Preheat oven to 180 degrees C. Lightly spray two 20 x 10 x 5cm bread tins with a non-stick cooking spray.
Combine the fat free sour cream, egg whites, vanilla, bananas and margarine then mix on medium speed of electric mixer until smooth, creamy and well blended.
Sift the flour, baking powder, bicarbonate of soda, salt and light brown sugar into the banana mixture. Stir with a spoon until combined. Add more flour if necessary until a thick and rather resistant dough is formed. Fold in the optional nuts and sultanas. Mix for 1 minute on the low speed of an electric mixer. Divide dough evenly between the two bread tins.
Bake at 180 degrees C until golden and the centre tests done. Remove breads from tins immediately and allow to cool on a rack before slicing.