For nearly a half century, America has been on a witch hunt to find the ingredient that is making us fat. In the 1980s, the culprit was fat itself. Next it was carbs. Today, sugar is the enemy—unless you’re caught up in the war on gluten. And none of it has worked. Obesity is now closing in on smoking as our No. 1 preventable cause of death. The U.S. has rarely failed at anything the way it has failed at weight loss.