Association between relative fat mass and cognitive impairment in older adults: A cross-sectional study using NHANES...

Relative fat mass (RFM), derived from height and waist circumference, may better reflect overall and central adiposity than body mass index. This study examined the association between RFM and cognitive impairment among older adults in the United States. We conducted a cross-sectional analysis using National Health and Nutrition Examination Survey 2011-2014 data, including 2730 participants aged ≥ 60 years with valid anthropometric data, cognitive assessment data, and covariate information required for the primary analysis. Cognitive function was evaluated using the Consortium to Establish a Registry for Alzheimer Disease (word learning & delayed recall tests), animal fluency, and digit symbol substitution tests. Cognitive impairment was defined as performance below the education-specific 25th percentile of a standardized global cognition z-score. Weighted logistic regression models estimated associations between RFM and cognitive impairment, adjusting for demographic, socioeconomic, lifestyle, and clinical factors. Nonlinearity was tested using restricted cubic spline and two-piecewise regression analyses. In the base-adjusted model, RFM was not significantly associated with cognitive impairment. After full adjustment for demographic, socioeconomic, lifestyle, dietary, and clinical factors, higher RFM was associated with increased odds of cognitive impairment (odds ratio = 1.05; 95% confidence interval: 1.02-1.09). Sensitivity analyses excluding participants with very high body mass index, trimming RFM outliers, modeling log-transformed RFM, and using waist circumference as an alternative adiposity measure yielded directionally consistent results. The relationship was linear (P_overall = .042; P_nonlinear = .253), with stronger associations in participants aged < 70 years, and those with hypertension or prior stroke. Hypertension (10%) and physical inactivity (9.4%) partially mediated this relationship, whereas depression and diabetes showed minimal effects. Higher RFM was associated with greater odds of test-based cognitive impairment among United States older adults after multivariable adjustment. The observed associations involving hypertension and physical activity status were exploratory and should not be interpreted as causal. Future longitudinal studies are needed to clarify temporality and evaluate the potential clinical utility of RFM in cognitive risk assessment.