In a large bowl, beat banana, yogurt, margarine, egg, egg white, Equal, and vanilla at medium speed until blended; beat at high speed 1 minute.
Add combined flour, baking powder, baking soda, and salt, mixing just until ingredients are moistened. Stir in walnuts, if using.
Pour batter into one 8 1/2 x4-inch greased and floured loaf pan or three 5 5/8 x3 1/4-inch loaf pans.
Bake until bread is golden and toothpick inserted in center comes out clean, 55 to 65 minutes for large loaf, 35 to 40 minutes for small loaves. Cool 10 minutes in pans on wire rack, remove from pan and cool completely.