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A week before the 2015 Boston Marathon, Jonathan Scheiman, then a postdoctoral researcher in the lab of geneticist George Church at Harvard Medical School in Boston, Massachusetts, was racing around the city in a hire car collecting faeces. He gathered stool samples from runners planning to participate, as well as those from non-runners — mostly fellow postdocs. After the race, he revisited his donors to collect more samples. Four years later, his efforts were rewarded with a paper describing one of the first attempts to establish a causal link between the symbiotic community of microorganisms living in our guts and athletic performance1.