How your sport shapes your gut bacteria

Exercise in general helps your gut bacteria, and that part of the story is covered elsewhere on this site. What's newer is this: the specific sport you train for seems to grow its own bacterial community. A cyclist's gut doesn't look like a runner's, and researchers can now see it in the data.

Cyclists and runners carry different signatures

A 2024 study compared 58 people: 18 competitive cyclists, 22 runners, and 18 people who don't train at all.¹ The two athlete groups differed from each other, not just from the non-athletes. Runners showed greater overall bacterial diversity than cyclists. Six bacterial types rose and fell in step with how much the cyclists rode and how much oxygen they used during effort. Among runners, eight different bacteria tracked their training in a similar way.

A transplant experiment that points toward cause and effect

A 2025 study went further, asking whether an athlete's gut bacteria can change how a body works, not just reflect it. Researchers took gut bacteria from cyclists and soccer players with unusually high exercise capacity and transplanted it into ordinary mice.² The mice's running endurance didn't improve, so this isn't a shortcut to becoming an athlete. But their cells got better at pulling sugar out of the blood, and they stored more glycogen, the fuel muscles burn during exercise. Moving bacteria into a different body, with no training involved, was enough to shift how that body handled fuel.

Different sports seem to favor different bacterial jobs

One 2025 study compared the functional genes carried by gut bacteria across sports with different energy demands.³ In cyclists, the dominant bacteria carried more genes for breaking down fat and carbohydrate, matching the steady, oxygen-burning effort that long-distance riding demands. In weightlifters, the bacteria leaned toward genes tied to amino acid and protein pathways, more in line with muscle repair. The blood chemistry differences between the two groups were even more pronounced than the bacterial differences, a reminder that this research is still filling in the details. For a closer look at how sustained training blocks shape gut bacteria over time, see our piece on gut health and endurance training.

How often you train seems to matter too

Sport choice isn't the only factor. In one of the earlier cyclist studies, riders training more than 11 hours a week were far more likely to carry high levels of Prevotella than riders training less, and the effect scaled with hours logged.⁴ A similar frequency pattern has shown up in studies of martial arts athletes.⁵ Consistency, not just the sport itself, appears to shape what ends up living in your gut.

Where the evidence still needs to catch up

A scoping review focused on swimmers found only a small number of studies have looked at how swimming affects gut bacteria, compared with the research base for running or cycling.⁶ The reviewers called it an acknowledged gap. If someone tells you confidently what swimming does to your gut bacteria, treat that with some skepticism for now, because the research just isn't there yet.

Whatever sport you play, the pattern holds up so far: consistent training in one activity appears to grow a bacterial community suited to that demand, the same way it builds specific muscles and endurance. The field is still emerging, and most of these studies are small, so treat the findings as a promising direction rather than settled fact.

What to do with this

This content is for educational purposes only and is not a substitute for professional medical advice.