Can you improve your gut microbiome by picking up bacteria while playing outside?

Structured outdoor play in nature did in fact change the composition of the gut microbiome in 54 preschool children after ten weeks, particularly the amount of the bacterium Roseburia. At the same time, measured stress levels fell and serotonin concentrations in the stool rose. However, this is just one small randomised study, and the researchers themselves stress that follow-up research is needed to confirm whether the change in gut microbiome is truly the cause of that improvement in behaviour.

The idea that playing outside exposes you to a greater variety of bacteria does appear to hold up. Children who play on real rocks come into contact with 67 species of bacteria characteristic of that environment. On artificial rubber play mats, there were only three. Whether those environmental bacteria actually make it into the gut and do anything useful there was not demonstrated in that study. There are therefore two open questions: do they reach the gut, and what do they do there?

There is one more interesting caveat regarding those environmental bacteria. The Proteobacteria that are abundant on the play rocks are the same bacterial group that, when they overgrow in the gut, are actually associated with inflammatory diseases and a disrupted gut microbiome. What that means when children take them in through outdoor play has not been studied. This warrants attention but does not yield a conclusion.

The well-known and well-established factors influencing your gut microbiome are mode of birth, infant feeding, and antibiotic use. In adults, medication use turns out to be the single most important predictor. Early effects such as mode of birth are no longer detectable in the gut microbiome by adulthood. Playing outside as a strategy for your gut has by comparison been barely studied and cannot in any case replace those proven factors.