How healthy is playing outside? Freezers in Maastricht filled with baby poop hold the answer

Do young children who play outdoors more often actually have a lower risk of developing allergic conditions such as asthma and eczema later in life? Maastricht microbiologist Niels van Best is trying to find out using baby poop and playground mud.

As a father of three young children, Van Best knows better than anyone: babies put everything in their mouths. Let them play outside in nature, and they ingest quite a bit of soil through their hands and toys. “According to one study, up to 60 milligrams a day, though I wouldn’t be surprised if it’s even more,” the researcher laughs. “As a parent, you sometimes feel the urge to intervene, but I usually just let it happen. Evolutionarily speaking, there must be a good reason why young children have that tendency.”

After all, the motto for young children is “The dirtier, the better,” says Van Best, an assistant professor at the Faculty of Health, Medicine, and Life Sciences (FHML). The bacteria children are exposed to early on play a significant role in determining the composition of their gut microbiota. This can influence their health later in life, for example, in the development of allergies. “In principle, a more diverse gut flora is better.”

More allergies

Van Best has been studying for years which factors influence that diversity. “The method of delivery—natural or cesarean—plays a role, as does whether the baby is breastfed or bottle-fed. But the environment is also important: are there siblings or pets bringing ‘dirt’ into the house, and does the child attend daycare? And consequently: does the child come into contact with microbes from the soil by playing outdoors?”

It is therefore not surprising that researchers suspect that the modern lifestyle, in which children spend most of their time indoors, is linked to the increase in conditions such as food and dust mite allergies, eczema, hay fever, and asthma. Still, the conclusion that “playing outside prevents allergies” is a bit premature, says Van Best. “Last year, there was a lot of media attention surrounding a report from the Lung Foundation about a Finnish study that supposedly showed that children who play in nature more often are less likely to develop asthma. But that was too simplistic; you can’t make such a definitive statement based on those research results. There’s a lot we still don’t fully understand: which microorganisms children pick up, where, and at what age, and what the exact effect of that is. We’re now trying to investigate that thoroughly.”

The answer lies in baby poop. Since 2018, Van Best and colleagues from Maastricht (Evgenia Dikareva, John Penders, Monique Mommers) have been following babies in South Limburg during their first year of life as part of the LucKi study; the count now stands at around 150. At various intervals, parents send a scoop of stool from the diaper to the health Campus labs at Randwyck (“We now have several large freezers full of poop here”). They also fill out questionnaires about things like what their baby eats, how often they get sick, and how often they go outside. “In the lab, we then investigate which microorganisms are present in the intestines and what function they serve there.”

To determine the exact effect of digging in the dirt, Van Best is comparing the bacteria from baby feces with the microbes found in soil samples taken from playgrounds. “This helps us figure out which organisms from the environment are actually ingested by children while they play, and at what age. We already have a good understanding of this.” The next step in the study—for which Van Best received a Veni grant last year from the Netherlands Organization for Scientific Research (NWO)—is to examine how immune cells react to these bacteria in the lab, and thus what role they play in preventing allergies.

Greener Playground

That knowledge is useful for developing interventions, says Van Best. “Think of a program for daycare centers where children go to play in the woods every week. Or making playgrounds greener, bringing nature closer. Then it helps to know at what age this has the greatest effect, or what type of soil or vegetation you want in that green playground.”

He hopes that South Limburg will become a living lab in the future for testing such interventions. “This is the perfect place for this. In the Mining Region, for example, there are more cases of asthma. These are caused by the poorer air quality of the past—asthma can be genetically inherited—but also by the environment: neighborhoods with a low socioeconomic status, with many high-rise buildings and no gardens to play in. It’s interesting to measure the differences with other regions. The proximity to Germany and Belgium is also useful. In the Netherlands, we send children to daycare at a relatively young age, often after just a few months, whereas in neighboring countries this usually doesn’t happen until after a year. That can also make a difference, which you need to take into account when developing measures.”

The COVID-19 Effect

The fact that the environment influences the gut microbiota of young children became clear during the COVID-19 pandemic. “In our LucKi study, we observed that babies born during the lockdowns had a distinctly different gut microbiome composition after one year compared to those born before and after the pandemic,” explains Van Best, who recently published this finding with colleagues in the journal Genome Medicine. “That doesn’t say anything about health effects, though; it’s still too early to measure those. Take asthma, for example: although it likely develops in the first year of life, you can’t measure it until age six.” It doesn’t yield any conclusions about playing outside. Measures such as the closure of daycare centers also played a major role, and perhaps people actually spent more time in nature with their children during lockdowns.

Source: Observant. Author: Dennis Vaendel. Photo: Shutterstock

About Niels van Best, NUTRIM researcher

Niels van Best is an Assistant Professor in Host–Microbe Interactions and Microbial Ecology at Maastricht University (Medical Center). His research focuses on how the early-life gut microbiome influences immune development. Trained through a joint PhD at RWTH Aachen University, he combines fundamental research, multi-omics approaches, and epidemiology to study how environmental microbial exposures shape the risk of childhood allergies. His current work explores whether contact with natural environments can help prevent allergies in children, with the aim of translating findings into practical solutions.