Someday, in the very distant future, athletes might be able to boost their performance by tapping into a precious, albeit unconventional, resource: the poop of elite runners.
That possibility is still a long way off. But scientists may now have a few clues on which gut microbes give marathoners the runs (no, not that kind).
According to a study published today in the journal Nature Medicine, bacterial “probiotics” extracted from marathoners’ feces can boost the endurance of mice on tiny treadmills. The microbial magic appears to rely on the ability of these bacteria to do what mammalian cells can’t: convert a common byproduct of muscle metabolism into a performance-enhancing compound.
The treatment has yet to be tested in humans, and there’s nothing to suggest that bacteria alone could make or break an athletic career. But if the findings pan out, future studies of athletes’ fleet-footed feces might just contain a whiff of speedy success.
“This study presents a really fascinating hypothesis on the ways in which the microbiome can contribute to [human] physiology,” says Ariel Brumbaugh, a microbiologist at Stanford University who was not involved in the study. “That said...this alone shouldn’t encourage people to immediately start a round of probiotics.”
Nowadays, there are few trendier topics in the world of biomedical research than the gut microbiome—the diverse, teeming population of single-celled organisms that inhabits the large intestine. Though they’re physically restricted to the colon, these industrious critters, which number in the trillions, are capable of performing a variety of functions that nourish and protect human cells, affecting just about every vital organ system in the body.
Years of research have shown that perturbing the gut microbiome can accelerate the onset of ailments ranging from inflammatory bowel disease to certain types of cancer. But in a departure from the disease-centric approach of many microbiome researchers, study author Jonathan Scheiman, then a researcher at Harvard University, decided to test how gut microbes could enhance wellbeing in already healthy individuals—and, perhaps, push the limits of human performance.
Scheiman and his colleagues began their study by investigating how the gut microbiome responded to intense exercise. For this team of Boston-based scientists, that meant leveraging the city’s low-hanging fitness fruit: long-distance runners, competing in the annual marathon.
It was a task that required a lot of poop—and a lot of patience. In weeks bookending the 2015 Boston Marathon, Scheiman spent close to 90 hours battling Boston traffic, ferrying samples on dry ice after 15 elite runners and 10 more-sedentary controls dropped their daily deuce.
But the work paid off. An analysis of the runners’ pre- and post-marathon feces suggested the event had spurred an uptick in a bacterial genus called Veillonella—a group that also seemed to be more prevalent in the average runner than in their non-racing counterparts.
It’s hard to know which came first, the microbes or the athleticism, says study author Aleksander Kostic, a microbiologist and immunologist at Harvard University’s Joslin Diabetes Center. But these associations hinted that something about the runners’ bodies might make them especially good homes for these bacteria.
That something turns out to be lactate, one of the major byproducts of the reactions that take place in hardworking muscles—and a main source of energy for Veillonella.
A quick aside: The word “lactate” (often erroneously referred to as “lactic acid”) may sound familiar, as it’s still commonly cited as the culprit behind sore, fatigued muscles. But that myth was busted long ago, explains Victoria Vieira-Potter, an exercise physiologist at the University of Missouri who was not involved in the study. Instead of causing aches, this molecular scapegoat is actually an important source of fuel that cells can use to push through the pain.
What lactate does do is build up as muscles tire out. And if that’s a common occurrence, that’s probably good news for lactate-loving Veillonella. In return, Veillonella pumps out a molecule called propionate—a compound thought to have a bevy of beneficial effects on the mammalian body, including regulating blood pressure and revving up metabolism.
To see if this microbial pathway had any performance-enhancing effects, the researchers orally injected two groups of 16 mice with either a strain of Veillonella isolated from one of the marathoners’ fecal samples, or Lactobacillus, another innocuous microbe that’s commonly found in yogurt. Five hours later, the researchers had the rodents hop onto a set of miniature treadmills and run until they were too exhausted to keep going.
It took around 16 or 17 minutes for the Lactobacillus mice to crap out. The Veillonella mice, on the other hand, lasted closer to 19 minutes—a 13 percent increase in endurance. Even when the researchers leapfrogged the Veillonella and administered propionate directly, the mice maintained a comparable edge over others who received only squirts of salt water.
Thirteen percent might not sound like much, Kostic says. But for an athlete in the midst of training, even the smallest gains in performance can make an enormous difference. Besides, he adds, this whole pathway constitutes a metabolic bonus: During exercise, there’s often lactate to spare—but if there’s Veillonella around, it might just get recycled into something more useful.
This symbiotic feedback loop, in which human and microbe alike benefit from repurposing each other’s byproducts, is “the coolest thing about this paper,” says Carolina Tropini, a microbiologist at the University of British Columbia who was not involved in the study. It’s also an outside-of-the-box way to think about what these bacteria are sensitive to, she adds. “We always think about influencing our gut microbes through diet,” she says. “But this is a way of shaping the microbiome that has nothing to do with what we’re eating.”
Figuring out how propionate improves performance is the clear next step, but Kostic’s gut feeling is that the perks of harboring Veillonella go beyond having an in-house propionate factory. Scheiman, who has since left Harvard, and several of the paper’s other authors now run the sports biotechnology company FitBiomics, which they hope will eventually pioneer a line of fitness-enhancing probiotics (this time for humans) that may include Veillonella and similar strains.
A lot will need to happen before then, Brumbaugh says. The data so far is intriguing, she adds, but because several of the findings rely on behavioral data in mice, they may be difficult to reproduce in humans. Additionally, larger studies in more diverse populations are needed to figure out how generalizable these results are to other types of exercise, as well as people with different levels of fitness, Vieira-Potter says. Tropini advises caution as well, noting that a human probiotic could have unexpected side effects, or simply no effect at all—especially in the long term.
Even if a human probiotic passes muster, things could easily get murky if microbes begin to venture into the realm of performance-enhancing drugs. Bacterial “doping” may eventually become something humankind has to grapple with, and “this is an area in which we should tread with caution,” Vieira-Potter says.
For now, it’s worth keeping in mind that plenty more goes into athleticism than what’s living in your gut. Even in the study, Veillonella was by no means universal among athletes, and not every mouse that received it responded to its effects.
But we humans still have a lot to learn about the bacterial cells that share our bodies. If the past several decades of research have taught us anything, it’s that our gut microbes are full of surprises.
Who knows? Someday, going that extra mile may truly come down to going with your gut.