Association between red blood cell distribution width-to-albumin ratio and lumbar spine bone mineral density: a cross-sectional analysis among US adults, 2015–2018

Background Red blood cell distribution width (RDW) and albumin levels are associated with bone metabolism. However, the relationship between the ratio of the two (RAR) and lumbar spine bone mineral density (BMD) remains unclear. This study aims to explore the association between RAR and lumbar spine BMD and the potential nonlinear relationship. Methods Multivariate logistic regression, restricted cubic spline (RCS) regression, receiver operating characteristic (ROC) analysis, and sensitivity analyses were used to examine the relationship between RAR and lumbar spine BMD based on NHANES data from 2015–2018. The study also used subgroup analyses and interaction tests to explore whether the relationship was stable across populations. Results Elevated RAR is significantly associated with reduced lumbar spine BMD (fully adjusted model β = -0.309, 95% CI: -0.327 to -0.291, P < 0.001). RCS analysis revealed an L-shaped nonlinear association between the two (P for nonlinearity < 0.001), with an inflection point at RAR = 4.25. Below the inflection point, RAR was negatively correlated with BMD (β = -0.410, P < 0.001), while above the inflection point, it was positively correlated (β = 0.438, P < 0.001). Trend analysis showed that increasing RAR quartiles were associated with decreasing BMD (Q4 vs. Q1: β = -0.239, P < 0.001; trend P = 0.024). Subgroup analysis showed consistent results across subgroups of gender, age, and race (interaction P > 0.05), but there were modifying effects in subgroups of education level, BMI, sleep duration, PIR, and hypertension (interaction P < 0.05). Conclusions Elevated RAR is an independent risk factor for reduced lumbar spine BMD, with a threshold effect of 4.25. RAR may serve as a potential biomarker for assessing bone health. Clinical trial number: Not applicable.