Finnish children who played in formerly gravel-covered urban day care center yards renovated with natural forest floor developed more diverse microbiomes (gut and skin bacteria) and signs of better-regulated immune systems within just one month, according to a new study in the October 16 issue of Science Advances.
After 28 days of playing on the forest floor, children in these day care centers had more diverse skin and gut bacteria and a stronger presence of anti-inflammatory proteins in their bodies than children in day care centers without the intervention.
The research, which included a total of 75 children between 3 and 5 years old, suggests it may be possible to improve immune systems in urban communities simply by granting the youngest residents daily access to green spaces. In other words, city kids might be healthier if they spent more time playing in the dirt.
"We were surprised that the findings were so clear even though we did not get as many participants as we had hoped," said Aki Sinkkonen, a research scientist at the Natural Resources Institute Finland in Turku and an author of the study.
City residents are not known for their flourishing gut microbiomes. Frequent antibiotic use and exposure to urban pollutants are believed to take a toll on the structure of city dwellers' inner microbial communities. A highly processed Western diet is certainly no friend to gut bacteria, either, as demonstrated by professor of genetic epidemiology and writer Tim Spector's son, a college student who completed a 10-day McDonald's anti-cleanse in the name of science.
People living in built environments may simply have less exposure to diverse microbiota from the outdoors. In a 2018 study, Sinkkonen and colleagues collected doormat debris from 30 rural and 26 urban households, finding that people living in a more developed area had fewer, less biodiverse microbial organisms on their doorsteps.
Previous studies have also shown that urban populations, with their less diverse gut microbiota but greater exposure to pathogens, are more susceptible to immune-mediated diseases. While the prominent "biodiversity hypothesis" blames lack of exposure to diverse microbiota, scientists had yet to procure evidence to support this hypothesis.
"I am interested in the synergy between biodiversity conservation and actions supporting human health," said Sinkkonen. "We wanted to find new ways to help urban children sustain health-supporting microbiota."
To accomplish this while testing the biodiversity hypothesis, Marja Roslund, a researcher at the University of Helsinki and the lead author of the study, and her colleagues worked with a total of 10 day care centers. Out of seven standard day care centers largely surrounded by concrete, four were randomly chosen to receive the new yards, while three nature-focused centers (where children took daily excursions to nearby forests) provided a yardstick for comparison.
The Experiment
The team worked with a commercial provider to install transplanted patches of natural forest floor (kuntta, in Finnish) in the barren day care center yards. The forest floor arrived complete with about 20 centimeters of topsoil and a rich spread of native vegetation, including dwarf shrubs, blueberries, crowberry, and mosses. Within the course of an evening, the day care grounds were transformed into green oases.
While it may have taken a little time for the children's microbiomes to pick up on the change, the children's reactions were immediate and enthusiastic.
"One of the daycare teachers told us that when children in the evening nap day care center went out, they stared for a moment and then ran to the yard," said Sinkonnen. "This had never happened before."
The children played in the newly renovated yards for an hour and a half each day over a 28-day period, with nurses at the day care centers leading guided activities that included planting vegetation, crafting natural materials, and playing games in the greenery. The researchers took blood samples and measured the children's skin and gut microbiota just before the vegetation arrived and again at the end of the experiment, storing the samples on dry ice in a lab.
When they analyzed the samples, Roslund and colleagues found that the microbiota of the children at standard day care centers that received the intervention shifted, becoming more like the microbiomes of children who attended the nature-oriented day care centers. Children at the intervention centers had more diverse Proteobacterial and Gammaproteobacterial communities on their skin after the trial, while the relative abundance of Clostridiales bacteria in their guts decreased and the diversity of Ruminococcaceae increased. Similar changes were not observed in children at the standard, unmodified day care centers.
Importantly, children at the intervention day care centers also developed a higher ratio of the anti-inflammatory protein IL-10 to the pro-inflammatory protein IL-17A, suggesting that playing in the forest dirt stimulated their immunoregulatory pathways. In contrast, children at standard centers without the intervention did not experience a similar immune boost.
Sinkonnen noted that the researchers plan to extend their trials to other countries and vegetation types, adding that trampling-resistant vegetation may provide a better alternative to forest floor in future interventions.
"Natural Resources Institute Finland is developing techniques to produce biodiversity for urban environments at a larger scale," said Sinkonnen. "Additionally, the government of Finland is economically supporting day care greening based on findings by us and other major groups in Finland. I hope major research funders will open calls for research combining biodiversity and health."
[Credit for associated image: Aki Sinkkonen]