For millions of adults in the United States, involuntary loss of bowel control, or fecal incontinence (FI), has a major negative influence on their quality of life and mental health. Although it is believed that obesity has an impact on bowel function, it is still unknown how body mass index (BMI), the usual measure of obesity, and FI are related.
Investigating more accurate indicators of obesity, like as body composition and fat distribution, as opposed to BMI alone, may provide more insight into how obesity affects FI.

In a cross-sectional examination of the U.S. population, researchers from Brigham and Women’s Hospital, a founding member of the Mass General Brigham healthcare system, showed that waist circumference-to-height ratio (WHtR), a marker of central obesity, was a greater predictor of FI than BMI.

Using data collected between 2005-2010 by the National Health and Nutrition Examination Survey, the researchers calculated WHtR and BMI for more than 7500 participants and assessed them for prevalence and risk of FI. They found that FI occurred in just over 9% of the study cohort and that higher prevalence was linked to higher WHtR, but only weakly associated with higher BMI.

After adjusting for potentially confounding factors like sociodemographics, metabolic comorbidities, diet and physical activity, the research team observed that higher WHtR, but not BMI, remained consistently correlated with FI. Sex-specific analyses also revealed more significant associations between WHtR and FI in men.
“We know that central adipose tissue distribution tends to be more prevalent in men than women,” said first author Brent Hiramoto, MD, a senior fellow in the Division of Gastroenterology, Hepatology and Endoscopy. “These sex-based differences could indicate the importance of central obesity in the pathogenesis of FI.”

“Our findings suggest that risk of bowel continence may depend on how body mass is distributed,” said senior author Walter Chan, MD, MPH, of the Division of Gastroenterology, Hepatology and Endoscopy. “BMI is a pragmatic measure for obesity but does not distinguish between fat and muscle mass or assess distribution of adiposity. WHtR may contribute to FI through increased intra-abdominal pressure affecting the pelvic floor or anal sphincter, or through higher visceral fat with the associated inflammatory and oxidative stress. Further research is needed to better understand why FI occurs.”