During WWII, Keys studied starvation, and he noted that in Europe where people suffered from food rationing, during the war, that one result of the lesser diet was a reduction in the incidence of heart disease.
The leap from diet to cardiovascular disease involves at least two steps. First, that an increase in fats in the diet, particularly saturated fats, increases total cholesterol in the blood. Second, that the increased total cholesterol causes the accumulation of hardened fatty deposits in blood vessels called atherosclerosis.
In two studies based on published national data, Keys believed that he could show this direct relationship between diet and cardiovascular disease. (Look up: "Seven Country Study") From this arose the idea of a Mediterranean diet, and research that showed that if saturated fats were replaced with vegetable oils, total cholesterol was lowered.
There was no controlled scientific testing to prove this. Ancel Keys was very articulate, and his theory seemed to make sense, and in a short time it was widely accepted. Sadly, this led to the misdirection of human dietary research for the next 40 years. Ancel Keys has been proven wrong. Dietary control of cholesterol is a failure because your body creates it's own cholesterol. It's now believed that atherosclerosis is caused by inflammation, and that the cholesterol in those deposits is the body's attempt to repair the damage.
When the Framingham Heart Research Study finally showed that total cholesterol was not a good marker for future risk of atherosclerosis, the idea that cholesterol was bad didn't die. Framingham suggested that HDL-Cholesterol was protective, and that LDL-Cholesterol might or might not be harmful. Hence, the idea of good cholesterol and bad cholesterol, became popular. If LDL-Cholesterol is "bad," it's suggested that the small dense particles cause the problem.
So for years we've been trying to eat a low-fat high-carbohydrate diet, based on Ancel Key's idea that saturated fats, caused high cholesterol which in turn caused heart disease. We now KNOW that this theory is wrong.
To correct the problems of the old, but respected Framingham Heart Study, The Women's Health Initiative was established, the "Rolls Royce" of diet-heart studies. It didn't last even 10 years. It was clear from the improved data collection, that the "healthy diet" in fact had no health benefits. This astounding result, really confused the researchers. The study was closed in 1998-1999, and the findings were not released until 2006. No matter how they massaged the numbers, they never produced the expected result.