The humble sandwich is the saboteur of the American diet. Most Americans consume too much sodium, sugar and saturated fat, according to government survey data. Sandwiches—which almost half of Americans eat on any given day—are a primary culprit. Nutritionists, doctors and public-health officials are trying to nudge people to make their sandwiches healthier, believing that even simple changes can improve health. Sandwiches are the number one source of sodium and saturated fat in Americans’ diets, making up about one-fifth of our daily sodium intake and 19% of our daily saturated fat calories, according to an analysis of federal survey data....