JONATHAN SCHEIMAN SWINGS open the door of a giant freezer at the Wyss Institute at Harvard. Gas billows out like an effect from a cheesy sci-fi movie. Inside are dozens of boxes stored at minus-80 degrees Celsius, each crusted with snowy ice.

Scheiman holds an ice-caked box in one gloved hand and with the other withdraws a small plastic vial. At its bottom, a tiny hint of brown.

"This is fun stuff," he says. "This freezer is literally boxes full of s---."

Each specimen in this freezer is a fecal sample from an athlete, mostly from marathon runners. The collection process is rather simple. Participants take a tube with a little plastic tip and dip the tip into a piece of used toilet paper, then place it right back into the tube. "It's not invasive, no time commitment, totally done in private," Scheiman says. And -- this is key -- donors don't have to touch any stool.

Scheiman went to great pains to obtain the roughly 400 samples in this freezer. Each one started with a somewhat uncomfortable conversation with the athlete -- he approached them noting that he's a scientist and that they'd be helping with an experiment about the microbiome, all before circling around to the request for a fecal sample. He then spent hours every day driving around the greater Boston area to pick up samples from donors' homes. Each of the samples contains millions of bacteria; he put them in the freezer to keep them viable for later study, the way you might store a leftover lasagna.

Scheiman is one of a handful of researchers investigating whether there is a link between athletic performance and the gut microbiome -- the ecosystem of hundreds of types of bacteria that live in the intestines and play a key role in almost all of our body's essential processes. The potential link makes sense. Many of the functions of microbes in the gut, including breaking down food into energy and protein, modulating our immune function, even regulating our thoughts and moods, correspond directly to elements of athleticism: endurance, strength, mental toughness and recovery.

A wave of new studies appeared in 2017. Lauren Petersen, a researcher-cyclist at the Jackson Laboratory in Connecticut, was the first author on a study published in the journal Microbiome that cataloged the species of microbes in the guts of elite cyclists. (This study, like many others about the gut microbiome, suggests a correlation between the prevalence of a microbe, in this case of the genus Prevotella, and aspects of athletic performance, in this case muscle recovery -- but does not prove a direct cause-and-effect.) The same year, a team in Ireland published a similar study in the journal Gut, focusing on high-performing rugby players. And in August 2017, Scheiman presented his first study of marathon runners, in which he identified a bacteria whose high numbers seemed to correlate with athletic exertion, at the annual conference of the American Chemical Society.

So far, the field hasn't arrived at any one, concrete conclusion. Some research teams, such as Petersen's and Scheiman's, have identified specific types of bacteria that they believe are tied to aspects of athletic performance. Others, including the Irish team, have found that greater microbial diversity in the entire ecosystem of the gut makes a better athlete. Some interested but skeptical scientists who study the microbiome, however, doubt that the fledgling body of performance enhancing microbiome research has produced any clear results at all.

Right now, most researchers working in this field assume that their work will lead to some new probiotic -- a pill that contains one or several types of bacteria to seed the microbiome -- that will shake up the booming supplements industry. The probiotics industry is expected to top $75 billion by 2025, and most of the probiotics currently on the market are derived from just two strains of bacteria. Meanwhile, there are millions of microbes in the human gut, almost none of which have been harnessed or monetized. Scheiman and others are betting they are on the bleeding edge of an industry with potential that is just barely being tapped.

Jonathan Scheiman has collected (and frozen) about 400 fecal samples to better understand the gut microbiome. "This freezer is literally boxes full of s---." Courtesy of Wyss Institute at Harvard University

But there are blossoming communities that are unwilling to wait for the scientific establishment to clear up the probablys and maybes. They congregate online to share tips about how to remodel the gut microbiome on their own terms, selling and purchasing untested, homemade probiotics, or even taking the drastic step of fecal transplants. It's a bold plan, and a risky one -- and the hypothesis behind it might also be completely bogus.

ANDERS CORBETT WAS working at Brooks Brothers in a Boston suburb when he recognized the distinctive salt-and-pepper beard of one of his customers. It was the famous geneticist George Church. Corbett had read Church's popular book "Regenesis" and struck up a conversation. "He gave me great advice: 'Put yourself in places you don't belong because that's where you learn the most.' I take those words very seriously," Corbett says.

