This process, called hydrogenation, changes the structure of the fat in some places along its carbon backbone. Thus the name trans fats. The process is done to improve the texture and keeping quality of oil-based fats. Trans fats also occur naturally, mainly in meat and dairy products. Overall, trans fats make up from 2 to 4 percent of the calories in our diet, compared to 12 percent for saturated fats and 30 percent for total fats.
Hydrogenated fats are nothing new.
In 1911, cottonseed oil was first hydrogenated in the United States to produce vegetable shortening as a replacement for lard. The manufacturing process became more established in the 1930s as margarine entered the scene. As diets high in saturated fat became more strongly associated with high blood cholesterol and elevated risk of heart disease, more people began switching to lower saturated-fat products like vegetable margarine. And, research has shown this was a good change. In the past 30 years since low saturated fat diets have been advocated, there has been steady progress in reducing the overall risk of developing heart disease among Americans.