Preheat the oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan or line it with parchment paper.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, whisk the granulated sugar and vegetable oil until combined.
Add the eggs one at a time to the sugar and oil mixture, whisking well after each addition.
Stir in the vanilla extract, lemon zest, and fresh lemon juice until evenly mixed.
Add the grated zucchini to the wet ingredients and stir to combine.
Alternately add the dry flour mixture and buttermilk to the wet ingredients, starting and ending with the flour mixture. Stir gently until just combined; do not overmix.
Pour the batter into the prepared loaf pan and smooth the top with a spatula.
Bake in the preheated oven for 50 to 55 minutes, or until a toothpick inserted into the center comes out clean.
Remove the loaf from the oven and let it cool in the pan for 10 minutes.
Transfer the loaf to a wire rack and let it cool completely before slicing.