The People You Train with and the Microbes You Carry

Friday Stairs fills its 250 weekly spots in Florida within minutes, and the Tampa waitlist is close to 400, according to Athletech News. The workout is free, it starts early, and it means running up stairs until your legs complain. People fight to get in anyway. Now the city that started it is bringing it to Los Angeles.

Most coverage treats this as a fitness story. We think part of it is a gut story, and the best evidence for that part comes from an odd place: baboons.

What baboons and married couples have in common

In 2015, researchers studying wild baboons in Kenya found that how often two animals interacted predicted how similar their gut microbes were, even after controlling for diet, kinship, and shared environment (Tung et al. 2015). Later work found that a male who joined a new group slowly came to resemble the long-term residents (Grieneisen et al. 2017). The longer he stayed, the closer the match.

Humans show a similar pattern. Using data from the Wisconsin Longitudinal Study, researchers found that spouses have more similar gut microbiota than siblings do. Married people had more diverse microbial communities than people living alone, and couples who described their relationships as close had the most (Dill-McFarland et al. 2019). In a separate study of 287 people in Fiji, the strongest signs of shared bacterial strains were within households and between spouses (Brito et al. 2019).

What this doesn’t show

These are associations. The Fiji authors couldn’t tell which direction the bacteria were moving, or whether the transfer was direct. An analysis in wild baboons (Debray et al. 2025) showed that shared environments can mimic social transmission, so some of what looks like contact may just be people eating the same food in the same place.

And none of these studies involved people who work out together. Spouses share a kitchen. A stair class shares a stairwell for an hour. Nobody has measured the microbiomes of a group fitness class, so we can’t tell you Friday Stairs changes yours.

The exercise half

The evidence here is mixed too, and newer work suggests exercise intensity is a big part of why. A study of professional rugby players (Clarke et al. 2014) found higher gut microbial diversity than in matched controls. But the players also ate far more protein, and diversity tracked with diet as much as with training. The authors called the relationship complex, which is fair.

Two reviews published this year give a more specific picture. A narrative review in Nutrients concluded that moderate continuous training has the most consistent record of raising diversity and enriching butyrate-producing bacteria (Alsinani et al. 2026). High-intensity work is less clear. Interval training can temporarily increase intestinal permeability in untrained people, though once the body adapts, the same training appears to boost butyrate production. At the far end, one review proposes that exertion above roughly 85% of maximal oxygen uptake may tip the gut toward dysbiosis, but its authors note that this still needs direct testing (Yang et al. 2026). So the idea that very hard training can work against your microbes is plausible, not proven. The effects also fade when people stop training.

Why we still find it interesting

The brain-gut-microbiome system responds to more than food. Regular movement, social contact, and diet all feed into it, and most people get them in separate places. A weekly class that puts hundreds of people through the same effort in the same place, followed by the same coffee line, is one of the few settings where all three overlap.

There is also a simpler benefit that doesn’t depend on microbes. In a small pilot of a live group exercise program for older adults (Coletta et al. 2024), participants named social connection as one of the things that made it work. People keep showing up to things where they know someone.

What you can do with this

Eat after class, and eat with people. Berries, cocoa, olive oil, and tea are all sources of polyphenols, and your gut bacteria are what convert them into compounds your body can use. Breakfast at the top of the stairs counts.

Sources

Study details are from PubMed abstracts.

1. Tung J, et al. Social networks predict gut microbiome composition in wild baboons. eLife, 2015. doi:10.7554/eLife.05224
2. Grieneisen LE, et al. Group living and male dispersal predict the core gut microbiome in wild baboons. Integr Comp Biol, 2017. doi:10.1093/icb/icx046
3. Dill-McFarland KA, et al. Close social relationships correlate with human gut microbiota composition. Sci Rep, 2019. doi:10.1038/s41598-018-37298-9
4. Brito IL, et al. Transmission of human-associated microbiota along family and social networks. Nat Microbiol, 2019. doi:10.1038/s41564-019-0409-6
5. Debray R, et al. Shared environments complicate the use of strain-resolved metagenomics to infer microbiome transmission. Microbiome, 2025. doi:10.1186/s40168-025-02051-8
6. Clarke SF, et al. Exercise and associated dietary extremes impact on gut microbial diversity. Gut, 2014. doi:10.1136/gutjnl-2013-306541
7. Alsinani Y, et al. Exercise and the gut microbiome: from mechanisms to clinical applications. Nutrients, 2026. doi:10.3390/nu18101565
8. Yang G, et al. Exercise-driven remodeling of the gut microbiome: reciprocal regulation of structural and functional adaptations in elite athletes. Curr Issues Mol Biol, 2026. doi:10.3390/cimb48100989
9. Coletta G, et al. A live online exercise program for older adults improves depression and life-space mobility. PLoS One, 2024. doi:10.1371/journal.pone.0312992
10. Rehfeldt C. Viral Tampa workout Friday Stairs expands to new cities across US. Athletech News, Oct 19, 2026