Step 1Heat the oven to 350 degrees. Grease and flour a 12-cup bundt pan and set aside.
Step 2Sift together the all-purpose and whole-wheat flours, the baking soda, nutmeg and cinnamon. Set aside.
Step 3Beat together the oil, eggs, brown and granulated sugars and vanilla until creamy. Add the flour mixture and mix well. Stir in the grated zucchini and nuts and pour into the pan.
Step 4Bake the bread until a toothpick inserted in the center comes out clean, 50 to 60 minutes. Let cool in the pan 10 to 15 minutes, then run a knife around the edges of the pan to loosen the bread. Turn it onto a rack. Dust the top with a little powdered sugar passed through a strainer, if desired. Serve warm.