Researchers from Finland added greenery and soil to nursery outdoor play areas.
They monitored the skin and gut microbiota of the children playing in the yards.
The team found the kids kept diverse microbiota and got an immune boost.
It is thought that exposure to microbes challenges the body's immune system.
This prevents autoimmune conditions like asthma, eczema and type 1 diabetes.
Playing among forests and parklands — rather in than concrete and gravel yards — helps children to foster a stronger immune system, a study has concluded.
Previous studies have proposed that city-dwellers may be at a greater risk of immune-mediated diseases thanks to a lack of exposure to diverse microbiota.
This fails to challenge the immune system and paves the way for conditions like asthma, eczema, inflammatory bowel disease, multiple sclerosis and type 1 diabetes.
Researchers from Finland renovated the outdoor play areas of four nurseries with plants, grasses and soil — finding that it had a positive impact in just one month.
The children who played in the greener spaces maintained more diverse skin and gut microbiota, the team said — along with signs of a better-regulated immune system.
The findings suggest that the bodily defences of city-dwelling children could be boosted by providing daily access to green spaces and soil to play in.
In their study, environmental researcher Aki Sinkkonen of the University of Helsinki and colleagues altering the outdoor play spaces of four nurseries in Finland.
They overhauled the previously 'bare' concrete-, sand- and gravel-covered yards with the introduction of wood-like elements including grass, mosses, small shrubs, planting boxes, and natural forest floor.
Over the course of 28 days, the children attending the nurseries — each aged between 3–5 — spent 1.5 hours each day in the green renovated spaces playing games, planting vegetation and crafting with natural materials.
The renovations cost less than €5,000 (£4,524 / $5,860) to implement — less than each yard's annual maintenance budget.
Skin swabs for microorganisms were taken from each of the children both before and after the study period — along with blood and stool samples — and the researchers also analysed soil or sand samples from the yards before and after.
The team compared these nurseries with three 'standard' childcare centres whose yards were left bare, as well as three 'nature-orientated' establishments where children were taken to nearby forests on a daily basis.
In total, the study included 75 children across the 10 nursery centres.
The team found that the children attending the four renovated nurseries maintained a high diversity in their skin microbiota across the length of the study.
The kids also developed a higher ration of anti-inflammatory cytokine IL-10 to the pro-inflammatory cytokine IL-17A — suggesting that exposure to the more natural environment and dirt had stimulated their immunoregulatory pathways.
While similar results were observed from the three centres where children were routinely taken out to play in nature, those attending the three centres whose yards were left more bare got no immune boost — and their microbiota declined.
'When we saw the results, we were very surprised because they were so strong,” Dr Sinkkonen told the Guardian.
'Our study can pave the way for new preventive practices to cut the global epidemic of immune-mediated diseases.'
'It is wonderful forward-looking work, immunologist Graham Rook of the University College London told the Guardian.
'Many of the disorders that are increasing in western urbanised populations are due to failure of the mechanisms that supervise the immune system,' he added.
'This study shows that exposing children to a biodiverse natural environment boosts several biomarkers of the essential control mechanisms.'
'These Finnish research groups have been leading the way in applying this understanding in a practical way.'
The full findings of the study were published in the journal Science Advances.
Researchers now estimate that a typical human body is made up of about 30 trillion human cells and 39 trillion bacteria.
These are key in harvesting energy from our food, regulating our immune function, and keeping the lining of our gut healthy.
Interest in, and knowledge about, the microbiota has recently exploded as we now recognise just how essential they are to our health.
A healthy, balanced microbiome helps us break down foods, protects us from infection, trains our immune system and manufactures vitamins, such as K and B12.
It also sends signals to our brain that can affect mood, anxiety and appetite.
Imbalances in the gut are increasingly being linked to a range of conditions. Last year, scientists at California Institute of Technology found the first ever link between the gut and Parkinson's symptoms.
The composition of our gut microbiota is partly determined by our genes but can also be influenced by lifestyle factors such as our diet, alcohol intake and exercise, as well as medications.