Trans fats (also known as trans fatty acids) are formed when hydrogen atoms are added to liquid oils (in a process called "partial hydrogenation") to form more solid fats like shortening and hard margarine.
Partially hydrogenated vegetable oils work better in certain food applications (e.g., frying, baking, cooking) because they are more solid than oils. Trans fats can most often be found in fried foods and baked goods such as doughnuts, cakes, pie crusts, crackers and stick margarines and other spreads. Trans fats are easy and inexpensive to use and give foods the desired texture and taste people desire. A small amount of trans fat also occurs naturally in foods such as beef, veal, lamb and foods containing milk fat, such as butter, whole milk, cream, cheese and ice cream. There have not been sufficient studies to determine whether naturally occurring trans fats have the same bad effects on cholesterol levels as trans fats from food processing.
Before 1990 very little was known about trans fats' impact on health. In the 1990s, research suggested that unlike other fats, trans fats raise LDL ("bad") cholesterol levels and lower HDL ("good") cholesterol levels. High LDL cholesterol levels in combination with low HDL cholesterol levels increase the risk of heart disease. In June 2015 the FDA determined that Partially Hydrogenated Oils (PHO’s and a source of trans fats) were no longer generally recognized as safe and were required to be removed from food processing by June of 2018.
The 2015-2020 edition of the Dietary Guidelines for Americans recommends keeping the amount of trans fat consumed as low as possible, especially by limiting foods that contain synthetic sources of trans fats, such as partially hydrogenated oils, and by limiting other solid fats. Similarly, the American Heart Association recommends limiting intake of trans fat.
You can determine the amount of trans fats in a particular packaged food by looking at the Nutrition Facts Panel. You can also spot trans fats by reading the ingredient line and looking for the ingredients referred to as “partially hydrogenated oils.” It is important to note that trans fat will appear on a label where they occur naturally in small amounts such as in meat and dairy.