Overview: Kellogg's, the world's largest cereal maker, has seen its biggest drop in sales since the 1970s. Food companies are selling off their struggling bread divisions. It's all because best-selling health evangelists say that wheat is causing everything from fat bellies to schizophrenia. But do they have science on their side? Mark Kelley takes a hard look at what's driving a movement that is dramatically changing the way we eat.