How often you poop may be (partly) written in your genes

Genes passed down by your parents may influence how often you poop, a new study suggests, and these key pooping genes may hold clues as to what causes poorly-understood gastrointestinal disorders, such as irritable bowel syndrome (IBS).

That said, the new research comes with a number of caveats, said Dr. Emeran Mayer, a professor, gastroenterologist and neuroscientist at University of California, Los Angeles, who was not involved in the study. For instance, while this and other studies hint that IBS may have a genetic component, other factors, such as a person's diet and stress levels, likely have a bigger effect on the disorder's symptoms, Mayer told Live Science in an email.

The new study, published Dec. 8 in the journal Cell Genomics, included genetic sequences and health data from 167,875 individuals with records in the U.K. Biobank, a huge biomedical database, and four smaller databases curated by the Netherlands, U.S., Belgium and Sweden. Some of these patients had IBS, which involves intestinal symptoms, such as recurrent abdominal pain, bloating, gas and changes in bowel function, including constipation, diarrhea or both, according to UCSF Health. On top of providing their DNA for analysis, these individuals answered a critical question: "What is the average number of times you open your bowels per day?" In other words, how often do you poop? 

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Based on the participants' answers and genetic sequences, the team found that how often a person poops shows a "modest but detectable heritability," meaning it's at least partly influenced by genetics. In determining this, they identified 14 stretches of the genome that seemed linked to stool frequency. 

Those who pooped less frequently than other participants showed similar patterns of genetic variation within those 14 areas, and those who pooped more frequently also shared a similar genetic make-up in those gene regions. Overall, most participants reported pooping once, twice or three times a day; fewer people reported four, five or six times, and a smaller group reported huge numbers, up to 20 times a day, D'Amato said. (There's a range for what stool frequency is considered "normal," but 20 times a day would not be normal under any circumstances, he added.)

To see if they could predict which participants experienced IBS based on these distinct genetic profiles, the researchers calculated a "polygenic risk score" for each person. That risk score indicated how likely each participant was to have high stool frequency, based on their genetic make-up. They found that those in the top 1% of scores showed a fivefold higher risk of IBS than the rest of the participants, specifically the subtype of IBS that causes increased diarrhea (IBS-D). 

But why would that be? To start to answer that question, the team looked more closely at which specific genes appeared in the 14 stretches of DNA and how they may relate to defecation and IBS. 

Each of these regions of DNA contains several genes, and in looking at the genes with known functions, "some of them made a lot of sense," said senior author Mauro D'Amato, formerly head of the Gastrointestinal Genetics Lab at CIC bioGUNE, a health science research center in Spain, and now a professor of medical genetics at the Libera Universita' del Mediterraneo in Italy.

For example, one gene codes for brain-derived neurotrophic factor (BDNF), a protein that helps direct the function and survival of nerve cells throughout the body. Evidence suggests that increasing the level of BDNF in the gut can boost motility — or how quickly material moves through the gastrointestinal tract. In the past, it's been tested as a potential treatment for chronic constipation, according to a 2000 report in the journal Gastroenterology. 

Seeing the BDNF gene pop up in the new study hinted to the authors that they were on the right track, D'Amato said. At the moment, it's unclear exactly how different versions of the BDNF gene might influence stool frequency, but it gives the team a good starting point for future research.

Related: Genetics by the numbers: 10 tantalizing tales

A few other interesting genes cropped up in their analysis, including some that code for neurotransmitters, hormones and other molecules that help control the nerve cells involved in intestinal peristalsis, the wave-like movements that push poop through the intestine. Many of these molecules also play roles in the brain. 

This finding may somewhat align with an earlier study by D'Amato and his colleagues, published Nov. 5 in the journal Nature Genetics, which hints that the genetic risk factors for IBS overlap with those for anxiety and depression. This genetic data also coincides with clinical observations of IBS patients, who often experience elevated anxiety levels prior to their IBS symptoms. Research has started to suggest that IBS involves both the brain and gut, rather than the gut in isolation, Mayer said.  

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Looking forward, D'Amato and his colleagues plan to study in more detail each stretch of DNA identified, nailing down which genes are most important to stool frequency and to the symptoms of IBS. They also hope to address some of the limitations of the current study, including the fact they didn't have any data on participants' diets, which would obviously influence both their stool frequency and potential IBS symptoms.

Another limitation is that study participants were asked how many times they poop each day, rather than each week or month. "So we are missing those people who may have, you know, one bowel movement per week," D'Amato said. "Because of the way the question was posed to the participants, you're sort of looking at the right half of the spectrum." For this reason, the current study is more useful for investigating IBS-D, and more data will need to be gathered to perform a similar study of IBS with constipation (IBS-C), he said.

But even if the team can get better stool frequency data, it's unclear whether a person's self-reported bowel habits really reflect their overall gut motility and all the factors that affect it, Mayer said. Some IBS patients' bowel habits switch from diarrhea to constipation without apparent cause, and it's unlikely that an average measure of stool frequency would capture that. 

Despite its limitations, the current study could help scientists uncover the molecular underpinnings of IBS and eventually come up with new pharmacological treatments for the disorder, D'Amato said. 

But for now, "patients should be aware that even though genetics may play a very small role in the overall disorder, there are many behavioral and lifestyle factors that are more important and that can be modified," Mayer said. Namely, dietary changes, therapies to help with stress reduction and drugs that relieve the intestinal symptoms can all be very helpful to IBS patients, he said. 

Originally published on Live Science.