Eating two slices of buttered toast a day can double the risk of diabetes, warns new research.
A study of more than 3,000 people found those who consumed just 12 grams (0.42 ounces) were twice as likely to develop the disease within the next five years.
Scientists say the finding underlines the importance of switching to a Mediterranean style diet.
It is rich in legumes, whole grain cereals, fruits, vegetables and nuts and low in animal based foods like red meat and pastries.
They added that increasing evidence is suggesting plant based diets benefit health and also have less impact on the environment.
Butter is rich in unhealthy saturated fatty acids and trans fats and has been linked to a high risk of suffering type 2 diabetes, the form linked with obesity.
So the international team of researchers evaluated the associations between the amount of fat, and the type, consumed by 3,349 people in the PREDIMED (Prevention With Mediterranean Diet) and their risk of diabetes.
At the start the participants, who were all Spanish, were free of diabetes but at high risk of heart disease or stroke.
After four and a half years 266 of them had diabetes and this was twice as likely among those who consumed higher amounts of saturated fatty acids and animal fat.
The consumption of whole fat yogurt was associated with a lower risk, reports the American Journal of Clinical Nutrition.
Dr Marta Guasch-Ferre, of Harvard University, said: "These findings emphasise the healthy benefits of a Mediterranean diet for preventing chronic diseases, particularly type 2 diabetes, and the importance of substituting saturated and animal fats, especially red and processed meat, for those found in vegetable sources such as olive oil and nuts."
Diabetes UK says as well as being protective against type 2 diabetes, Mediterranean diets rich in fruit, vegetables and fibre can help people with diabetes to control their blood sugar levels.
Previous large-scale studies have linked a Mediterranean diet with a lower chance of developing diabetes.
A traditional Mediterranean diet is principally composed of oily fish, poultry fresh fruit and vegetables, legumes, fresh bread, pasta and olive oil.