[Researchers gathered] stool samples every day from 15 middle-of-the-pack racers, some male, some female, some meat eaters and some vegans, starting a week before the 2015 marathon and continuing for a week after. They also collected similar samples from 10 sedentary people.
Next, they looked at the composition of the microbes in the samples and found a striking difference. After the marathon, the runners’ guts teemed with far greater numbers of a bacteria called Veillonella. Their intestines had contained more of this microbe than the guts of sedentary people before the race, but afterward, the numbers soared. No other germ showed similar gains.
And this led to the theory that more Veillonella meant a better, stronger athlete. This led to a study of mice in which some mice were injected with the bacteria and then become better mouse athletes.
This bacteria has not yet been injected into any humans to see if it’s some secret Captain America serum for us lesser athletes, but this type of experiment may be on the horizon. Of course, we’ve been reading about these types of studies long enough to guess that you may have to wait quite a while for a human study.