Children raised on farms and in the rural environment grow up to have more stress-resilient immune systems and have a lower risk to mental illness.
According to new research from the University of Ulm in Germany and CU Boulder in the US, children raised on farms, surrounded by animals and bacteria-laden dust, grow up healthier than their city counterparts.
The study adds to mounting evidence supporting the "hygiene hypothesis," which posits that overly sterile environments can breed health problems.
The study recruited 40 healthy German men between 20 and 40 years old. Half had grown up on a farm with livestock, and half had grown up in a large city without pets.
On test day, the men were asked to give a speech in front of a group of stone-faced observers and then asked to solve a difficult maths problem while being timed.
Blood and saliva were taken five minutes before and five, 15, 60, 90 and 120 minutes after the test.
Those who grew up in cities had significantly higher levels of immune system components called peripheral blood mononuclear cells (PBMCs) after the stressful experience.
They also showed 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 co-author Christopher Lowry, a professor of integrative physiology at CU Boulder.
"This exaggerated inflammatory response is like a sleeping giant that they are completely unaware of."
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 immunoregulatory response to stress develops in early life and is shaped largely by the microbial environment.
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 note.
"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," Mr Lowry added.
Research leader Stefan Reber, a University of Ulm Professor said: "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."