Trans fats are a type of unsaturated fat.  They can be found naturally in meats and dairy products from ruminants (hooved animals that digest food first, by eating the raw material and regurgitating a semi-digested form known as cud, then eating the cud). In the US, we commonly consume foods from cattle, sheep, and deer, which are ruminants. The amount of trans fat in these foods is only about 2-5% of total fat and considered to be relatively small.

Most of the trans fat that people in the US eat comes from processed foods that contain partially hydrogenated vegetable oils.  Hydrogenation of oils changes the molecular structure so that they have a better flavor and longer shelf-life. These properties are beneficial for mass production and distribution of foods.

Trans fats are not required by humans and have no health benefit.  In fact, they have been deemed harmful to our health. Although they are beneficial for food production, they are not beneficial for our health. Trans fats, not only increase LDL, they also decrease HDL (“good cholesterol”).

According to a report published by FDA,

“Scientific evidence shows that consumption of saturated fat, trans fat, and dietary cholesterol raises low-density lipoprotein (LDL), or "bad cholesterol," levels, which increases the risk of coronary heart disease (CHD).”

A law was enacted on July 11, 2003 requiring food manufacturers to list the amount of trans fat in all their products by January 2006.  It is now listed on the Nutrition Facts panel directly under the line for saturated fat. Consumers now have access to the trans fat content of all foods required to include the Nutrition Facts panel.

Because trans fats are so common in the foods available to Americans, it would be nearly impossible to completely eliminate them from our diets.  The Institute of Medicine/National Academies of Science (IOM/NAS) recommend that “consumption of trans fats be as low as possible while consuming a nutritionally adequate diet.”

It is important for consumers to become aware of the highest sources of trans fats and use food labels to choose foods that contain as little as possible.  The FDA makes the following recommendations:

• Check the Nutrition Facts panel to compare foods because the serving sizes are generally consistent in similar types of foods. Choose foods lower in saturated fat, trans fat, and cholesterol. For saturated fat and cholesterol, use the Quick Guide to %DV: 5%DV or less is low and 20%DV or more is high. (Remember, there is no %DV for trans fat.)
• Choose Alternative Fats. Replace saturated and trans fats in your diet with mono- and polyunsaturated fats. These fats do not raise LDL (or "bad") cholesterol levels and have health benefits when eaten in moderation. Sources of monounsaturated fats include olive and canola oils. Sources of polyunsaturated fats include soybean, corn, sunflower oils, and foods like nuts.
• Choose vegetable oils (except coconut and palm kernel oils) and soft margarines (liquid, tub, or spray) more often because the combined amount of saturated and trans fats is lower than the amount in solid shortenings, hard margarines, and animal fats, including butter.
• Consider Fish. Most fish are lower in saturated fat than meat. Some fish, such as mackerel, sardines, and salmon, contain omega-3 fatty acids that are being studied to determine if they offer protection against heart disease.
• Limit foods high in cholesterol such as liver and other organ meats, egg yolks, and full-fat dairy products, like whole milk.
• Choose foods low in saturated fat such as fat free or 1% dairy products, lean meats, fish, skinless poultry, whole grain foods, and fruit and vegetables.

Sources:

Wikipedia. Trans Fats.

FDA. Revealing Trans Fats. FDA Consumer magazine. September-October 2003 Issue. Pub No. FDA05-1329C.

CFSAN Office of Nutritional Products, Labeling and Dietary Supplements. Questions and Answers about Trans Fat Nutrition Labeling. July 9, 2003; Updated March 3, 2004, June 25, 2004, August 1, 2005, September 6, 2005, and January 1, 2006.