Apolipoprotein A-I (OR 0.990, 95% CI 981–0.999) and glutamine (OR 0.985, 95% CI 0.972–0.997) had protective causal effects on depression, whereas acetoacetate (OR 1.021, 95% CI 1.009–1.034), glycoproteins (OR 1.005, 95% CI 1.000–1.009), isoleucine (OR 1.013, 95% CI 1.002–1.024), and urea (OR 1.020, 95% CI 1.000–1.039) had an anti-protective effect on depression. The gene discussed is APOA1; the disease is major depressive disorder.