Hemoglobin-to-Red cell distribution width ratio and asthma risk: cross-population validation and vitamin D mediation analysis.

Subgroup analyses confirmed the robustness of the HRR–asthma association in older adults (OR = 0.15, 95%CI: 0.03–0.85, P = 0.032), while the association in younger adults did not reach statistical significance (all P > 0.05) ( Supplementary Table S5 ). 5.5 Mediating role of vitamin D (NHANES cohort) Mediation analysis showed vitamin D fully mediated the HRR-asthma association: Total effect of HRR on asthma: β =−0.015( P = 0.44); Indirect effect via vitamin D: β =0.011( P = 0.001); Direct effect: β =−0.02

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