Elite athletes really are different from you and me. Or, at least their gut microbes are.

Scientists who analyzed stool samples of 15 runners a week before and after they competed in the 2015 Boston Marathon found unusually high levels of one particular microbe compared to 10 non-athletes. Levels of the microbe in question, Veillonella, spiked after an intense workout and bloomed even more after the marathon.

That was a lightbulb moment for the scientists because the bacterium is known for breaking down and eating lactate, a metabolite even ordinary runners equate with fatigue.  

Veillonella acts like a “metabolic sink” for lactate, the scientists suspect, converting into fuel the by-product of hard-working muscles runners blame for their aching legs in the latter part of long-distance races. The bacteria turn lactate into propionate, a short-chain fatty acid that’s a source of energy and can also be an anti-inflammatory, a Harvard-affiliated team of scientists reported Monday in Nature Medicine.

In theory that sets up a feedback loop: When the athletes are exercising, they are constantly producing lactate. They are also creating a niche in their microbiome for organisms that can use lactate as a primary energy source.

After confirming in 87 other athletes what they found in the marathoners, the researchers then showed — just in mice — that colonizing the lab animals with a strain of Veillonella taken from one of the elite athletes extended their endurance while on treadmills by 13%. That improvement may not sound impressive, but athletes striving to shave seconds off their personal bests would likely call it huge, should larger studies bear out the findings.

“This study represents a hypothesized but as of yet unproven mechanism by which the microbiome can influence host physiology during extremes of activity. We actually proposed a similar mechanism in the past,” said Jack Gilbert, a professor at the School of Medicine at the University of California, San Diego. He was not involved in the research. “We already know that microbes can ferment food and produce metabolites that can improve muscle fitness and strengthen muscles. This study provides compelling evidence to support lactate metabolism as a key component of extreme athlete performance.”

In her own research, Orla O’Sullivan of University College, Cork, in Ireland, has also seen levels of the Veillonella family of bacteria rise in rugby players. “What this paper does is add to the previous results that have shown that athletes do have different microbiomes than sedentary people,” she said, adding that there’s a long way to go to see why this is and how it happens. Does your microbiome help determine if you can be an athlete, or does being an athlete influence your microbiome? Will the improved endurance seen only in mice show up in people, too?

O’Sullivan also wonders if the athletes’ post-marathon bump in Veillonella helped their recovery rather than endurance because of propionate’s anti-inflammatory action. She said it would also be interesting to see whether the phenomenon takes place only in endurance athletes or across all athletics.

Some of the Nature Medicine study’s co-authors are pursuing two divergent tracks based on their early results, which they say need to be tested in larger trials.