Now there's another reason to get back to nature. A new study reveals that people who grow up in more rural environments are less likely to develop allergies. The reason may be that environments rich with species harbor more friendly microbes, which colonize our bodies and protect against inflammatory disorders.
"We are proposing that contact of people, particularly children, with the natural environment and biodiversity could be really important for the development of the immune system," says Ilkka Hanski, an ecologist at the University of Helsinki and lead author of the study.
Hanski and his colleagues investigated the biodiversity hypothesis, or the assertion that the global decline in biodiversity and decreasing contact with it is linked to the escalating prevalence of chronic inflammatory and autoimmune diseases. To test whether or not biodiversity does indeed create a shield against such conditions, the team investigated the microbial diversity of 118 teenagers. The study participants, who had lived in the same houses their whole lives, were chosen at random from a 100-by-150-kilometer block in eastern Finland. Some kids lived on rural, isolated farms, while others lived in larger towns. The researchers controlled for factors such as whether family members smoked, if pets lived in the house, and what type of allergens the subjects were sensitive to ensure that correlation with the bacteria's health benefits wasn't driven by a single allergen.
The group then took microbial samples of an area on their subjects' forearms and sequenced the DNA to figure out which species of microbes were present. They also surveyed all of the types of plants growing around the adolescents' homes. The participants were part of a separate long-term allergy study, so the researchers took advantage of that data to investigate the connection between biodiversity and allergies.
Though individuals with allergies lived throughout the study area, the authors found that allergies were tied to the amount of biodiversity around the teenagers' homes; the more forest and agricultural land, the lower the prevalence of allergies. On the other hand, kids living near bodies of water or in urban centers had significantly higher levels of allergies.
The rural kids were surrounded by more biodiversity than the urban kids, and as a result, they had more species of microbes than the urban kids, too. In particular, the researchers report online today in the Proceedings of the National Academy of Sciences, the number of species of a certain group of flora—uncommon native flowering plants—was 25% higher in healthy kids' yards than their allergic counterparts. Whether there is just something special about Finland's native plants or whether this finding can be applied around the world is still an open question, Hanski says. "Many research groups worldwide could easily attain these data from their study populations, and then we'd know how general these results might be."
In addition to higher levels of plant biodiversity around their homes, compared with urban kids, nonallergic individuals also sported a larger number of microbial species on their skin. One group of bacteria called gammaproteobacteria was especially more prevalent. Acinetobacter, a member of this group that is often found in soil, has been linked to higher levels of an anti-inflammatory marker in the blood of healthy study participants, meaning these bacteria may be responsible for convincing the immune system to ignore allergens. These particular types of bacteria seem to play an important role in explaining why children develop allergies or not, says Thomas Abrahamsson, a pediatrician at Linköping University in Sweden who was not involved in the study.
Allergies aren't the only thing at stake here, says Hanski. He thinks the diversity of microbes living with us "absolutely" influences other diseases, such as type 1 diabetes, asthma, and even depression. "We're not claiming that contact with nature and biodiversity is the only important thing, but it could be a significant contributing factor." Whether or not people living in cities could just inoculate themselves with ointments of gammaproteobacteria—for example by spreading a bacteria-laced lotion on their skin or consuming a probiotic beverage teeming with gammaproteobacteria—as a substitute for natural exposure remains to be seen, he says.
Abrahamsson thinks the study would have been even more compelling had Hanski's group analyzed the teens' gut microbiota as well, which his group and others have previously tied to allergies.
Obviously, Hanski acknowledges, not everyone can become a farmer. But this study and others like it should raise serious questions about how we want development to occur on an increasingly crowded and allergy-ridden planet, he says. "Urbanization can't be stopped, but perhaps we should take the planning of green spaces in cities more seriously."