Preheat your oven to 350°F (175°C) and prepare your loaf pan with grease and flour.

In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and ground cinnamon until well blended.

In a large bowl, beat together the vegetable oil, granulated sugar, and brown sugar until smooth, then add eggs one at a time and mix thoroughly before adding vanilla extract.

Gently fold in the grated zucchini until evenly distributed.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined, and fold in optional nuts if using.

In a small bowl, mix the sugar and cinnamon for the swirl.

Pour half of the batter into the prepared pan and sprinkle with half of the cinnamon-sugar mixture. Top with remaining batter and finish with the remaining cinnamon sugar.

Gently swirl the batter and cinnamon sugar together to create a marble effect.

Bake for 55-65 minutes until golden brown and a toothpick comes out clean.

Let the zucchini bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.