Illustration by Kris Mukai

What if Number 2 Makes You Number 1?

Elite athletes have a greater variety of bacteria in their guts than couch potatoes do, raising the prospect of “poop doping.”

Poop doping” is an unlikely phrase that began appearing in headlines this summer, thanks to a scientist who claimed that a do-it-herself fecal transplant from a competitive cyclist had improved her health and athleticism. Other researchers objected to the lack of evidence for this claim. But it was too tantalizing for many news outlets to ignore. UC Davis microbiologist Jonathan Eisen, who tracked the coverage on his blog, lamented, “The crap keeps flowing.”

The microbiome — the vast community of microorganisms living in and on our bodies — is still a new field of study. Scientists are only starting to understand the complex connections between these bugs and our health. A few studies have found differences in the gut microbes of athletes and non-athletes, but researchers haven’t teased apart causes and effects. Our diets, exercise routines, microbes, and other factors may be tangled together like cyclists in a peloton.

One of those studies came from Orla O’Sullivan, a computational biologist at Teagasc Food Research Centre in Cork, Ireland, and her colleagues. They scrutinized the gut bugs of a professional rugby team for a 2014 study. The researchers analyzed the bacteria in fecal samples from the 40 athletes and 43 control subjects; they also looked at other factors such as diet and body composition.

“We were very excited to see the athlete microbiome is distinct from the controls,” O’Sullivan says. “And what was even more exciting is that there was a huge increase in diversity of the athlete microbiomes.” In other words, when compared to control subjects, athletes had a greater variety of species in their gut ecosystems.

It matters, O’Sullivan says, because “people with higher diversity tend to be healthier.” Hers wasn’t the only study to find a link between exercise and microbial diversity. Mehrbod Estaki, a PhD student at the University of British Columbia, led a 2016 study of 39 men and women, looking at their fitness and feces. That study found that people in better shape (as measured by their VO2 peak, the maximum amount of oxygen used during an exercise test) had more diverse gut microbes.

A 2017 study of 40 women compared the microbiomes of those who did less than 1.5 hours of physical activity a week to those who did three hours or more. The researchers found that certain types of bacteria were more common in the active women. Unlike in the other studies, they didn’t find higher biodiversity in exercisers. But author Mar Larrosa of European University in Madrid, Spain, says this may depend on the intensity of the exercise.

“Microbiome work is a perfect breeding ground for snake oil.”

O’Sullivan stresses that her studies and others have only found correlations between exercise and patterns in the microbiome. The researchers don’t know whether one thing causes the other. Even if rugby players have healthier microbiomes, O’Sullivan doesn’t think people should start giving themselves transplants of rugby players’ feces. “God, no!” she says.

It’s tricky to separate the effects of exercise from those that come from diet. “People who embark on an exercise plan tend to also change their diet,” O’Sullivan says. Maybe they start eating more healthily, or they eat more because they’re hungrier, or they reward themselves with candy bars after gym visits. In the guts of the rugby players, microbial diversity correlated both to exercise and to how much protein people ate.

Although diet likely affects an athlete’s microbial makeup, exercise itself could have an impact too. Maybe exercise encourages or discourages the growth of certain bacteria by speeding up the rate at which food passes through the gut, O’Sullivan says. (This could be what’s happening when long-distance runners get diarrhea). Or perhaps exercise changes the physical structure of the gut just like it changes the muscles, bones, and blood vessels, Estaki says, and this affects the microbiome.

Additionally, some other factors beyond diet and exercise probably influence athletes’ microbiomes. Their genes, for example. Or their exposure to the world. Estaki suggests that fitter people might change their microbiomes by spending more time on athletic fields or in locker rooms, or simply by enjoying the outdoors.

“Every single thing that we do will have some kind of impact on the microbiome,” O’Sullivan says.

It makes sense that exercise could change our gut microbes. “Perhaps the most interesting question,” Estaki says, is the opposite one: Can our gut microbes change the way we exercise?

This was what the poop doping story claimed, albeit without evidence. But it’s not impossible. The gut is linked to the brain and behavior. Microbes in your intestines can alter your body’s levels of neurotransmitters such as serotonin, and they produce their own molecules that can interact with the brain. Maybe certain people are more athletic because their microbes help motivate them to exercise, Estaki speculates. Or perhaps their microbes help them get more benefit from the same amount of work. Larrosa says gut microbes can produce molecules that affect muscle physiology. A study in Taiwan of germ-free mice — lab animals with no gut microbes at all — found that these mice were weaker endurance swimmers.

To start teasing apart the relationships between diet, exercise, and microbes, O’Sullivan’s group just wrapped up a study that assigned inactive subjects to start exercising, eat more protein, or both. The researchers will also look into the microbes of athletes besides rugby players — including rowers, track and field athletes, and cricketers.

Microbiome research is “very hot right now,” says Eisen, who debunked the poop doping story on his blog. Excitement about the field, combined with the difficulty of actually studying or understanding the microbiome, can lead to oversold results. He says the most common issue is that people misinterpret correlations as causal relationships. A group of cyclists has a certain microbial profile, say, and a researcher suggests that the microbes make their hosts better athletes — even when they’re transplanted. “Sadly,” Eisen says, “microbiome work is a perfect breeding ground for snake oil.”