So the United States Food Administration embarked on a campaign to get Americans to voluntarily give up some of their favorite foods - including white bread made from wheat flour. By the 1910s white bread was ingrained (no pun intended) in the American diet and culture. It held onto its associations with wealth and refinement long after white flour became affordable and abundant. In addition, the conventional wisdom of nutrition science at the time elevated carbohydrates as a valuable source of energy. Which meant that both white bread and refined white sugar were considered healthful and important sources of the newly-discovered calories. Getting people to give up their favorite breakfast, side dish, and anytime (including midnight) snack was not going to be easy.
This pair of propaganda posters produced by the USFA illustrate the same primary point - that if everyone gave up a little, the compound effect would be enormous. You'll note they don't focus on getting Americans to stop eating white bread - just to consume less. The implication of both posters is that be reducing consumption by as little as one slice a day would really add up.
Despite these challenges, the fact that the United States was able to feed their military, and the Allies, on the same 1916 wheat harvest suggests that Americans did reduce their wheat consumption in 1917.
Today we know that refined white flour is a little too efficient a carbohydrate, and that the vitamins and minerals in whole grain flour, and the added fiber, are generally much better for human health. For wheatless recipes from the First World War, check out this article from North Carolina State University Libraries.