Restricted cubic spline analyses identified linear dose–response relationships: ALT/AST ratio positively correlated with depression risk (P < 0.001), while lower levels of BUN (P < 0.001) and BUN/Cr ratio (P < 0.001) inversely associated with depression risk, with no significant heterogeneity across subgroups (interaction P > 0.05). The gene discussed is GPT; the disease is major depressive disorder.