Scientists said that people have been advised to reduce our intake of saturated animal fats, and eat more of the polyunsaturated vegetable fats found in margarine.
However, now US scientists claim to have turned that conventional wisdom on its head, with a new analysis of a study carried out between 1966 and 1973.
The study followed 458 men aged 30 to 59 in Sydney who had recently had a heart attack or suffered from angina, the 'Daily Mail' reported.
As many as half were advised to cut their animal fat consumption and replace it with safflower oil - similar to sunflower oil - and safflower oil margarine.
Results showed that those who ate more of these products were almost twice as likely to die from all causes, including heart disease.
Most studies of dietary interventions have involved multiple changes, but the Sydney study looked solely at omega 6, the most prevalent polyunsaturated fat in most Western diets, also known as linoleic acid.
Researchers said it is found in large quantities in vegetable oils such as corn, sunflower, safflower and soybean and in margarines made from these oils.
It is converted into a chemical called arachidonic acid in the body, which can trigger the release of other chemicals leading to inflammation - a leading cause of a host of chronic diseases, including heart disease.
The researchers, from the National Institutes of Health in the US, say their findings could have "important implications for worldwide dietary recommendations".
However, other scientists have criticised the results, saying they did not provide enough evidence to suggest people should change their diets.
Professor Tom Sanders, of King's College London, said the study was "enormously underpowered", of "little relevance to diets today" and its findings had been refuted by recent better studies.
Vegetable oils and margarine are supposed to help lower cholesterol and blood pressure and increase weight loss and improve overall health.
But they are some of the most chemically altered foods in our diets, and critics say they should not be promoted as healthy.