Cross-sectional associations of low body mass index and being underweight with joint damage in Chinese patients with rheumatoid arthritis.

Objective: To characterize BMI in Chinese patients with RA US patients and examine its association with joint damage in Chinese patients.

Methods: Each of the 1318 patients from a real-world Chinese RA population was first stratified by gender and then individually age-matched with one American RA patient from the US National Health and Nutritional Examination Survey 1999-2018. Data on BMI, bilateral hand radiographs and risk factors at enrolment were collected but radiographs were unavailable for the American patients. Logistic regression was used to evaluate the association of BMI with radiographic joint damage (RJD) in Chinese patients.

Results: Chinese patients had a significantly lower BMI [(weighted) median 21.8 29.8 kg/m;  < 0.001] and a higher prevalence of being underweight (15.2% 1.1%;  < 0.05) than their American counterparts. Underweight Chinese patients (BMI <18.5) had higher modified total Sharp scores (median 17 10) and joint space narrowing (JSN) subscores (median 6 2) (both  < 0.05) than normal-weight patients (BMI ≥18.5-<24). After controlling for confounding, continuous BMI was cross-sectionally negatively associated with RJD [adjusted prevalence odds ratio (OR) 0.90 (95% CI 0.85, 0.96)] and JSN [adjusted prevalence OR 0.92 (95% CI 0.87, 0.96)]; being underweight normal weight was associated with RJD [adjusted prevalence OR 2.14 (95% CI 1.37, 3.35)] and JSN [adjusted prevalence OR 1.77 (95% CI 1.10, 2.84)].

Conclusion: Low BMI and being underweight were cross-sectionally associated with joint damage in Chinese RA patients, especially JSN, suggesting the clinical importance of identifying underweight patients and focusing on weight gain to prevent joint damage.