This was the conclusion of new research that was led by the University of Ulm in Germany and is now published in the journal PNAS.
This study is by no means the first to propose that growing up in urban settings lacking in microbe diversity can undermine physical health.
In that respect, it adds to the growing evidence in support of the theories that developed from the "hygiene hypothesis."
But the study is the first to suggest that a greater risk of psychiatric disorders — likely due to an "exaggerated immune response" — might be another unexpected consequence of growing up in an environment with fewer opportunities to interact with a variety of microbes.
"It has already been very well documented," says study co-author Christopher A. Lowry, who is a professor of integrative physiology at the University of Colorado Boulder, "that exposure to pets and rural environments during development is beneficial in terms of reducing risk of asthma and allergies later in life."
However, he adds that their study also "moves the conversation forward by showing for the first time in humans that these same exposures are likely to be important for mental health."
Losing contact with co-evolved microbes
Human existence is becoming increasingly urbanized. In 1950, only a third of the world's population lived in cities. By 2014, this figure had risen to 54 percent and is expected to rise to 66 percent by 2050.
The idea that increasing urbanization and the changes in lifestyle that accompany it might increase risk of certain diseases because of reduced interaction with a variety of microbes stems from the hygiene hypothesis.
The theory has its roots in 30-year-old research that suggested that a lower rate of infection among young children was the reason that rates of asthma and allergy-related diseases shot up in the 20th century.
However, it has become apparent that interaction with microbes goes beyond this original scope, and it has even been suggested that the term hygiene hypothesis is a misnomer and should be abandoned.
In their study paper, senior author Stefan O. Reber, a professor in molecular psychosomatics at the University of Ulm, and his team use the term "old friends" to refer to the microbes that co-evolved with humans.
Prof. Lowry and colleagues previously discussed how "progressive loss of contact with organisms with which we co-evolved" may be to blame for "much of the failure of regulation of inappropriate inflammatory immune responses" seen in many modern city-dwellers and inhabitants of wealthier nations.
Study tested men with various upbringings
The new study probes this link further by comparing stress-related responses in young adults who were raised in rural settings where they had plenty of contact with animals with those of people who were raised in urban settings "in the absence of pets."
The researchers enrolled 40 healthy male volunteers aged 20–40 years who were resident in Germany.
Half had been raised on farms where they frequently handled animals, and the other half had been raised in pet-free city environments.
To create the stress condition, all of the participants completed two tasks. In the first, they gave a presentation to an audience that showed no reaction, and then, they had to solve a difficult mathematics problem under time pressure.
The volunteers gave samples of blood and saliva 5 minutes before the test, and then again 15, 60, 90, and 120 minutes afterward.
'Exaggerated immune response'
The results showed that the young men raised in cities without pets had a "pronounced increase" in levels of "peripheral blood mononuclear cells." These cells form a large part of the immune system.
Meanwhile, members of the city upbringing group also had sustained higher levels of interleukin 6 and "suppressed" levels of interleukin 10. Interleukin 6 is a compound that promotes inflammation, while interleukin 10 is a compound that reduces it.
Prof. Lowry says that these results showed that "[p]eople 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 2-hour period."
What surprised the researchers was that although their bodies appeared to have a more sensitive response to stress, the men with a pet-free city upbringing reported lower feelings of stress than their counterparts who had been raised on farms.
Prof. Lowry likens the "exaggerated inflammatory response" of the city-raised men to "a sleeping giant that they are completely unaware of."
Contact with animals could be key factor
In discussing their findings, the authors mention previous research that showed that the way our immune system responds to stress is shaped in childhood by our interactions with microbes.
Other studies have suggested that an amplified inflammation response is linked to a higher rate of post-traumatic stress disorder and depression later on.
They also discuss how the presence or absence of animals might be an important factor in the findings.
They note how other researchers have found that "highly industrialized farming with low contact with farm animals" is more closely tied to conditions related to immune dysregulation — such as asthma and allergies — than "traditional farming with regular contact with farm animals."
This would suggest, they explain, that the "protective effect" of a rural upbringing with animals compared with a city upbringing without animals more likely comes from having contact with animals than the difference between rural and city life.
'Get a pet and spend time in nature'
The researchers now want to repeat their study with larger groups — both male and female — and with more varied upbringings in order to tease out the effects of animal contact and degree of urbanization.
They also acknowledge that their study did not take into account other factors that might impact childhood exposure to microbe variety.
These include, for example, the type of delivery at birth, breast-feeding compared with formula feeding, use of antibiotics, and diet.
In the meantime, the researchers suggest that city-dwellers get themselves a "furred pet," spend time in nature, and eat foods that are "rich in healthy bacteria."
"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."
Prof. Stefan O. Reber