As a father of three young children, Van Best knows all too well that babies put everything in their mouths. Let them play in nature and they are likely to ingest a fair amount of soil through their hands and toys. “One study suggests it’s up to 60 milligrams per day, but I wouldn’t be surprised if it’s more”, he says with a laugh. “As a parent, you sometimes feel the urge to step in, but I usually just let it happen. From an evolutionary perspective, there must be a reason why young children do this.”
When it comes to young children, it’s very much a case of “the dirtier the better”, explains Van Best, an assistant professor at the Faculty of Health, Medicine and Life Sciences (FHML). Early exposure to bacteria plays a key role in shaping the gut microbiome, which in turn can influence health later in life, including the development of allergies. “A more diverse gut microbiome is generally better.”
Increase in allergies
Van Best has spent years studying which factors influence this diversity. “The mode of delivery – natural birth or C-section – plays a role, and whether the baby is breast-fed or formula-fed. But environment matters too. Are siblings or pets bringing bacteria into the house? Does the child go to daycare? Or come into contact with soil microbes by playing in nature?”
Unsurprisingly, researchers suspect that modern lifestyles – with children spending much of their time indoors – may be linked to the increase in conditions such as food allergies, dust mite allergies, eczema, hay fever and asthma. But Van Best says it is still too early to conclude that outdoor play prevents allergies. “Last year, there was a lot of media attention for a report from Lung Foundation Netherlands (Longfonds) on a Finnish study suggesting that children who spend more time playing in nature are less likely to develop asthma. But that’s too simplistic; you can’t draw such firm conclusions from those findings alone. There’s still so much we don’t know – which microbes children pick up, where and at what age, and what their precise effects are. We’re now working to study this properly.”
Nappies
As it turns out, baby poo may hold the answer. Since 2018, Van Best and colleagues have been following babies in South Limburg during their first year of life as part of the LucKi study, which now includes around 150 babies. Parents periodically send in stool samples from nappies to Randwyck (“We now have several large freezers full of poo”) and complete questionnaires about diet, illness and time spent outdoors. “In the lab, we analyse which microbes are present in the gut and what function they have.”
To determine the precise effects of soil microbes, Van Best compares the bacteria found in baby poo with microbes from soil samples taken from playgrounds. “That allows us to see which environmental microbes are actually picked up by children during play, and at what age. We’re starting to build a clear picture of that.” The next step in the study, for which Van Best received a Veni grant from the Dutch Research Council (NWO) last year, is to examine how immune cells in the lab respond to these bacteria – and, therefore, what role they may play in preventing allergies.
Greener playgrounds
This knowledge could help inform practical interventions, says Van Best. “Think of daycares taking children to play in the forest every week, or greener playgrounds that bring nature closer. It would help to know the age at which this has the greatest effect, or which types of soil or vegetation are most beneficial.”
He hopes that South Limburg will one day serve as a living lab for testing such interventions. “It’s an ideal setting. In the former mining region, for example, asthma rates are higher. That’s partly due to historically poorer air quality – asthma can be passed on genetically – but also environmental factors: neighbourhoods with lower socioeconomic status and lots of high-rise housing without gardens for children to play in. It would be interesting to compare this with other regions. The proximity of Germany and Belgium is also useful. In the Netherlands, children start daycare quite young – often within a few months – whereas in neighbouring countries it’s usually after a year. That may also make a difference, and needs to be taken into account when developing interventions.”