Trans fat is the common name for a type of unsaturated fat with trans- isomer fatty acid(s). Trans fats may be monounsaturated or polyunsaturated.

Chemically, trans fats are made of the same building blocks as non-trans fats, but have a different arrangement. In trans fatty acid molecules, the hydrogen atoms bonded to pairs of doubly bonded carbon atoms (characteristic of all unsaturated fats) are in the trans rather than the cis arrangement. This results in a straight, rather than kinked, shape for the carbon chain, more like the straight chain of a fully saturated fat.

How They Are Created

Most trans fats consumed today are industrially created by partially hydrogenating plant oils — a process developed in the early 1900s and first commercialized as Crisco in 1911. The goal of partial hydrogenation is to add hydrogen atoms to unsaturated fats, making them more saturated. These more saturated fats have a higher melting point, which makes them attractive for baking and extends their shelf-life. Another particular class of trans fats, vaccenic acid, occurs naturally in trace amounts in meat and dairy products from ruminants.

Where They Are Found 

Trans fat can be found in vegetable shortenings, some margarines, crackers, cookies, snack foods, and other foods made with or fried in partially hydrogenated oils. Unlike other fats, the majority of trans fat is formed when food manufacturers turn liquid oils into solid fats like shortening and hard margarine. A small amount of trans fat is found naturally, primarily in some animal-based foods.

Health Implications 

Unlike other dietary fats, trans fats are neither essential nor salubrious and, in fact, 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).

According to the National Heart, Lung, and Blood Institute of the National Institutes of Health, more than 12.5 million Americans have CHD, and more than 500,000 die each year. That makes CHD one of the leading causes of death in the United States.

Health authorities worldwide recommend that consumption of trans fat be reduced to trace amounts. Trans fats from partially hydrogenated oils are more deleterious than naturally occurring oils.

A comprehensive review of studies of trans fats was published in 2006 in the New England Journal of Medicine reports a strong and reliable connection between trans fat consumption and CHD, concluding that "On a per-calorie basis, trans fats appear to increase the risk of CHD more than any other macronutrient, conferring a substantially increased risk at low levels of consumption (1 to 3 percent of total energy intake)". This study estimates that between 30,000 and 100,000 cardiac deaths per year in the United States are attributable to the consumption of trans fats.