Obesity has been proposed as a risk factor for fecal incontinence (FI), but studies relying on body mass index (BMI) as the measure of obesity have had mixed results.
Increasing intra-abdominal pressure is a potential mechanism of the impact of obesity on bowel continence. Therefore, Brent Hiramoto, MD, a fellow in the Division of Gastroenterology, Hepatology and Endoscopy at Brigham and Women’s Hospital, Walter W. Chan, MD, MPH, director of the Center for Gastrointestinal Motility at the Brigham, and colleagues hypothesized that central adiposity, not elevated BMI alone, may have a role in FI.
Based on nationally representative U.S. data, they report in Clinical Gastroenterology and Hepatology that waist circumference-to-height ratio (WtHR) is a stronger predictor of FI than BMI.
Methods
The data source for the study was the National Health and Nutrition Evaluation Survey (NHANES), in which people are interviewed in their homes and undergo physical examinations and laboratory tests in mobile clinics. This analysis included 7,530 adults (≥20 years old) who participated between 2005 and 2010, completed the Bowel Health Questionnaire, and had full data available on potential covariates.
Primary Analysis
The prevalence of FI, defined as involuntary loss of mucus, liquid, or solid stool at least once during the past 30 days, was 9.2%.
On multivariable analysis, WtHR was significantly associated with FI:
- 4th quartile vs. 1st—OR, 1.75 (P=0.021)
- Continuous variable—OR, 9.4 (P=0.01)
BMI was not associated with FI whether analyzed by quartiles or clinical categories (underweight/normal, BMI <25 kg/m2; overweight, 25–29.9 kg/m2; class 1 obesity, 30–34.9 kg/m2; class 2 obesity or higher, ≥35 kg/m2).
Secondary Analysis
The researchers also examined the effect of WtHR on FI using a more stringent definition of FI, limited to liquid and solid leakage:
- The association between WtHR and FI was strengthened—4th quartile vs. 1st: OR, 2.66 (P<0.001)
- The association between BMI and FI became statistically significant—4th quartile vs. 1st: OR, 1.93 (P=0.011); class II or greater obesity: OR, 1.92 (P=0.019)
It’s worth noting the magnitude and strength of the association between BMI and FI were lower than between WtHR and FI.
Optimized Cutoff Values
The team determined cutoffs for WtHR that optimized the prediction of FI:
- Overall, WtHR >0.592 was most predictive of FI with a sensitivity of 59%, specificity of 54%, and area under the receiver operating characteristic curve of 0.57
- In participants ≥65 years old, optimized cutoffs were >0.662 for men and >0.784 for women
Because NHANES intentionally oversamples underrepresented groups, these results can be extrapolated to the general U.S. population.