Freezers Full of Baby Poo Are Unlocking Secrets of Outdoor Play and Allergy Risk

Somewhere in the Netherlands, freezers are packed with hundreds of nappy samples from babies under one year old. The person responsible, microbiologist Niels van Best, says the unorthodox collection method may hold the key to one of modern medicine's most pressing puzzles: why have rates of asthma, eczema, and food allergies climbed so sharply in recent decades, and could playing in the mud actually help?

Infant gut microbiome development and the allergy epidemic

The gut microbiome, the community of bacteria, fungi, and other microorganisms that colonises the intestinal tract in the first weeks of life, is not just a digestive accessory. Early-life gut colonisation plays a pivotal role in shaping the immune system and influences long-term health outcomes, with imbalances linked to allergic diseases and metabolic disorders. In other words, what moves into an infant's gut in the first months of life may set the immune system's thermostat for years to come.

The numbers paint a troubling picture. Allergic conditions including dust mite sensitivity, hay fever, eczema, and asthma have all risen in frequency, and the leading hypothesis points toward modern clean lifestyles. Children today spend more time indoors, encounter fewer farm animals and garden soils, and live in smaller households than previous generations. Each of those shifts reduces the microbial variety that reaches the developing gut. Disrupted early-life microbiomes have been linked to childhood inflammatory bowel disease, ADHD, and obesity, not only allergies, suggesting that the consequences stretch well beyond the lungs and skin.

How the LucKi study tracks babies from nappy to playground

Van Best and colleagues designed the LucKi study to follow approximately 150 babies in South Limburg, Netherlands, from birth through their first year of life, starting in 2018. Parents periodically collect stool samples from nappies and log data on diet, illness, and time spent outdoors. The researchers then sequence the DNA of the microbes in those samples and compare the results with microbes found in soil from the playgrounds where the children play.

The methodological core of the current publication involves whole metagenome sequencing performed on 808 stool samples drawn from 139 infants across nine time points during their first 14 months. Whole metagenome sequencing reads the genetic material of every microorganism in a sample, not just a single marker gene, which provides a richer portrait of microbial community structure than earlier approaches relying on 16S rRNA amplicon sequencing. The team also compared playground soil samples directly with infant stool samples to identify specific environmental microbes children acquire during outdoor play.

One striking finding sits at the centre of van Best's interest: infants ingest a notable amount of soil when playing in nature. Research on playground environments suggests children who play in microbially diverse outdoor settings show measurable shifts in gut microbiota, including increases in butyrate-producing bacteria thought to reduce inflammation. Van Best himself is relaxed about the habit. "One study suggests it's up to 60 milligrams per day," he said, "but I wouldn't be surprised if it's more. 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."

COVID-19 pandemic as an accidental experiment in microbial exposure

The LucKi study gained an unexpected research tool in 2020. The COVID-19 pandemic abruptly altered the social and hygiene behaviours of households across the Netherlands: families isolated at home, reduced contact with extended family and daycare, and ramped up hand hygiene and surface disinfection. For 36 infants in the LucKi cohort who had samples collected during the pandemic, the team developed an exposure index to quantify variations in household-level pandemic-related behaviours, including social distancing and hygiene measures.

The results suggest the lockdown left a measurable mark on infant gut microbiology. Pandemic-related behavioural changes shaped infant gut microbial development from around six months of age onward. Specifically, the study found reduced abundance of Faecalibacterium prausnitzii at 14 months and increased levels of Alistipes putredinis and Parabacteroides johnsonii. Faecalibacterium prausnitzii is widely regarded as a marker of gut health in older children and adults, and its reduced presence in pandemic-era infants is notable, though researchers caution against over-interpreting a single cohort finding.

What this research cannot yet tell us about allergy prevention

Van Best is careful not to overstate what the data can currently support. The follow-up period in the LucKi study covers only the first 14 months of life, which means the researchers do not yet know whether the microbiome differences they observe will translate into different rates of asthma, eczema, or food allergy by school age. Establishing that causal chain will require longer follow-up and larger samples.

He is also sceptical of headlines that jump the gun. Last year, a Finnish study attracted media coverage for suggesting that children who spend more time playing in nature are less likely to develop asthma. Van Best's assessment: "That's too simplistic; you can't draw such firm conclusions from those findings alone." The same honest caution extends to his own work. A more diverse gut microbiome is generally better, he says, but confirming that outdoor play produces that diversity, and that the diversity in turn reduces clinical allergy rates, requires evidence that the LucKi team has not yet collected.

Factors like birth mode (vaginal delivery versus caesarean section), feeding method (breastfeeding versus formula), siblings, pets, and daycare attendance all shape the infant gut microbiome independently, making it difficult to isolate the specific contribution of outdoor play.

Greener playgrounds and the practical path forward

Despite those caveats, the research trajectory is pointing toward concrete applications. If specific environmental microbes acquired during outdoor play do consistently appear in infant stool and correlate with healthier immune development, the findings could inform how urban planners and schools design play spaces. Van Best and colleagues are interested in whether deliberately increasing the microbial diversity of playground soils, through greening initiatives, could translate into measurable changes in children's gut microbiomes and, eventually, their allergy risk.

The farm effect, the well-documented observation that children raised on farms develop allergies at lower rates than urban peers, provides circumstantial support for the idea that microbially rich environments shape immunity early in life. Soil bacteria such as Mycobacterium vaccae have even shown antidepressant-like effects in animal models, hinting that the benefits of dirt exposure may extend beyond immune calibration. The LucKi study is still accumulating data, the freezers are still filling up, and the children are still eating soil at the playground. The science is catching up.