Scientists know that gut bacteria are important for good health.
A thriving, diverse gut microbiome doesn’t just bolster gut health; it has benefits throughout the body.
Changes in gut bacteria are associated with with many health problems, including inflammatory gut conditions, metabolic diseases like diabetes, cardiovascular disease, and even neurological problems.
Scientists also know that bacteria make up around one-quarter to one-half of the dry weight of poop.
So, the idea of transferring poop from someone with a healthy gut microbiome to someone with a medical condition seems worth investigating.
This technique — known as a fecal microbiota transfer, poop transplant, or fecal transfer — has attracted a fair amount of scientific interest.
A brief history
Incredibly, the concept of fecal transfer is very old. There are records of Chinese doctors trying it around 1,700 years ago.
Documents show that a doctor called Ge Hong used fecal suspensions to treat diarrhea and food poisoning.
Apparently, it worked. ZOE does not recommend this approach, in case you were wondering.
Jumping through time, the first mention in medical literature came in 1958. The paper describes how scientists helped treat a type of Clostridium difficile infection with a fecal transfer.
Today, treating C. diff is the most common use of a fecal transfer. We’ll cover that in more detail later on.
Some research in animal models has yielded impressive results.
However, before we dive in, we must mention that what happens in animals doesn’t necessarily translate into humans.
This is certainly the case when it comes to fecal transfers, as we’ll see. Nevertheless, the results are quite eye-opening.
For instance, scientists have shown that if you transfer poop from an obese mouse into a mouse that lacks gut bacteria, it becomes obese.
Similarly, using a mouse model of diabetes, scientists showed that a fecal transfer improved the animals’ insulin sensitivity.
Will the same thing necessarily happen in humans? No. But these studies show that fecal transfers are a technique worth pursuing in humans.
And, at least in treating one condition, poop transplants have been quite successful.
C. diff and fecal transfers
C. diff is a bacterium, and it’s often present in the gut microbiome. In some cases — for instance, after a course of antibiotics — C. diff can take over and cause problems.
Symptoms of a C. diff infection can include fever, cramps, and diarrhea.
It causes around half a million infections each year in the United States. And in older adults, it can sometimes be fatal.
This infection often returns after standard treatment. In some people, this causes colitis, which is inflammation of the colon.
For these people, doctors sometimes recommend a fecal transfer.
What’s the procedure like?
First, a healthy donor provides some poop, which is filtered to remove undigested food and other particles.
There are a number of ways to get donated poop into a recipient.
Most commonly, the transplant is delivered by colonoscopy. This involves a medical professional guiding a tube through your butt and into your intestines.
As they pull the tube out, they release liquid containing fresh or frozen feces.
Less commonly, medical professionals insert the tube into the nose — a nasogastric tube — and gently guide it down to the intestines.
Other options include capsules containing feces that you swallow or have as an enema.
Does it treat C. diff?
Overall, fecal transfers appear to work well against repeated C. diff infections.
A 2019 review, for instance, concluded that this is “a safe, economical, and effective method.”
The study's authors also point to unanswered questions about the best ways to do these procedures.
For instance, does frozen or fresh poop work best? And do multiple fecal transfers work better?
Doctors often treat recurrent C. diff infections with antibiotics. But because antibiotic-resistant strains of C. diff are increasingly present, an effective new treatment can’t come soon enough.
However, some scientists have questioned whether fecal transfers are better than drug treatments and raised safety concerns. We’ll touch on some of the safety aspects later on.
How does a fecal transfer work?
In short, experts aren’t 100% sure. But it seems that the transfer of a healthy selection of gut bugs restores some balance to a disordered gut microbiome.
Experts are thrashing out more precise explanations, but they have some theories. We’ll outline a couple here.
The importance of bile
Some scientists think bile might be involved — C. diff needs bile to thrive.
Some species of “good” gut bacteria convert bile into secondary bile acids. And these acids might hinder C. diff.
In this way, an infusion of “good” gut bugs might reduce the amount of bile available to C. diff while increasing levels of secondary bile acids slow its progress.
Short-chain fatty acids
Short-chain fatty acids (SCFAs) might also help fecal transfers fight off C. diff. SCFAs are produced by “good” gut bacteria as they feed on dietary fiber.
They play several beneficial roles in the body, including nourishing your gut lining.
And there’s some evidence from lab studies that an SCFA called valerate might help fend off C. diff.
So, reinstating some “good” bugs that produce SCFAs might be another way that fecal transfers help treat this recurrent infection.
We should also remember that poop is not pure bacteria. Viruses and fungi, among other microbes, might play a part. But scientists know very little about how these elements of the gut microbiome influence health.
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Fecal transfer for other conditions
Right now, a recurrent C. diff infection is the only condition that’s routinely treated with a fecal transfer.
But scientists are interested in whether the technique might be useful further afield.
Inflammatory bowel disease (IBD) is an umbrella term for a number of conditions, and one of the most common is ulcerative colitis.
This chronic condition involves inflammation in the colon and rectum. And small ulcers appear in the lining of the colon.
Current treatments usually include steroids. These are effective for some people, but they often have unpleasant side effects. So, doctors are eager for better ways to manage this condition.
