The study is the most detailed examination to date on how dietary fats impact health. Different types of dietary fat have different associations with mortality, the researchers found. Trans fats, found in margarine, takeaway and fast foods, had the most significant adverse impact on health. Every percent higher intake of trans fat was associated with a 16 percent higher chance of premature death during the study period. Higher consumption of saturated fats was also linked with greater mortality risk. 

Conversely, high amounts of unsaturated fats found in fish and soy and canola oils, were associated with lower risk of premature death.

People who replaced saturated fats with unsaturated fats—especially polyunsaturated fats—had significantly lower risk of death overall, as well as lower risk of death from heart disease, cancer, neurodegenerative disease, and respiratory disease. 

People who replaced saturated fats with carbohydrates had only slightly lower mortality risk. This was because carbohydrates in the American diet tend to be primarily refined starch and sugar, which have a similar influence on mortality risk as saturated fats.

“Our study shows the importance of eliminating trans fat and replacing saturated fat with unsaturated fats, including both omega-6 and omega-3 polyunsaturated fatty acids. In practice, this can be achieved by replacing animal fats with a variety of liquid vegetable oils,” said senior author Professor Frank Hu.