Association between A body shape index and bone mineral density in middle-aged and elderly adults: a retrospective analysis of NHANES 2005-2018.

INTRODUCTION

Despite accumulating evidence on central obesity and osteoporosis, the role of a body shape index (ABSI), a nonlinear index quantifying body shape via body mass index (BMI), waist circumference (WC), and height, remains controversial and underexplored. Although recent meta-analyses suggest central obesity may modulate fracture risk bidirectionally, no research has comprehensively compared ABSI with traditional adiposity metrics, such as BMI, WC, and waist-to-height ratio (WHtR), to predict site-specific changes in bone mineral density (BMD) across anatomical regions.

METHODS

This study utilized National Health and Nutrition Examination Survey (NHANES) data from 2005 to 2018, involving 12,421 participants. ABSI was computed using the formula: ABSI = WC/(BMI²/³ × Height¹/²). BMD was assessed at four sites-the total femur (TF), femoral neck (FN), trochanter (TR), and intertrochanter (IN) regions-via dual-energy X-ray absorptiometry (DXA). The association between ABSI and BMD was analyzed via multiple regression models and a generalized additive model (GAM). To compare ABSI's predictive efficacy with conventional adiposity indices, regression analyses juxtaposed ABSI against BMI, WC, and WHtR in assessing correlations with site-specific BMD.

RESULTS

After full covariate adjustment, a significant negative association was observed between ABSI and BMD in four femoral regions (P< 0.01). Smoothed curve fitting revealed a significant nonlinear relationship and threshold effect between ABSI and BMD among middle-aged and older individuals. Additionally, an inverted J-shaped curve was observed between ABSI and BMD in all four femoral regions. Meanwhile, ABSI showed significant negative associations with BMD across all femoral sites (β = -0.27 to -0.31, p-trend< 0.000001), whereas BMI, WC, and WHtR exhibited positive correlations (WHtR showing the strongest effect: β = 0.41-0.69). This highlights ABSI's ability to detect central adiposity-related bone loss obscured by conventional obesity metrics.

CONCLUSION

ABSI's robust inverse associations with femoral BMD (β = -0.27 to -0.31), persisting across nonlinear threshold analyses, establish it as a novel biomarker of central adiposity-related skeletal fragility. Unlike conventional indices reflecting mechanical loading benefits (BMI β = 0.008-0.012; WC β = 0.003-0.005; WHtR β = 0.41-0.69), ABSI specifically captures visceral fat-driven metabolic disorder-a critical pathway for osteoporosis risk stratification in normal-weight and obese populations.