A review published in 2019 concluded that “Fecal microbiota transplantations were associated with significant clinical remission and response in ulcerative colitis patients.” So, there's potential.
However, fecal transfers didn’t seem to help people who had stopped taking steroids.
Another review, which agrees that there’s potential, outlines some important gray areas that need clearing up becomes fecal transfers become a mainstream ulcerative colitis treatment.
For instance, how big do the doses need to be? How long should the treatment last? What’s the best method to use? And whose poop would be best?
The other main form of IBD is Crohn’s disease. As with ulcerative colitis, there’s evidence that fecal transfers may work for people with Crohn’s. And it appears to be safe.
But again, scientists need to do more research to figure out the specifics.
"Metabolic syndrome" refers to a collection of conditions: diabetes, high blood pressure, and obesity. And it’s increasingly common.
Scientists have identified links between changes in the gut microbiome and metabolic syndrome, so could poop transfers be a viable treatment?
A 2019 review looked at studies of fecal transfers in people with metabolic syndrome. Although the authors could only find a handful of small studies, there were some encouraging results.
Two studies found that insulin sensitivity improved after 6 weeks in people who had a fecal transfer, compared with others who had a placebo.
Another study in the review found a decrease in average blood sugar levels 6 weeks after a fecal transfer. However, this change didn’t last for the duration of the 18-week study.
At this point, scientists still need to do much more research. Metabolic syndrome is incredibly complex, and many factors can influence how the syndrome manifests.
Irritable bowel syndrome
Irritable bowel syndrome (IBS) is a common condition affecting an estimated 10–15% of adults in the U.S. The symptoms can include stomach cramps, diarrhea, bloating, and constipation.
Some experts think fecal transplants might help, but the evidence is a little mixed.
A review from 2022 concluded that fecal transplants improved the quality of life for people with IBS, compared with those who had a placebo. But the team didn't see improvements in all measures.
And an earlier review, from 2019, found that fecal transfers didn’t work for people with IBS.
The authors describe several reasons why there was no apparent effect. For instance, they write that the placebo effect is particularly strong in people with IBS.
In other words, people who had a placebo reported almost as much benefit as those who had the fecal transfer.
The scientists noted that the method of transfer might make a difference. For instance, studies using colonoscopies or nasogastric tubes had better results than those using oral capsules.
Finally, IBS can vary from person to person — there are subtypes. And this type of intervention might work better for some than others.
But in this review, the authors didn’t have access to enough data to distinguish between subtypes.
A 2021 review found positive results in some studies but negative results in others.
The authors suggest that, among other issues, getting the right poop donor is vital.
As they note, studies have shown that some donors’ poop is more effective than others, but there's still no clear criteria for these “super donors.”
Most studies so far have investigated whether fecal transfers work for IBS, not which donors produce the most effective poop.
To understand whether IBS can be treated by poop transplants — without wanting to sound like a broken record — we need more research.
What about the brain?
Once you’re comfortable with the idea that gut bacteria are key to gut health, it’s not too much of a leap to imagine how a poop transplant might help treat gut conditions.
It takes an extra leap to understand how a transfer of feces might make a difference in mental health conditions.
Yet there are strong links between the gut and brain: These form the gut-brain axis.
So, is there any evidence that a poop transplant can help? A 2020 review of 28 studies looked at the effects of fecal transfers on people with a range of psychiatric conditions.
The team assessed a number of studies in which researchers transferred gut bacteria from humans with psychiatric conditions into animals that lacked gut microbiomes.
They found that the transfers “consistently” produced similar symptoms in the animals.
Turning to studies with human participants, they determined that all “found improvement in the symptoms of psychiatric disorders related to these disorders following [fecal transfer] from healthy donors.”
The studies were small, and the benefits often only lasted a few months, but the results are fascinating.
Because current treatments for mental health conditions don’t work for everyone, this is an exciting avenue of research.
Is it safe?
There are some safety concerns associated with poop transplants.
For instance, although donors are thoroughly screened for disease-causing bacteria, they could host an unknown pathogen that medical checks wouldn’t detect.
As an extreme example, in one case, a patient received a fecal transplant containing an antibiotic-resistant strain of Escherichia coli and died from the infection.
Also, when doctors use a nasogastric tube, there’s a very small risk that the tube might accidentally be maneuvered into the lungs, which would cause a serious lung infection.
However, medical professionals have procedures in place to ensure that this doesn’t happen.
Overall, poop transplants do seem to be quite safe.
What does it all mean?
In short, poop transplants are effective for treating recurrent C. diff infections.
Beyond that, there are some encouraging results, but we need to wait for more data.
Gut bacteria have wide-ranging effects on human health, and scientists are still only scratching the surface of these relationships.
As of yet, we haven’t even named most of the microbes in our gut.
So, even with the most careful screening, there’s a risk of passing along microbes that negatively affect the recipient.
At ZOE, we understand that looking after your gut microbiome helps you care for your whole body.
So, while fecal transplants aren’t on the cards for most conditions anytime soon, making sure you have a gut-healthy diet is a great way to help maintain good health.
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