Individuals with obesity are significantly more likely than their peers of normal weight to have chronic diarrhea, and the relationship does not appear to be mediated by diet or obesity-associated risk factors, new research has shown.

In a comprehensive analysis of health and nutrition data from a nationally representative sample, Sarah Ballou, PhD, of Beth Israel Deaconess Medical Center in Boston, Massachusetts, and colleagues investigated the relationship between body mass index (BMI) and bowel habits in US adults. They found that the prevalence of diarrhea increased gradually with BMI. The investigators adjusted for demographic and psychological factors, lifestyle, laxative use, dietary factors, and comorbid diabetes.

The findings, which are reported in an article published online September 18 in Alimentary Pharmacology and Therapeutics, "indicate that a change in diet may not be the solution to chronic diarrhea in these patients," Ballou said in an interview with Medscape Medical News. "Obesity is associated with many comorbidities and health risks and requires multidisciplinary management, and management of bowel habits needs to go beyond nutritional support."

Although previous studies have investigated the association between bowel habits and obesity, the current study is one of the few to include data on diet, physical activity, and other potential confounders, the authors write.

They extracted data from the 2009–2010 National Health and Nutrition Examination Survey (NHANES). Participants had completed the bowel health questionnaire, were at least 20 years of age, and had not reported any history of having been diagnosed with inflammatory bowel disease, celiac disease, or colon cancer.

The BMI-based weight categories were defined as follows: underweight, BMI <18.5; normal weight, BMI 18.5–24.9; overweight, BMI 25.0–29.9; obese, BMI 30–34.9; and severely obese, BMI >35. Stool consistency was determined on the basis of respondents' self-reported Bristol Stool Form Scale types.

The study sample comprised 5126 respondents, including 70 (1.40%) who were underweight, 1350 (26.34%) who were of normal weight, 1731 (33.77%) who were overweight, 1097 (21.40%) who were obese, and 878 (17.13%) who were severely obese.

In the weighted sample, 4300 respondents reported having normal bowel habits, 414 reported having diarrhea, and 385 reported having constipation. More than half of respondents with diarrhea were obese (25.82%) or severely obese (27.68%), the authors report. In contrast, the combined percentage of obese and severely obese individuals was 35.6% in the group that had normal bowel habits and 29.8% in the constipation group. "Similarly, up to 8.5% of obese and 11.5% of severely obese individuals had chronic diarrhoea, compared to 4.5% of normal weight individuals," the authors write.

In the first stepwise regression model, obesity and severe obesity were significantly associated with diarrhea; obese and severely obese individuals, respectively were two times and three times more likely to have chronic diarrhea. The significantly increased risk persisted after controlling for demographics/lifestyle, laxative use, dietary factors, and self‐reported diagnosis of diabetes and/or self‐reported use of medication to manage blood sugar. In the final adjusted model, the prevalence odds ratio (POR) for diarrhea among severely obese individuals was 1.93; the POR for constipation in this group was 0.55, compared with normal weight peers.

"The direction of this relationship makes sense, especially when considering lifestyle and other medical factors associated with being obese/severely obese," Ballou said. "What is interesting here is that this association remained significant even after controlling for diet, laxative use, and medical comorbidities. Although an association does not necessarily imply causation, it would make much more sense that obesity would be associated with underlying factors that might cause diarrhea rather than that diarrhea might directly or indirectly cause obesity."

Further research is needed to identify mechanisms behind this association. Some possibilities include various physiologic changes that have been associated with obesity, such as bile acid malabsorption; faster colonic transit; increased intestinal permeability; microbial dysbiosis and endotoxemia; and increased ratio of Firmicutes to Bacteroidetes organisms, which is also seen in diarrhea-predominant irritable bowel syndrome, the authors write.

Another possible explanation, according to Ballou, is the link between obesity and chronic low-grade inflammation.

Despite some limitations related to study design, the findings are important and confirm the hypothesis that dietary and medical comorbidities alone cannot explain the relationship between obesity and chronic diarrhea. "These findings might motivate physicians to screen more carefully for altered bowel habits in obese patients," Ballou said. "Furthermore, they indicate that a change in diet may not be the solution to their chronic diarrhea."

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