How much whole wheat flour do we have to use in a bread formulation to label a loaf "whole wheat bread"?

The Food and Drug Administration (FDA) has established a standard. The total flour used in the dough must be 100 percent whole wheat flour in order to label a loaf "whole wheat bread".
Tenbergen, Klaus. 2004July. The Workbench. Modern Baking 18: 10..

Updated: Wednesday, May 23, 2012.