How often you poop could suggest more than you think, study finds

How often you poop may influence more than whether you’re uncomfortably bloated. The frequency may also affect your gut microbiome and risk of chronic disease, a new study has found.

Gut bacteria that digest fiber, for example, seemed to thrive in participants who pooped 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 were enriched in those with diarrhea or constipation, respectively.

The authors also found younger people, women and participants with a lower body mass index had less frequent bowel movements.

“Many people with chronic diseases, including Parkinson’s and chronic kidney disease, report having had constipation for years prior to the diagnosis,” said the study’s senior author, Dr. Sean Gibbons, who has lost family members to Parkinson’s disease.

“However, it is unclear whether or not these bowel movement frequency … aberrations are drivers of disease, or simply a consequence of disease,” Gibbons, associate professor at the Institute for Systems Biology in Seattle, added via email.

This quandary is what motivated the researchers to study the associations between bowel movement frequency and genetics, the gut microbiome, blood plasma chemistries and blood metabolites — small molecules that are participants in and products of metabolism — to assess whether the pattern may be negatively affecting the body prior to a disease diagnosis, Gibbons said.

The authors examined the health and lifestyle data of more than 1,400 healthy adults who had participated in a scientific wellness program at Arivale, a consumer health company that operated from 2015 to 2019 in Seattle. Participants, nearly 83% of whom were White, answered questionnaires and consented to blood and stool sampling.

Self-reported bowel movement frequency was separated into four groups: constipation (one or two bowel movements per week), low-normal (three to six weekly), high-normal (one to three per day) and diarrhea.

The authors also found that several blood metabolites and blood plasma chemistries were linked with different frequencies. Byproducts of protein fermentation 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 chemistries linked with liver damage were higher in people with diarrhea, who also had more inflammation.

“What’s exciting to me about this study is that we have long known about a connection between constipation and chronic kidney disease, but the potential mechanisms have never been well understood,” said Dr. Kyle Staller, director of the Gastrointestinal Motility Laboratory at Massachusetts General Hospital and associate professor of medicine at Harvard Medical School, via email.

“This study provides one pathway by which future studies could investigate this connection over time … to learn whether people with low bowel movement frequency produce more potentially toxic metabolites and subsequently develop kidney disease,” added Staller, who wasn’t involved in the study.

Understanding gut health

“There are some important limitations here that make the findings less translatable to the average person,” Staller said, including that the study doesn’t prove a cause-and-effect relationship. The data result from participants studied at a single point in time, so other factors could be at play. It’s also possible a person’s gut microbiome could be influencing bowel movement frequency.

Bowel movement frequency also isn’t the most ideal measure of bowel function, he said.

“We know that normal bowel movement frequency ranges from (three) bowel movements per week to (three) bowel movements per day, but the best measure of how fast things are moving through our gut is stool form,” Staller added. “That is, when stools are harder, they have been sitting in the colon for longer — what we call a longer transit time.

“When stools are softer, the opposite is true. Thus, a more ideal measure of bowel function would be stool consistency rather than frequency.”

Additionally, many of the findings are based on the group with low to normal bowel movement frequency — three to six times per week — with few derived from those who were constipated or had diarrhea, experts said.

“Ideally, we would see kind of a ‘dose-response relationship,’ where the worse the constipation, the worse the kidney function and the higher number of these potentially harmful metabolites in the blood as a marker.”

What’s more, bacterial species in participants’ stools were detected by a type of technology that indicates only a larger group, or genus, the bacteria belong to, rather than specific species — which could have differential effects even when from the same group, he added.

Participants with generally normal bowel movement frequency also had important differences in lifestyle, such as consuming 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 wasn’t involved in the study.

Regarding the potential processes behind the authors’ hypothesis, “prior work has established that gut microbes undergo a switch between fiber and protein fermentation, depending on gut transit time,” Gibbons said via email. “Over normal transit times (normal BMFs), gut microbes ferment dietary fibers into healthy organic acids that maintain intestinal homeostasis.

“However, if stool stays in the gut for too long (constipation), the microbes start to run low on fiber and they switch to fermenting protein (and eating our mucus layer, which is also protein rich),” Gibbons added. “Protein fermentation in the gut gives rise to these toxic compounds that are found in the blood.”

All considered, Staller doesn’t think anyone should see the findings as a reason to worry about how fast their gut is moving, he said. “Much of the data is derived from people that we physicians would consider normal, and there are not enough truly constipated individuals from which we can draw hard conclusions.”

What’s more important, he added, is the fact that 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 growing by leaps and bounds daily,” Staller said.

“However, any attempt to simplify this knowledge into cultivating the ‘ideal’ gut microbiome is a ways off,” he added. “Our knowledge in this field is just too underdeveloped to make sweeping changes to our lives based on the findings of a study like this just yet.”