Association between fat-to-muscle ratio and secondary osteoporosis in rheumatoid arthritis: a cross-sectional study at a tertiary hospital in China.

objectivesTo investigate the correlation between fat-to-muscle ratio (FMR) or other body composition and secondary osteoporosis (OP) in patients with rheumatoid arthritis (RA) and to develop a predictive model using FMR and related clinical factors.

designCross-sectional observational study with machine learning-based risk modelling.

settingTertiary hospital in eastern China, secondary care level.

participantsA total of 670 hospitalised RA patients (135 males and 535 females; aged 58.00 (50.00-67.00) years; disease duration 8.00 (2.00-16.00) years) and 126 healthy controls were recruited between October 2019 and October 2022. There were no differences in basic indicators such as gender, age distribution and body mass index between the two groups. RA diagnosis followed American College of Rheumatology (ACR) 1987 or ACR/European League Against Rheumatism 2010 criteria. Exclusion criteria included major organ dysfunction, endocrine disease, infection or long-term hormone or psychotropic drug use. PRIMARY AND SECONDARY OUTCOME MEASURES: Primary outcomes included total skeletal muscle mass, fat mass, FMR measured by bioelectrical impedance analysis and bone mineral density measured by dual-energy X-ray absorptiometry. Secondary outcomes included RA disease activity scores (clinical disease activity index (CDAI), simplified disease activity index, disease activity score in 28 joints (DAS28)) and glucocorticoid use. Logistic regression and four additional machine learning algorithms were used to build predictive models for OP.

resultsThe RA group (age, 58.00; duration, 8.00; DAS28, 5.03; rheumatoid factor, 104.75; C-reactive protein, 25.65; erythrocyte sedimentation rate (ESR), 59.00) exhibited reduced total skeletal muscle mass (19.49 vs 25.38, p<0.001), hip bone mineral density (0.90 vs 1.15, p<0.001) and L1-4 bone mineral density (0.86 vs 1.08, p<0.001), alongside increased total fat mass (18.33 vs 16.37, p=0.020) and FMR (0.98 vs 0.68, p<0.001). Total fat mass was positively correlated with simplified and CDAI (p<0.001). Total skeletal muscle mass was negatively correlated with ESR (p=0.001) and positively correlated with both L1-4 and hip bone mineral density (p<0.001). FMR showed a positive correlation with clinical disease activity index (p<0.001). There were significant differences in total fat mass and FMR among RA patients with varying disease activity levels (p<0.001). RA patients with concomitant OP or using glucocorticoids had a higher total fat mass and FMR than their respective control groups, with only total skeletal muscle mass levels being lower (p<0.01). We developed predictive models using multiple machine learning algorithms, which identified that both age and FMR were key factors associated with secondary OP in RA patients. Subgroup analysis identified an interaction effect between FMR and gender and restricted cubic spline fitted the dose-response relationship between FMR and OP.

conclusionFMR may serve as a useful clinical indicator of secondary OP in RA patients. A model based on FMR and associated risk factors can predict the possibility of secondary OP.