Preheat the oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan.
In a large bowl, whisk together the sourdough starter, vegetable oil, granulated sugar, eggs, and vanilla extract until smooth.
In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, salt, ground cinnamon, and ground nutmeg.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined.
Fold in the grated zucchini until evenly distributed throughout the batter.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake for 55 to 60 minutes, or until a toothpick inserted into the center comes out clean.
Remove the bread from the oven and allow it to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing.