More than 50 per cent of the world's population now lives in urban areas, meaning humans are exposed to far fewer micro-organisms than they evolved with, the authors noted
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Children raised without pets in an urban setting are more vulnerable to mental illness than those raised in a rural environment, surrounded by animals and bacteria-laden dust, says a new study.
Kids raised in rural settings may grow up to have more stress-resilient immune systems and might be at lower risk of mental illness, according to the study published in the journal Proceedings of the National Academy of Sciences (PNAS).
The study adds to mounting evidence supporting the "hygiene hypothesis," which posits that overly sterile environments can lead to health problems. The research also suggests that raising kids around pets might be good for mental health.
"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 study co-author, Christopher Lowry, Professor at the University of Colorado Boulder in the US.
"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," Lowry added.
For the study, led by Professor Stefan Reber of the University of Ulm in Germany, the scientists recruited a small group of healthy German men between ages 20 and 40. Half had grown up on a farm with farm animals. Half had grown up in a large city without pets.
On test day, all were asked to give a speech in front of a group of stone-faced observers and then asked to solve a difficult math problem while being timed. Blood and saliva were taken five minutes before and five, 15, 60, 90 and 120 minutes after the test.
"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," Lowry said.
Previous studies have shown that those with an exaggerated inflammatory response are more likely to develop depression and post-traumatic stress disorder (PTSD) later in life.
Research has also shown that our immunoregulatory response to stress develops in early life and is shaped largely by our microbial environment. More than 50 per cent of the world's population now lives in urban areas, meaning humans are exposed to far fewer micro-organisms than they evolved with, the authors noted.
"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," Lowry said.
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