To find the reasons for the discrepancy, Keys launched what would become the Seven Countries Study. He and his team of researchers dispatched themselves to select destinations in Europe, Japan, and North America. They asked men in those locales what they were eating, measured serum cholesterol in their blood, and noted rates of disease.
Causation or correlation?
Keys and company found that men in Italy and Greece had lower cholesterol, seemed to drop dead less often, and said they ate less red meat than their American counterparts. Keys extrapolated his anti-meat message from there, and his crusade landed him on the cover of TIME.
Despite the great publicity, Keys had found an interesting fact pattern but nothing more. Even amateur scientists know that correlation is not causation.
Men in Italy and Greece had less coronary heart disease than American men -- they also had less use for the English language. Doesn’t mean speaking English causes heart disease. In most scientific experiments, observational studies lead to hypotheses, along the lines of “meat may cause coronary heart disease,” and then the real work begins. To test a hypothesis, researchers design a randomized, controlled study and complete it. And this is where the advice on meat falters.