Everyone poops, but how often people go could say a lot about their long-term health, according to a study published Tuesday in the journal Cell Reports Medicine.
The study, which included more than 1,400 healthy adults, found that people who had fewer bowel movements also showed signs of decreased kidney function. People at the other end of the spectrum, who had much more frequent bowel movements than normal, showed signs of impaired liver function.
“It’s well known that problems like constipation are associated with chronic disease,” said Sean Gibbons, a study co-author and associate professor at the Institute for Systems Biology in Seattle.
What is less clear is which comes first: constipation or chronic disease? Does early constipation cause chronic disease in healthy people, or is constipation the result of chronic disease?
To try to answer this question, Gibbons and his team used data from Arivale, a now-defunct wellness company. The study participants were healthy white adults living in the Pacific Northwest. The researchers analyzed blood and stool samples, body mass index measurements, and responses to questionnaires about eating and exercise habits and mental health.
From the blood samples, the team was able to detect chemicals in the blood that indicated dysfunction of organs, including the kidneys and liver. From the stool samples, the researchers were able to genetically sequence each person’s gut microbiome, allowing them to learn the composition of beneficial and potentially harmful microbes living there.
The researchers divided the participants into groups based on how often they had bowel movements: one or two bowel movements per week, three to six bowel movements per week, one to three bowel movements per day, and diarrhea, which they defined as four or more times per day.
The study found that the ideal number of bowel movements for optimal health was one or two per day. Younger people, women, and people with lower BMIs tended to have less frequent bowel movements. Chronic constipation (two or fewer bowel movements per week) was linked to decreased kidney function, while diarrhea (four or more bowel movements per day) was associated with decreased liver function.
Hungry microbes in search of food
The gut is filled with colonies of different microbes that feed on the nutrients in the stool and ferment them. Their favorite food is fiber, but if the stool sits in the gut for too long, as is the case with chronic constipation, the microbes deplete their reserves. They then turn to protein, which is usually the protein-rich mucus layer that lines the gut.
“If we don’t feed our microbes, they start eating us,” Gibbons said.
This creates a three-pronged problem: When these microbes feed on protein, they produce toxic metabolites, some of which are associated with kidney and liver dysfunction. The more bacteria must rely on dietary sources of protein, the more protein-preferring bacteria there will be and the fewer fiber-eating beneficial bacteria there will be.
Eventually, microbes can begin to break down the lining in their search for food, and the gut can become “leaky.” This allows bacteria and metabolites from the gut that are toxic to other organs to leak into the bloodstream, where they can cause inflammation that leads to heart, liver and kidney disease, said Dr. Phillipp Hartmann, an assistant professor of pediatric gastroenterology at the University of California, San Diego, who studies the interaction between the gut microbiome and other organs.
“What we can say for sure is that intestinal permeability at least contributes to the disease,” said Hartmann, who was not involved in the new study. “It may not be the only factor, but it often makes the disease worse.”
A similar chain of microbial events can occur if a person has diarrhea, although in this case it is likely inflammation that breaks down the mucous membrane lining the intestine, allowing toxins to enter the bloodstream.
“When you have diarrhea or constipation, microbes accumulate and produce toxins that influence the disease,” said Joseph Petrosino, chairman of the department of molecular virology and microbiology at Baylor College of Medicine, who also was not involved in the new research.
The study found that people who regularly suffered from constipation or diarrhea had more protein-producing gut bacteria, while people who had bowel movements once or twice a day had more fiber-fermenting bacteria.
Gibbons said it’s not clear why the study found constipation to be associated with kidney dysfunction and diarrhea to be associated with liver dysfunction, but it may have to do with the fact that diarrhea prevents the intestine from absorbing bile acids, the product of cholesterol metabolism, leaving more room for the liver to process them. The kidneys are more affected by metabolites produced by protein-eating gut microbes.
Because the study was not a randomized clinical trial that included an intervention, the new research can’t conclude with certainty whether having more or less than one or two bowel movements per day is linked to chronic disease, but Gibbons said it appears that it may be.
“We hypothesized that we would see more protein-derived toxins in the blood in people with less frequent bowel movements or constipation, and that’s what happened,” he said. “These toxins damage the liver and kidneys.”
Petrosino said that while it’s not always within a person’s control, many lifestyle factors can help prevent constipation and diarrhea.
“First of all, it’s important to have a healthy diet, to eat foods that fertilize and promote the growth of good bacteria,” he said.
That means eating plenty of fruits and vegetables and cutting back on alcohol and red meat. In the study, people who reported eating a high-fiber diet, staying hydrated and exercising regularly tended to have healthier stool frequencies. Probiotics can be helpful if a person has chronic conditions such as stomach upset, constipation or diarrhea, Petrosino said.
“There are probiotics that can help normalize things, but if you’re already healthy, I wouldn’t recommend starting them,” he said. “Don’t fix what’s not fixed.”