Nonstick vegetable oil spray
1 3/4 cups all purpose flour
2 1/4 teaspoons baking powder
1/2 teaspoon salt
1 cup sugar
3 tablespoons vegetable oil
2 tablespoons (1/4 stick) unsalted butter, room temperature
4 large egg whites
2 tablespoons fresh lime juice
1 tablespoon grated lime peel
1 teaspoon vanilla extract
2/3 cup low-fat buttermilk

Preheat oven to 350°F .Lightly spray 8 1/2 x 4 1/2 x 2-inch loaf pan with vegetable oil spray. Mix flour, baking powder and salt in medium bowl. Using electric mixer, beat sugar, vegetable oil and butter in large bowl until well blended. Add egg whites 1 at a time, beating to blend after each addition. Beat in lime juice, grated lime peel and vanilla extract. Add dry ingredients alternately with buttermilk, beginning and ending with dry ingredients and beating just until combined after each addition. Transfer cake batter to prepared pan.

Bake until top of cake is golden brown and tester inserted into center comes out clean, about 55 minutes. Transfer pan to rack. Cool cake in pan on rack 20 minutes. Turn cake out onto rack and cool completely. (Can be prepared 1 day ahead. Wrap tightly in plastic; store at room temperature.) Cut into slices.