One part of the world Ancel Keys used in his study was eastern Finland. High fat diets, high levels of heart disease.

However, that is the same area of Finland that back then also used to have high incidence levels of goiter. Due to the fact that Finnish soil, especially on the eastern parts of the country, has hardly any iodine in it, so back when Keys did his study people in that area were mostly chronically deficient. And guess what, iodine deficiency is also connected to cardiovascular disease.