For decades fad diets, professional nutritionists, and the USDA’s food pyramid have issued warnings about the fat in our diets. With greater fat intake, says the common wisdom, comes increased risk of heart disease and even some cancers. That advice is reflected on countless labels boasting fat-free ingredients, low-fat dietary alternatives, and even medical opinion that suggests only a certain percentage of one’s diet should come from fat calories.

Fats were not always dietary boogeymen, though, and over the past few years, nutritional science has slowly started to exonerate certain types of fats from having specific dangers. According to Walter Willett, chair of the Department of Nutrition at Harvard School of Public Health, dietary studies in the 1950s and 1960s indicated that some fats actually lowered blood cholesterol. As the rate of heart disease across the country began to increase in the 1970s, though, nutritionists and scientists—afraid that differentiating between good and bad fats would be too complicated for the general public—suggested that Americans decrease overall fat consumption. Less fat is better, people were told, regardless of whether these fats were dangerous trans fats or polyunsaturated fats, which can be beneficial. Public health campaigns like Project LEAN—a marketing plan launched in the late 1980s—were designed to help spread this new dietary advice. LEAN’s agenda was to get Americans to limit calories from fat to 30 percent of their diet. “You can actually have a healthy diet that is quite high in fat,” Willet says—as long as it’s the right kind.

As nutritionists steered Americans away from fat, low-fat and fat-free foods became increasingly popular. But the replacement for fat tends to be refined carbohydrates, which can be worse than a moderate amount of dairy fat.

Doctors at the Harvard School of Public Health believe that in the near future the USDA will stop recommending a limit on the number of calories from fat one should consume. One study that Willet has been following—ongoing since the 1980s— has monitored roughly 300,000 participants and helped further demonstrate that all fat is not the dietary enemy, only certain kinds of it.

Despite a long history of contrary advice, polyunsaturated fats like those found in olive oil, avocados, fish, and some nuts actually lower blood cholesterol and help protect against certain forms of cancer and heart disease. With a greater understanding of fat types among the general population, low-fat salad dressings packed with refined carbohydrates may soon be a thing of the past.

“There is no good evidence that fats are harmful,” says Willett. “Even dairy fat like cheese is fine in modest amounts. It’s the 2,000-calorie cheese pizzas that are terrible.” Willett says there are even high-fat diets that help decrease risk of heart disease. A 2008 study in Italy of 1.5 million people offered evidence that a Mediterranean diet rich in fatty foods like olive oil, legumes, and nuts significantly lowered the mortality rates of participants during the study.

Decades of PR telling us that all fats are bad—along with the fact that “fat” is also the word for our least favorite part of ourselves—may make the story of “healthy fat” difficult to believe, but our triglyceride and cholesterol levels have been telling that story all along.

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