A famous old study that helped start the idea that eating fat causes heart disease picked countries in a way that might have skewed the results, and it didn’t properly count how much saturated fat people ate when they were fasting for religious reasons.

A famous old study that tried to link eating butter and meat to heart disease might have gotten it wrong because it picked countries in a biased way and asked people what they ate during times when they were fasting, which doesn’t show their normal eating habits.

People say this old study proved that eating fatty foods causes heart disease, but it only picked countries where both fat intake and heart disease were low—and ignored places where people ate lots of fat but still had low heart disease. That makes the conclusion unfair.

The Seven Countries Study selectively reported data from only seven of 22 analyzed countries to falsely imply a causal relationship between saturated fat intake and cardiovascular disease, omitting data from 15 countries showing no association.

Back in the 1960s, some people started saying that eating foods high in saturated fat (like butter and meat) causes heart disease, but they didn’t have strong proof from experiments—just observations and loud opinions.