Fatty acids that are produced when food manufacturers hydrogenate fats and oils. This not only changes their texture, but also their effects in the body: In many ways, trans fatty acids resemble saturated fatty acids, and a diet high in trans fatty acids is thought to contribute to high cholesterol and heart disease.
What we commonly call fat is actually a collection of molecules called fatty acids. Fatty acids are chains of carbon atoms, with varying numbers of hydrogen atoms attached to them.
In saturated fatty acids, hydrogen atoms are attached at every point possible along the carbon chain — the molecule is completely “saturated” with hydrogen atoms. Saturated fats are generally solid at room temperature and are thought to promote heart disease.
The unsaturated fats — monounsaturated and polyunsaturated fats — contain less than the maximum number of hydrogen atoms. They are liquid at room temperature and are thought to be less harmful to your health than saturated fats.
When substituted for saturated fat in the diet, both polyunsaturated fats and monounsaturated fats tend to lower blood cholesterol levels. Monounsaturated fats have recently gained popularity among diet experts because, unlike polyunsaturated fats, they seem to reduce LDL (bad) cholesterol without lowering HDL (good) cholesterol.
Trans fats are unsaturated fatty acids that have undergone the process of hydrogenation — hydrogen atoms have been added to fill the empty slots on the carbon chain. Manufacturers hydrogenate liquid vegetable oils to make them firmer and extend their shelf life.
To avoid trans fatty acids in the diet, the American Heart Association (AHA) recommends using unhydrogenated oils whenever possible and looking for processed foods made with unhydrogenated oils rather than hydrogenated or saturated fats. The AHA still advises using margarine instead of butter, but suggests choosing soft margarines over the harder stick forms, which contain more trans fatty acids. Furthermore, it recommends shopping for margarine with no more than 2 grams of saturated fat per tablespoon and with liquid vegetable oil listed as the first ingredient. The AHA also says that if people limit their daily intake of fats and oils to about 5 to 8 teaspoons per day, they are unlikely to get too many trans fatty acids.