Skeletal muscle index together with body mass index is associated with secondary osteoporosis in patients with rheumatoid arthritis.

Objective: The objective of this study was to explore the associations of body mass index (BMI), fat mass index (FMI), skeletal mass index (SMI) and secondary osteoporosis (OP) in patients with rheumatoid arthritis (RA).

Methods: The bone mineral density (BMD) at sites of the femur neck (Neck), total hip (Hip) and lumbar vertebrae 1-4 (L1-4) was measured by dual-energy X-ray absorptiometry. The skeletal muscle index, body fat percentage and mineral content were measured by biological electrical impedance for calculating BMI, FMI and SMI.

Results: A total of 433 patient with RA and 158 healthy controls were enrolled. The BMDs at each site of the RA patients were lower compared with those of the healthy controls (p < 0.0001), and the prevalence of OP (36.1%, 160/443) and sarcopenia (65.2%, 288/443) in the RA patients were higher than those in the controls (12.7%, 20/158, p < 0.0001; 9.0%, 14/156, p < 0.0001). Significant differences in the BMD, FMI, SMI, mineral content, body fat percentage and skeletal muscle mass were found among the RA patients in the different BMI groups (p < 0.05). In RA patients with BMI < 18.5 kg/m, the prevalence of OP in the RA patients with sarcopenia was similar to that in those without sarcopenia (44.4% vs. 66. 7%, χ = 0. 574, p = 0.449). In the RA patients with a normal BMI or who were overweight or obese, prevalence of OP in the RA patients with sarcopenia was significantly higher than that in the RA patients without sarcopenia (42.8% vs. 21.7%, χ = 10.951, p = 0.001; 61.1% vs. 13.0%, χ = 26.270, p < 0.0001). In the RA patients without sarcopenia, the prevalence of OP in the RA patients in the different BMI groups was different (p = 0.039). In the RA patients with sarcopenia, there was no significant difference in the prevalence of OP among the RA patients in the different BMI groups (p = 0. 128). The linear correlation analysis showed that the SMI in RA patients was positively correlated with the BMD of each site measured and BMI and FMI (p < 0.0001). However, there was a negative linear correlation between SMI and disease duration (p = 0.048). The logistic regression analysis found that SMI (OR = 0.569, p = 0.002, 95% CI 0.399-0.810), BMI (OR = 0.884, p = 0.01, 95% CI 0.805-0.971) and gender (1 = female, 2 = male) (OR = 0.097, p < 0.0001, 95% CI 0.040-0.236) were protective factors for OP in RA, while age (OR = 1.098, p < 0.0001, 95% CI 1.071-1.125) was the risk factor.

Conclusion: BMI and SMI are associated with the occurrence of OP in RA patients, and both SMI and BMI are important protective factors for OP secondary to RA.