Some three months later, in February 2015, Corbett got a meeting with Church and asked him for a job. He began working as a fellow on Church's team at Wyss before joining Scheiman's microbiome project that fall. Though Corbett and Scheiman worked together recruiting study participants and processing their samples, the two men had different ideas about how to arrive at viable scientific conclusions. Corbett favored aggressive methods, while Scheiman always wanted to do things by the book. For example, when Scheiman and his team traveled to Rio for the 2016 Summer Games, Corbett wanted to set up a port-a-potty outside the Olympic Village and "just watch and wait to collect samples," which Scheiman "outright rejected." Scheiman insisted that "all study participants were properly consented" into the study and all work "was approved by the Institutional Review Board on human subjects research."

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No matter their differences, Corbett believed in the work they were doing together. A collegiate rower, Corbett introduced his friends in the rowing community to Scheiman so they could donate samples. He even contributed his own sample to Scheiman's experiments. (Scheiman says that Corbett was enrolled in the project but that none of his samples was used for publication.)

But eventually, the tensions between their approaches proved insoluble. Corbett says that he asked Scheiman to be on the patent and that Scheiman refused.

"Anders was never involved in any intellectual property discussions nor involved in the creative process of any of the lab's studies," Scheiman says. Frustrated, Corbett left the Wyss Institute in February 2017. (A representative for Wyss says all personnel matters are confidential and declined to comment.)

About a month later, Corbett launched his own venture, Craft Microbiome, a small business selling probiotics extracted from Corbett's own feces. He bummed lab space where he could, freeze-drying the bacteria he cultivated for later use in his pills. (The U.S. Food and Drug Administration's rules about selling products derived from stool samples are complex. A source at the FDA, speaking on background, said Corbett's probiotics could fall under looser guidelines [those for foods or dietary supplements] or stricter ones [for drugs or biological products] depending on their intended use.)

Corbett harnessed his own microbiome because he has been a high-performance athlete for years. As a rower, he has trained with the national team at the Olympic Training Center and competed at the 2012 World Rowing Championships. More recently he has dedicated his efforts toward the pentathlon, competing in the Laser Run World Championships in Dublin last year.

So far, a few dozen people have taken Corbett's probiotics -- most made from Corbett's gut bacteria, though a few had probiotics made from bacteria distilled from their own guts. Corbett used to sell pills on a website, though most of his sales have come via word of mouth. He tried some pop-ups at running-shoe stores in Madison, Wisconsin, which were a failure. Said one business owner who turned Corbett down, "I didn't want to jump in with someone who works with livestock." (A reference to pig feces-derived probiotics Corbett had made for a pig farmer to help him boost the size of his animals.)

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Corbett's brother Carl, a co-founder of Craft Microbiome, took some of Anders' stuff and claims he gained 15 pounds, "putting on muscle and reducing fat." Philip Brown, for whom Anders Corbett designed probiotics derived from Brown's own gut, took a 10-day regimen with the intention of "putting on some good, lean weight"; he says he didn't notice any dramatic effects, though he later helped Corbett find other buyers.

Corbett says he has the probiotics tested twice by a third-party genetic sequencing company called Genewiz -- to make sure that they contain the type of bacteria they're supposed to and that they're not contaminated, and to check how many bacteria colonies are in each pill. The claims made by Corbett and anyone else who has taken the probiotics have not been documented or rigorously tested. And it's not totally clear what's actually in his pills -- he initially told me he was only isolating species of Lactobacillus, the type of bacteria most commonly found in probiotics, which wouldn't alone account for the dramatic results he claims.

"Sometimes, I feel like a snake oil salesman," Corbett says.

Corbett gives buyers a light warning before they take the pills. "I do not specify any of the dangers. But I caution [them] that this is completely untested, to take at your own risk," he says, adding that he doesn't typically have much follow-up with people who buy pills from him. His website contains no warning about the risks of taking the pills, nor any promises about the effects these pills could have.

"I guess what I'm finding is that people aren't deterred by that," Corbett says.

