Preheat oven to 350 degrees Fahrenheit.
Combine all-purpose flour, baking soda, salt, and baking powder. Mix well. Set aside.
Beat-in egg whites in a separate bowl. Gradually add the bananas, vegetable oil, sugar, lemon zest, and vanilla extract. Continue beating until all the ingredients are well distributed.
Gradually fold-in the flour mixture, and then the walnuts. Mix well.
Arrange muffin cups on a muffin tray. Scoop the mixture and arrange in each muffin cup. Make sure that you only fill about less than 3/4 of the cup's volume.
Place in the oven and bake for 25 minutes, or until it passes the toothpick test.
Remove from the oven and let cool by placing on a wire rack.
Serve. Share and enjoy!