The mystery of BMI in Sjögren’s disease: unravelling complex relationships between BMI and multiple laboratory parameters

The relationship between body mass index (BMI) and laboratory parameters in Sjögren's disease (SjD) remains unclear. We evaluated associations of BMI with selected serological and haematological parameters and explored its relationship with the EULAR Sjögren's Syndrome Disease Activity Index (ESSDAI). This single-centre retrospective cross-sectional study included 554 unique hospitalised patients with SjD. Multivariable linear regression models were adjusted for age, sex and disease duration. Effect estimates were expressed per 5 kg/m² higher BMI, and Benjamini-Hochberg false discovery rate correction was applied across all estimable laboratory outcomes. Generalised additive and segmented regression models examined nonlinear relationships. Natural-log-transformed CRP and TNF-α analyses and an ESSDAI score excluding both biological and haematological domains were used as sensitivity analyses. After adjustment for age, sex and disease duration, each 5 kg/m² higher BMI was associated with lower IgG and higher C3, HGB, WBC and RBC after false discovery rate correction. CRP showed a nominal inverse association (P = 0.038, q = 0.076). TNF-α showed no linear association, but U-shaped patterns were observed in raw and log-transformed analyses. BMI was weakly inversely correlated with ESSDAI (ρ=-0.160, P < 0.001) and very weakly with ESSDAI excluding the biological and haematological domains (ρ=-0.085, P = 0.044). BMI was associated with several laboratory parameters and weakly with ESSDAI in this hypothesis-generating cross-sectional study. The underlying mechanisms remain uncertain.