It’s one of the best-established medical facts: Lowering your LDL (“bad”) cholesterol cuts your risk of a heart attack. How to do it? Replace bad fats (red meat, cheese, butter, coconut oil, fatty sweets, etc.) with good fats (oil, salad dressing, mayo, nuts, fish, avocado, etc.).

How do we know that saturated fats raise—and unsaturated fats lower—LDL (“bad”) cholesterol?

Early observational studies like the Seven Countries Study reported higher rates of heart disease in people who ate more sat fat.

“But what really made an impression were two parallel series of trials testing which fats raised or lowered blood cholesterol,” says Martijn Katan, a cardiovascular disease expert and emeritus professor of nutrition at Vrije Universiteit in Amsterdam.

“By 1965 at the latest, it was beyond a reasonable doubt that if you replace saturated fats with polyunsaturated fats, you get a substantial lowering of total cholesterol,” says Katan.

Then, in the early 1970s, researchers started to look at LDL cholesterol separately. By 2016, a World Health Organization report had looked at 91 trials.

“There wasn’t one single experiment,” says Katan. “There was a mass of well-organized experiments that all showed the same thing: If you replace saturated fats with polyunsaturated fats, LDL goes down.”

And is there any reason to think that lowering LDL might not protect the heart?

“Absolutely not,” says Katan. “The effect of LDL on heart disease risk is one of the best established facts in the whole of medical science.”

The data testing statins or other drugs that slash LDL is massive.

“The latest summary included 27 trials involving 174,000 patients,” says Katan. “That’s a staggering number.”

But there’s even more evidence that lower LDL means less heart disease. “There’s a whole bunch of genetic variants that raise or lower LDL, and they all raise or lower coronary risk,” says Katan.

“So the evidence is coming from all directions, and there’s really no way to explain it all, except by assuming that lowering LDL lowers the risk of coronary heart disease.”

It’s not just the cholesterol trials

Several randomized clinical trials from the 1950s, ’60s, and ’70s looked not just at LDL but also at heart attacks and strokes.

“If you look at the four highest-quality trials together, they provide direct evidence that replacing a diet high in saturated fat with a diet high in polyunsaturated fat prevents heart attacks and strokes,” says Frank Sacks, professor of cardiovascular disease prevention at the Harvard T.H. Chan School of Public Health.

(To learn more about what we can learn from different types of studies, click here.)

Photo:  Adapted from "The Facts on Fat," American Heart Association.