Protein is the hot ingredient of the moment in processed foods, joining the terms “organic” and “low-fat” as a major selling point to pitch to health-conscious shoppers. While carbohydrates and some fats are still widely regarded as suspect, protein is being proudly added to products throughout the grocery store, especially the snack aisle, which has made way for a new star: the protein bar. But a closer look shows that its blessings are mixed.