Associations between five anthropometric indices and fecal incontinence: A cross-sectional study based on the 2005 to 2010 NHANES data

Fecal incontinence (FI) is a common condition that impairs quality of life. While the association between general adiposity, measured by body mass index (BMI), and FI is inconsistent, central adiposity may be more relevant. Indices such as the waist-to-height ratio (WHtR) and novel anthropometric indices, including the Conicity Index (CI), A Body Shape Index (ABSI), and relative fat mass (RFM), aim to better quantify central fat distribution. However, their association with FI is not well characterized. We aimed to comprehensively evaluate and compare the associations of these 5 indices with FI. This cross-sectional study analyzed data from the 2005 to 2010 cycles of the National Health and Nutrition Examination Survey, including 10,097 participants. FI was defined based on the Bowel Health Questionnaire as any involuntary loss of mucus, liquid, or solid stool within the previous month. Multivariate logistic regression was used to evaluate the association between the 5 anthropometric indices and FI. We used restricted cubic splines for dose–response, conducted sensitivity and subgroup analyses, and compared association strengths using the area under the receiver operating characteristic curve. The prevalence of FI was 8.4%. After full adjustment for confounders (Model 3), each standard deviation increase in the z -score of CI (odds ratio = 1.28, 95% CI: 1.18–1.39), ABSI (1.28, 1.18–1.39), RFM (1.29, 1.13–1.46), and WHtR (1.15, 1.06–1.24) was positively associated with FI, but not BMI (1.05, 0.98–1.13). The dose–response relationships were linear ( P -value for nonlinearity: CI = .08, ABSI = .63, RFM = .31, WHtR = .25). Subgroup analyses indicated interactions by age, BMI, and chronic diarrhea, but positive associations persisted in most subgroups. CI and ABSI demonstrated modest discriminatory ability for FI (area under the receiver operating characteristic curve = 0.620 and 0.613, respectively), exceeding WHtR (0.595), RFM (0.585), and BMI (0.548). CI, ABSI, RFM, and WHtR, but not BMI, were positively associated with FI, with CI and ABSI showing relatively stronger associations. These findings suggest that central fat distribution is more strongly related to FI than overall obesity. Given their modest predictive accuracy, future studies should validate these findings and assess their clinical utility.