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How often you have a bowel movement may affect more than just bloating. Frequency may also affect your gut microbiome and your risk of chronic disease, a new study suggests.
Gut bacteria that digest fiber, for example, appear to thrive in participants who poop once or twice a day, according to the study published Monday in the journal Cell Reports Medicine. But bacteria associated with the upper gastrointestinal tract or protein fermentation are more numerous in people with diarrhea or constipation, respectively.
The authors also found that younger people, women, and participants with a lower body mass index had less frequent bowel movements.
“Many people with chronic diseases, including Parkinson’s disease and chronic kidney disease, report having constipation for years before diagnosis,” said the study’s lead author, Dr. Sean Gibbons, who has lost family members to Parkinson’s.
“However, it is unclear whether or not these aberrations in bowel movement frequency are disease drivers, or simply a consequence of disease,” Gibbons, an associate professor at the Institute for Systems Biology in Seattle, added by email.
It’s this dilemma that motivated researchers to study associations between bowel movement frequency and genetics, the gut microbiome, blood plasma chemistries and blood metabolites — small molecules that participate in and are products of metabolism — to assess whether this pattern might negatively affect the body before a disease diagnosis, Gibbons said.
The authors examined health and lifestyle data from more than 1,400 healthy adults who participated in a science-based wellness program at Arivale, a consumer health company that operated from 2015 to 2019 in Seattle. The participants, nearly 83% of whom were white, answered questionnaires and consented to blood and stool samples.
Self-reported stool frequency was divided into four groups: constipation (one or two stools per week), low-normal (three to six per week), high-normal (one to three per day), and diarrhea.
The authors also found that several blood metabolites and blood plasma chemistry parameters were linked at different frequencies. Protein fermentation byproducts such as p-cresol sulfate and indoxyl sulfate, known to cause kidney damage, were enriched in constipated participants. Blood levels of indoxyl sulfate were also associated with reduced kidney function. And chemistry parameters linked to liver damage were higher in people with diarrhea, who also had more inflammation.
The authors say their findings provide “preliminary support for a causal relationship between bowel movement frequency, gut microbial metabolism and organ damage,” according to a press release.
“What’s exciting to me about this study is that we’ve known for a long time about a link between constipation and chronic kidney disease, but the potential mechanisms have never been well understood,” Kyle Staller, MD, director of the gastrointestinal motility laboratory at Massachusetts General Hospital and associate professor of medicine at Harvard Medical School, said via email.
“This study provides an avenue by which future studies could investigate this link over time…to see whether people with low stool frequency produce more potentially toxic metabolites and subsequently develop kidney disease,” added Staller, who was not involved in the study.
Understanding Gut Health
“There are some important limitations here that make the results less translatable to the average person,” Staller said, including that the study doesn’t prove cause and effect. The data comes from participants studied at a single point in time, so other factors could be at play. It’s also possible that a person’s gut microbiome influences bowel movement frequency.
Frequency of bowel movements is also not the most ideal measure of bowel function, he said.
“We know that normal stool frequency ranges from three stools per week to three stools per day, but the best measure of how quickly things are moving through our gut is the shape of the stool,” Staller added. “In other words, when the stool is harder, it has been in the colon longer, which we call a longer transit time.”
“When stools are softer, it’s the opposite. So a more ideal measure of bowel function would be stool consistency rather than stool frequency.”
Additionally, many of the results are based on a group with low to normal stool frequency (three to six times a week), and few come from people who are constipated or have diarrhea, the experts said.
“Ideally, we would see some sort of ‘dose-response’ relationship, where the more severe the constipation, the worse the kidney function and the higher the number of these potentially harmful metabolites in the blood as a marker.”
Additionally, the bacterial species in the participants’ stools were detected by a type of technology that indicates only a broader group, or genus, to which the bacteria belong, rather than specific species — which could have differential effects even when they come from the same group, he added.
Participants with generally normal stool frequencies also had important lifestyle differences, such as eating more fruits and vegetables, being better hydrated and exercising more, said Dr. Rena Yadlapati, a professor in the division of gastroenterology at the University of California, San Diego, who was not involved in the study.
As for the potential processes behind the authors’ hypothesis, “previous work has established that gut microbes undergo a switch between fiber and protein fermentation, depending on intestinal transit time,” Gibbons said via email. “During normal transit times (normal BMF), gut microbes ferment dietary fiber into healthy organic acids that maintain intestinal homeostasis.”
“However, if stool sits in the gut too long (constipation), the microbes start to run out of fiber and start fermenting the proteins (and eating our mucus layer, which is also high in protein),” Gibbons added. “Protein fermentation in the gut gives rise to these toxic compounds that end up in the blood.”
Ultimately, Staller doesn’t think the study’s results should worry us about how fast our bowels are moving. “Most of the data comes from people we as doctors consider normal, and there aren’t enough people who are actually constipated for us to draw firm conclusions.”
More importantly, he added, the study reaffirmed the ability of dietary factors to also affect gut function.
“Experts’ understanding of the interactions between resident gut bacteria and bodily functions is advancing by leaps and bounds every day,” Staller said.
“However, any attempt to simplify this knowledge to cultivate the ‘ideal’ gut microbiota is still far from being feasible,” he added. “Our knowledge in this area is simply too poorly developed for us to make radical changes to our lives based on the results of a study like this.”
