The association between body roundness index and lumbar marrow fat content in postmenopausal women

To investigate the relationship between body roundness index (BRI) and lumbar marrow proton density fat fraction (PDFF) in postmenopausal women, with emphasis on potential nonlinear associations.

In this retrospective cross-sectional study of 186 postmenopausal women, lumbar PDFF was quantified using T2-corrected magnetic resonance spectroscopy (L3 vertebra), and bone mineral density (BMD) was assessed via dual-energy X-ray absorptiometry. Multivariable-adjusted linear regression and nonlinear analyses (smooth curve fitting, two-segment linear models) were employed to evaluate BRI-PDFF relationships, adjusting for age, menopause duration, physical activity, and BMD.

Both BRI and lumbar BMD exhibited significant inverse correlations with PDFF (BRI: r = -0.513; BMD: r = -0.462; both P < 0.001). Higher BRI quartiles were associated with elevated BMD at total hip, femoral neck, and lumbar spine (P < 0.05, Q4 vs. Q1-Q3) and a progressive reduction in adjusted PDFF (P_trend < 0.05). Linear regression confirmed dose-dependent inverse associations between BRI (continuous/quartile) and PDFF (adjusted β = -1.870 to -11.098, P < 0.05 for Q3-Q4 vs. Q1; P_trend < 0.001). Nonlinear analysis identified a threshold at BRI = 2.58: below this point, BRI strongly predicted PDFF reduction (β = -26.048, P < 0.001), whereas the association attenuated above it (β = -1.086, P = 0.024).

Increased BRI independently correlates with reduced marrow adiposity in postmenopausal women, with a nonlinear threshold effect (BRI = 2.58) suggesting distinct biological mechanisms at lower versus higher adiposity levels. These findings highlight BRI as a potential biomarker for marrow fat dynamics and a target for metabolic bone disease interventions.