Association between fat mass index and depressive symptoms in U.S. adults: A cross-sectional analysis of NHANES 2011–2016

Fat mass index (FMI) reflects adiposity normalized for height and may provide information beyond body mass index. Evidence on its relationship with depressive symptoms in U.S. adults remains limited. This cross-sectional study used data from adults participating in National Health and Nutrition Examination Survey 2011 to 2016. Fat mass index was calculated from dual-energy X-ray absorptiometry-derived fat mass divided by height squared. Depressive symptoms were defined using the Patient Health Questionnaire-9. Survey-weighted logistic regression, restricted cubic spline analysis, subgroup analyses, and machine-learning models were applied to evaluate the association and predictive performance. The final analysis included 7273 participants. After full adjustment, FMI was positively associated with depressive symptoms, with an odds ratio (OR) of 1.05 for each 1-unit increment (95% confidence interval [CI]: 1.03-1.08). In the quartile-based analysis, participants in Q4 showed greater odds of depressive symptoms compared with those in Q1 (OR = 1.50, 95% CI: 1.12-2.01). Restricted cubic spline analysis suggested an approximately linear positive association. In exploratory prediction models, FMI showed a higher AUC than body mass index, although the overall discrimination remained moderate. Higher FMI was associated with a higher prevalence of depressive symptoms among U.S. adults. FMI may serve as a useful adiposity-related indicator in epidemiological research, but prospective studies are needed before clinical predictive use can be recommended.