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Want to become a better runner? You’re probably going to have to run faster, smarter and longer. But a piece of the puzzle may lie in a surprising place: the bacteria in your gut.

We know . Now George Church at Harvard University and colleagues say that the microbiome may be a critical component of physical performance.

The Boston marathon is famously tough to get into, due to its ever-faster . The researchers took daily stool samples from 15 runners in the 2015 race, one week before and one week after they ran, along with a sedentary control group, to see what was in their gut.

A genetic analysis found a significant increase in one genus of bacteria, Veillonella, post-marathon. The results were then successfully replicated by analysing the stool samples of 87 ultramarathoners and Olympic trial rowers before and after exercise.

The correlation raises the prospect of a causal link between the bacteria and physical performance, the researchers say. To find out more, one strain of Veillonella taken from one of the Boston marathoners was then put in mice. It allowed them to run 13 per cent longer on a treadmill than a control group without it. Further tests saw the team put forward the idea that the bug has a role in breaking down lactic acid, which can lead to fatigue during running.

The research potentially points the way to a future where probiotic supplements could change your microbiome and make you a better runner. But it’s too early to tell if that will come to pass, Church says. “This is worth enjoying as pure science for now. Until we complete the human trials, it is merely an interesting correlation in humans and a much stronger cause and effect shown in mice.”

Marathon mice

We simply don’t know if Veillonella-boosted humans would be able to run longer like the mice, and there are reasons to be cautious given the large differences not just between humans and mice, but among humans too, says Church.

David Relman of Stanford University says he likes the fact the study is looking for a plausible chain of links between the microbiome and exercise performance. But he doesn’t think the bar has been cleared in terms of the observations to back up the paper’s conclusion. For example, there is no control group for the 87 rowers and ultramarathoners. “We don’t know that if running up and down the stairs here in a middle-aged white guy wouldn’t produce the same thing,” says Relman.

Fifteen Boston marathoners is a very small sample, says Patrick Schloss of the University of Michigan. He would like to see a study over a longer period of time, beyond just before and after exercise. His verdict is: “they need to do a lot [more] to show that it’s [the bacteria] enhancing performance.”

A tailored probiotic supplement doesn’t exist yet, and may be a long way off. In 2017, Jonathan Scheiman at sports biotech firm FitBiomics and part of the research  he hoped to have a product launched by the end of 2018, but that has come and gone.

Even if the research is right and this is a route to becoming a better runner, how big an effect would it actually have? “Maybe this is a part of the real story in people, but if it is, it wouldn’t surprise me if it was a pretty tiny effect,” says Relman. “If you were to feed people these organisms… you’d be hard pressed to measure the effect on exercise performance. I could be wrong.”