their name suggests, help curb immune activity, including inflammatory responses.
This then appears to make them more resilient to stressful events, such as a potential conflict with another, more aggressive mouse.
"We see very dramatic effects on stress resistance, even a month after the last injection," says Lauri.
Of course, mice are not the same as children, but it does offer some useful insights into the role played by certain microorganisms.
Some scientists have also expressed considerable excitement about the role of "helminths" - soil-dwelling parasites such as roundworms - mainly because of the moderated immune response they trigger in the host.
People infected with helminths appear to have a lower risk of inflammatory bowel diseases such as Crohn's disease.
Some trials have attempted to deliberately expose patients to the larvae of these parasites, with varying success.
These therapies, however, would have to come to terms with potential side effects when you have worms.
And we're not even mentioning the obvious "disgust factor" that will most likely dissuade many people from intentionally ingesting helminths.
More shockingly, Antonelli suggests that many spa therapies—including mud baths and thermal mineral water—can improve your health by joining beneficial organisms to the microbial communities on your skin.
Many species could be responsible for the positive side of this, including Staphylococcus epidermidis, which has been shown to have anti-inflammatory effects.
Forest transplants
Given the importance of early exposure to friendly bacteria, many scientists are investigating the positive side of interventions that encourage greater contact with nature in childhood.
Antonelli says the act of "forest bathing"—gentle meditative walks through the woods—has been shown to alleviate symptoms in children and teenagers with atopic dermatitis, for example.
By touching leaves and soil, they may have picked up friendly organisms that enriched their skin microbiomes, he says.
An ambitious project in Finland, at the time, tried to bring nature to children.
In four day stays, the researchers replaced the asphalt and gravel from the school yard with "transplanted" soil and vegetation taken from the soil of the northern forests.
The children were also given planter boxes to practice gardening.
"It encouraged further contact," says Aki Sinkkonen of the University of Helsinki, co-author of the study specializing in mud microbiomes.
A month later, the children showed signs of increased microbial diversity on the skin and in the gut, as well as improved immune function.
They had higher numbers of those regulatory T cells that can help moderate the body's reactions to harmful intruders, for example.
The proportion of anti-inflammatory molecules in the blood plasma also increased - again indicating a better regulated immune system.
In the future, Sinkkonen hopes to test the long-term effects of these changes.
"I think it will lead to less human disease -- at least that's our hypothesis."
Mud kitchens
Given the known psychological benefits, many daycare centers and schools already encourage more contact with nature - such as nature school, regular nature walks and making mud kitchens that encourage children to play in the dirt.
"In many kindergartens and schools, there is a growing awareness that children have less open spaces to play," says Marilisa Modena, an architect specializing in school design, and founder of Zeroseiplanet, an Italian research and training center that deals with the positive sides of play in in the open.
"And we're looking for ways to reintroduce activities that, up until fifty years ago, were a common experience for children."
Greater interest in outdoor play arose in Northern Europe, says Modena, but today it is spreading to many other countries.
With future research, it may be possible to enrich the soil in home gardens and schoolyards with the most beneficial organisms.
For now, parents and teachers will have to make do with what they have.
Mud kitchens, in particular, can be cost-effective and take up little space.
All you need is an old table and some pots recycled from the kitchen and filled with a mixture of soil and water.
More sophisticated structures can be equipped with cabinets and drawers full of rocks, sand and vegetation.
And as the little mud chefs get dirty creating new imaginative creations, they may be cultivating their minds and optimizing their immune systems, with potential benefits that will last for months or even years.
* David Robson is a writer from London. His latest book is The Expectation Effect: How Your Mindset Can Transform Your Life, published in early 2022. He's @d_a_robson on Twitter.
* Alessia Franco is an author and journalist who deals with history, culture, society, storytelling and their impact on people. She is @amasognacredi on Twitter.
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