You know why these don't get done? Because that means that you're grabbing a large cohort of people, fixing their diets, and following them until a good number

You know why these don't get done? Because that means that you're grabbing a large cohort of people, fixing their diets, and following them until a good number of them die. The timescales required here, not to mention the cost, means that such studies are virtually unfeasible.