The non-linear relationships between fat mass and lean body mass with arthritis

Abstract Introduction Body composition has been associated with various health outcomes, but its specific relationship with arthritis risk remains unclear. The study aimed to examine the associations between lean body mass (LBM) and fat mass (FM) with arthritis risk in men and women and to identify their threshold values. Methods The data were obtained from the CHARLS, a prospective cohort study from 2011 to 2018. Multivariate Cox regression models evaluated the associations between LBM and FM and arthritis risk. Smoothing curves and two-piece linear regression models were applied to identify the inflection points of LBM and FM associated with arthritis risk. Results A total of 6,761 participants were included in this study. During a mean follow-up period of 6.66 years, 944 participants (13.96%) developed new-onset arthritis, with an incidence rate of 20.72 per 1,000 person-years. Multivariate Cox regression analysis demonstrated a significant linear association between FM and the risk of new-onset arthritis in men. Individuals in the highest FM quartile (Q4) had the highest risk of developing arthritis (HR = 1.25, 95% CI: 1.03–1.51). Two-piece linear regression models revealed nonlinear relationships between LBM, FM, and arthritis risk. Specifically, in men, LBM was negatively associated with arthritis risk when it was below 43.79 kg (HR = 0.97, 95% CI: 0.95–0.99), but this association was no longer significant above this threshold (HR = 1.01, 95% CI: 0.98–1.03). In women, arthritis risk significantly decreased when LBM exceeded 39.04 kg (HR = 0.92, 95% CI: 0.87–0.96). Additionally, in women, FM exhibited a U-shaped relationship with arthritis risk, with the lowest risk observed at an FM level of 17.16 kg. Conclusions Among Chinese adults aged 45 and older, maintaining appropriate levels of LBM and FM may help reduce arthritis risk. Based on the nonlinear findings, it is recommended to maintain LBM below 43.79 kg for men, above 39.04 kg for women, and to keep FM at approximately 17.16 kg for women, which may be appropriate.