Corbett doesn't understand what his customers are looking for when they take his pills, though that hasn't stopped him from catering to them, and at this point the science is still so new that small-time operations like Corbett's can exist. "I have no idea what they want to get out of it. My feeling is they're into the coolness factor," Corbett says. "At this point, it's not a product. It's a dream we're selling to people. The dream is self-improvement."

OUR OBSESSION WITH microbiomes has been around nearly as long as we have. In 4th-century China, people struck with diarrhea or food poisoning would ingest a feces-derived "yellow soup" to bring them back to health, a practice that continued for centuries. Authors and scientists in 19th-century England often touched upon the link between the gut and the brain -- a relationship scientists have learned more about in the past few decades.

The term "microbiome" was first used in the 1950s, but the recent fascination took off with the explosion of scientific studies around the turn of this century, an indirect result of cheaper genetic sequencing technology needed to discover the species and genuses of bacteria. In the nearly 20 years since, researchers have discovered much about the microbiome. (Though we usually say "microbiome" to talk about the gut, there are in fact other microbiota elsewhere on the body, like in the nose, on the skin, in the vagina and in the mouth and throat.)

Still, for all its sampling, sequencing and experiments, the scientific community hasn't fully determined how to alter the microbiome for better health. And that's partially because scientists don't know what a healthy microbiome looks like.

Despite that uncertainty, a largely experimental intervention in gut health has emerged called fecal microbiota transplant. FMTs are minor procedures that involve gently winding a colonoscope up the patient's rear end, then using it to infuse a thin liquid of watered-down fecal matter. The whole thing takes about 10 minutes. The FDA allows doctors to perform fecal transplants in select cases for patients with Clostridium difficile infections, or C. diff, a life-threatening condition that can set in after patients have taken antibiotics for another condition. But experiments to test whether fecal transplants could treat inflammatory bowel disease, diabetes, liver disease and obesity have not returned conclusive results, and the FDA has not approved its use for these conditions, designating it an "investigational therapy." As such, fecal transplants are rarely used in the United States.

"Even with the impressive cure rates with C. diff infections, we still don't know why it works," says Lea Ann Chen, a gastroenterologist at NYU, where about 30 fecal transplant procedures are performed a year. "We hypothesize that it has something to do with bacterial diversity. But we don't know precisely."

For some searching for a miracle cure, the lack of evidence hasn't stopped them from turning to DIY fecal transplants. Proponents congregate on YouTube and Facebook and specialized sites like the now-defunct ThePowerOfPoop.com, sharing advice and material about how to conduct the procedure at home.

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Catherine Duff ran a number of the online groups. She's a good candidate to do so -- between 2005 and 2012, she had eight C. diff infections. Each time, Duff's doctors treated the condition and she would feel better, only to have a resistant strain return. It nearly killed her. "My doctors told me to get my affairs in order and to say goodbye to my family," Duff says. But her daughter learned about some early-stage research that suggested a fecal transplant might help. Duff broached the idea with five doctors, but only two had ever heard of FMT and none was willing to recommend the treatment. She decided to do it on her own.

Doing a fecal transplant at home looks something like this. You take a dose of antibiotics to wipe out whatever bacteria you've still got, ask a "healthy" friend to donate some stool, and buy some supplies on Amazon -- enema bags, a funnel, a kitchen strainer and a blender that you will never use again. Put the resulting slurry into your colon with the enema bags, assisted by a little lube. Then move around a bit to slosh the substance around your colon. Some of the material might come out in the process, and that's OK. Try very, very hard not to poop for a while afterward.

Duff used a fecal sample from her husband, and within six hours of her first self-conducted FMT, she says she felt almost completely cured. "I was ecstatic, happy and recovering, but I couldn't check the feeling that this is ridiculous. People are dying [from C. diff] every day, and no one has heard of this treatment."

She went on to found the Fecal Transplant Foundation, a nonprofit that advocates for increased fecal transplant research and funding for C. diff and other conditions. For Duff, this is personal. It was ignorance about FMT treatments that almost killed her. Duff says several athletes at the hobbyist level (she calls them "weekend warrior types") have contacted her asking for samples from elite athletes. In the DIY community, such athletes represent the "height of performance, health and lifestyle." They're referred to as superdonors.