According to the “hygiene hypothesis,” children need to be exposed to germs so that their immune systems can develop resistance to diseases. A recent study from the University of Colorado Boulder is providing more evidence to support this theory, finding that growing up in a rural environment around animals is linked to a more stress-resistant immune system and a lower risk of mental health issues.
“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. “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 investigation, which was led by Professor Stefan Reber of the University of Ulm, was focused on 40 healthy German males between 20 and 40 years old. Half of the individuals had grown up on a farm around animals and the other half had grown up in the city without any pets.
The study participants were tasked with giving a speech in front of a neutral audience before attempting to solve a challenging math problem while being timed. The researchers collected blood and saliva samples from the individuals before and after the challenges.
Those participants who had grown up in cities had a heightened immune response to the experience, including higher levels of peripheral blood mononuclear cells (PBMCs), prolonged elevation of the inflammatory compound interleukin 6, and muted activation of the anti-inflammatory compound interleukin 10.
“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,” said Lowry
Despite intense immune responses, the participants from urban backgrounds surprisingly reported feeling less stress after the tests compared to participants from rural backgrounds.
“This exaggerated inflammatory response is like a sleeping giant that they are completely unaware of,” said Lowry.
In past studies, exaggerated inflammatory response has been linked to a higher risk of depression and post-traumatic stress disorder (PTSD) later in life.
Previous research has also demonstrated that our immunoregulatory response to stress develops in childhood and is strongly influenced by our microbial environment.Humans are exposed to far fewer microorganisms in the city, which is now home to more than half of the global population.
“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,” said Lowry.
The researchers recommend eating probiotics, spending more time in nature, and getting a pet.
The study is published in the Proceedings of the National Academy of Sciences (PNAS).