I’ve written many times that losing weight is the second hardest thing I ask my patients to do. (Breaking an addiction like smoking or alcoholism is the hardest.) The frustrating thing is how little we know about how to lose weight successfully. But we are learning more all the time about why losing weight is so difficult. Much about dieting and weight loss is poorly understood, but let’s first lay out some facts that are well established. Weight loss and weight gain are caused by an i...