1. Preheat oven to 350°.
2. 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.
3. Spread batter into an 8 x 4–inch 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.