Great athletes not only have heart, they have guts — special guts. Boston Marathon runners and Olympic athletes have higher than usual amounts of Veillonella bacteria in their digestive tract following races, a new study finds. When mice consumed the Veillonella, their treadmill runs were better than those of mice not given the same bacterial diet, according to new research published Monday in the journal Nature Medicine.

Our digestive system, a group of organs that includes the stomach and colon, harbors nearly 100 trillion bacteria. Collectively known as the “gut microbiome,” these bacteria help our bodies break down complex carbohydrates and starches, while also producing vitamins and hormones. Gut flora, which collectively weigh up to four and a quarter pounds and contain more genes than our actual genome, have escaped observation until recently, when scientists began to study and sequence the 500 to 1,000 different species of bacteria that inhabit each of us.

Although no two people have identical microbiomes, the researchers hypothesized for their new study that elite athletes’ guts must contain similar bacterial species that help them perform and also recover.