They should know because there are lipids/ fats and carbohydrates/ sugars that can damage the body. If you eat a burger from a fast food restaurant, it is most likely packed with bad fats and carbs, but very little protein.
People should know so they know which foods to eat to help them actually do the diet they want. If they don't even know how much carbs or fat is in their foods that they're eating, the diet might not really work. But it's okay to eat fast foods once in a while. Not too much though
