Association between relative fat mass and chronic cardiorenal syndrome: A cross-sectional study based on NHANES 1999 to 2018

Relative fat mass (RFM) is an anthropometric index calculated from height, waist circumference, and sex (RFM = 64 - [20 × height/ waist circumference] + [12 × sex], where sex = 1 for females and 0 for males), designed to estimate whole-body fat percentage more accurately than body mass index. This study investigated the relationship between RFM and chronic cardiorenal syndrome (CRS) in the general population, further exploring the potential mediating role of inflammatory markers using mediation analysis. This study utilized data from the 1999 to 2018 U.S. National Health and Nutrition Examination Survey. Using a stratified sampling strategy and multivariable logistic regression models, we examined the association between RFM and chronic CRS while adjusting for key demographic characteristics and clinically relevant covariates. Additionally, we assessed whether the systemic inflammatory response index, neutrophil-to-lymphocyte ratio, and C-reactive protein acted as mediators in this relationship. Sensitivity analyses included multiple logistic regression after propensity score matching of participants in the chronic CRS and nonCRS groups. Among the 45,122 participants, 2110 participants (4.68%) were diagnosed with chronic CRS. Higher RFM levels were associated with an increased prevalence of chronic CRS when modeled as a continuous variable (odds ratio [OR] = 1.08; 95% confidence interval (95% CI): 1.05-1.10). Smoothed curve fitting and threshold effect analysis confirmed this positive correlation, which was more significant at RFM > 28.39. Mediation analysis revealed that systemic inflammatory response index, neutrophil-to-lymphocyte ratio, and C-reactive protein mediated 10.88%, 7.26%, and 18.05% of the association between RFM and chronic CRS, respectively. Sensitivity analyses indicate that the association between RFM and chronic CRS remained significant postpropensity score matching adjustment. RFM was positively associated with the prevalence of chronic CRS. Inflammatory markers may play a potential mediating role in this relationship; however, due to the cross-sectional nature of this study, the mediation effect should be interpreted with caution.