Weigh or lightly spoon flour into dry measuring cups; level with a knife.
Combine flour, sugar, baking soda, and salt in a large bowl; make a well in center of mixture.
Combine buttermilk, butter, and egg whites, stirring with a whisk.
Add to flour mixture, stirring just until moist.
Spread batter into an 8 x 4inch loaf pan coated with cooking spray.
Bake at 350 for 45 minutes or until a wooden pick inserted in center comes out clean. Cool 10 minutes in pan on a wire rack; remove from pan. Cool completely on wire rack.
