The connection boils down to bacteria: children raised in the countryside with animals are exposed to more bacteria-laden dust that strengthened their immune systems and make them resilient to stress.
People who grow up in cities without pets have a higher risk of mental illness and chronic stress, a new study has found.
The researchers at the University of Colorado Boulder say their findings are yet more evidence that kids should be spending more time playing in nature, since overly-sterile environments ironically breed health issues.
The authors say much more research is needed but that they can say with certainty that city-dwellers would benefit mentally from eating more probiotics, spending more time outdoors, getting a pet.
Scores of research has shown nature and animals boost immune strength. But this is the first study to show people raised in hyper-clean city environments have poorer mental health
Research has shown that our immune response to stress develops in early life and is shaped largely by our microbial environment.
An overt inflammatory response increases the risk of depression and post-traumatic stress disorder (PTSD) later in life.
However, while the downsides of hyper-cleanliness have been documented for years, this is the first study to make a clear connection between early life environment and mental health.
And it is more pertinent than ever: more than 50 percent of the world’s population now lives in a urban areas, meaning humans are exposed to far fewer microorganisms than they evolved with, the authors point out.
‘It has already been very well documented that exposure to pets and rural environments during development is beneficial in terms of reducing risk of asthma and allergies later in life,’ said co-author Christopher Lowry, a professor of integrative physiology at CU Boulder.
‘This study moves the conversation forward by showing for the first time in humans that these same exposures are likely to be important for mental health.’
The study was led by Germany’s University of Ulm, in collaboration with CU Boulder.
Lead author Professor Stefan Reber gathered 40 healthy German men, aged 20 to 40 years old.
Twenty of them were raised on farms surrounded by animals, while the rest were city babies, largely without pets.
To test their stress levels, each participant was asked to give a speech in front of a group of people who gave no reaction.
They were then asked to solve a complicated math equation under timed conditions.
In order to monitor their immune response, the team took blood and saliva samples taken five minutes before – as well as five, 15, 60, 90 and 120 minutes after the test.
The results were stark.
After the stressful test day, the men who grew up in cities had much higher levels of peripheral blood mononuclear cells (PBMCs), a key component of the immune system which rises under stress.
Even 120 minutes after their experience, they still showed elevated levels of an inflammatory compound called interleukin 6, and dulled activity of interleukin 10, which is meant to fight inflammation in times of stress.
‘People who grew up in an urban environment had a much-exaggerated induction of the inflammatory immune response to the stressor, and it persisted throughout the two-hour period,’ Professor Lowry said.
To the team’s surprise, city-dwellers seem quite unaware of their higher risk: even though city-born people had a much weaker immune response to stress in clinical tests, they claimed to be less stressed than their rural peers.
‘This exaggerated inflammatory response is like a sleeping giant that they are completely unaware of,’ Professor Lowry said.
He added: ‘If you are not exposed to these types of organisms, then your immune system doesn’t develop a balance between inflammatory and anti-inflammatory forces, and you can develop a chronic, low-grade inflammation and exaggerated immune reactivity that makes you vulnerable to allergy, autoimmune disease and, we propose, psychiatric disorders.’
Professor Reber said his next step is to study to larger samples, including women, and to look at other parts of the world to see if some places have higher or lower mental health risks than others.
In particular, he wants to quantify exactly how beneficial it is to be around animals and nature in early life.
‘A lot of research still needs to be done. But it looks as if spending as much time as possible, preferably during upbringing, in environments offering a wide range of microbial exposures has many beneficial effects,’ Professor Reber said.