GDNF and diabetes mellitus: These findings were upheld in multivariate regression, where BMI (β = 0.176, p = 0.023), diabetes (β = 0.5456, p = 0.012), smoking (β = 0.312, p = 0.038), and alcohol use (β = 0.421, p < 0.0001) emerged as independent predictors of reduced GDNF protein content (Table 4).