Ancel Keys was a US scientist, who mainly worked and lived in Minnesota and managed to become a 100 years old (respect!). He reached early international fame by doing the Minnesota Starvation Study to aid the populations on the globe hit by famine after WW II. Later on, he did the Seven Countries Study, for which study he however had data from 20 nations/populations, but only used the seven, which seemed to support his hypothesis.
From Wikipedia: After observing in southern Italy the highest concentration of centenarians in the world, Keys hypothesized that a Mediterranean-style diet low in animal fat protected against heart disease and that a diet high in animal fats led to heart disease. The results of what later became known as the Seven Countries Study appeared to show that serum cholesterol was strongly related to coronary heart disease mortality both at the population and at the individual level. As a result, in 1956 representatives of the American Heart Association appeared on television to inform people that a diet, which included large amounts of butter, lard, eggs, and beef would lead to coronary heart disease. This resulted in the American government (George McGovern) recommending that people adopt a low-fat diet in order to prevent heart disease.
Keys had concluded that saturated fats as found in milk and meat have adverse effects, while unsaturated fats found in vegetable oils had beneficial effects. This message was obscured for a 20-year period starting around 1985, when all dietary fats were considered unhealthy. This was driven largely by the hypothesis that all dietary fats cause obesity and cancer.