It would not be surprising to learn that an unhealthy diet is to blame for both obesity and abnormal bowel habits. But sometimes science surprises us.

Obesity is defined by one’s body mass index (BMI), a ratio of height to weight. If a person’s BMI is 30 or higher, he/she is considered obese—a condition that has long been known to increase risk for a variety of health problems, including heart disease, type 2 diabetes and certain types of cancer.

But there’s been scant evidence analyzing the connection between obesity and bowel habits, including diarrhea, until researchers at Beth Israel Deaconess Medical Center (BIDMC) in Boston recently published a new study that could change the way doctors treat bowel problems in people who are obese.

Study details: To begin their investigation, the researchers extracted data, including BMI, for the more than 5,000 individuals, age 20 and older, who had completed a bowel health questionnaire and had no history of irritable bowel syndrome, celiac disease or colon cancer. A statistical software program was then used to compare BMI with bowel habits, while also controlling for diet, physical activity, laxative use and psychological and medical conditions.

The results: The obese respondents were 60% more likely to have experienced chronic diarrhea than those with a healthy BMI. Overall, only 4.5% of healthy-weight individuals had chronic diarrhea compared with 8.5% of obese and 11.5% of severely obese individuals.

“Our research confirms a positive association between obesity and chronic diarrhea and reveals for the first time that this relationship is not driven by confounding factors such as diet or physical activity level,” explained study author Sarah Ballou, PhD, a health psychologist in the division of gastroenterology, hepatology and nutrition at BIDMC.

Even though the study, which was published in Alimentary Pharmacology & Therapeutics, did not address why obesity may be linked to diarrhea, the authors theorize that chronic low-grade inflammation may play a role. Obesity promotes inflammation, which also may contribute to diarrhea. Another theory focuses on whether an imbalanced gut microbiome, which is more common in people who are obese than in healthy-weight individuals, could also be a factor in the relationship between obesity and diarrhea.

Implication: Based on these findings, the researchers urge doctors to use a multidisciplinary approach to treat obesity and obesity-related health problems. Diarrhea can have a negative impact on quality of life, so an obese person’s increased risk for this type of abnormal bowel habit should be addressed as part of his/her medical care. With further study, the researchers hope to uncover the cause of the obesity-diarrhea link and reveal more effective ways to treat abnormal bowel habits in obese patients.