Association between relative fat mass and rheumatoid arthritis: A cross-sectional study.

Obesity has been recognized as a significant contributor to the development of rheumatoid arthritis (RA). Relative fat mass (RFM), a newly proposed anthropometric index, offers an alternative method for estimating body fat percentage. This cross-sectional study aims to elucidate the potential association between RFM and RA risk. Data were obtained from the National Health and Nutrition Examination Survey 1999 to 2018. Weighted multivariable logistic regression was used to assess the independent association between RFM and RA, and subgroup interaction analyses were performed to evaluate potential effect modification. Restricted cubic spline (RCS) models were applied to explore potential nonlinear relationships. Receiver operating characteristic (ROC) curve analysis was conducted to compare the discriminatory ability of RFM and body mass index (BMI). This cross-sectional study demonstrates a positive association between RFM and RA. Although RFM offers some improvement over BMI as an obesity-related indicator, its clinical utility for identifying RA risk remains limited. A total of 43,499 participants were included in the analysis, among whom 2557 had RA. Weighted multivariable logistic regression revealed a significant positive association between RFM and RA (odds ratio = 1.048; 95% confidence interval: 1.036-1.060). This association remained consistent when RFM was analyzed in quartiles, with individuals in the highest quartile showing a significantly higher risk of RA compared to the lowest quartile (odds ratio = 2.487; 95% confidence interval: 1.875-3.299). Subgroup interaction analyses revealed that this association was modified by age, educational level and smoking status. RCS analysis demonstrated a linear relationship between RFM and RA. RFM showed slightly better discrimination than BMI in ROC analysis, but the overall discriminatory performance remained modest (area under the curve = 0.614 vs 0.581).