Trans fatty acid is generated in a process called hydrogenation in manufacturing margarine or shortening. Hydrogenation is a chemical reaction to add hydrogen to unsaturated fatty acids using nickel or something as catalyst under a high temperature and a high pressure. Through hydrogenation, unsaturated fatty acids reduce their double bonds between carbon atoms. For example, alpha-linolenic acid transforms into linoleic acid, and then into oleic acid. During this reaction, however, some of the cis configurations of unsaturated fatty acids are converted into trans configurations, which is tarns fatty acid, or trans fat in short. In nature, the vast majority of unsaturated fatty acids have cis double bonds, whereas trans isomer molecules are rare.
Thus, trans fat is an artificial substance generated under a peculiar condition. With this perception, you would not feel that you could eat the substance before it is proved to be a human food. Nonetheless, an increasing number of studies found that trans fat is not a human food. At present, it is well established that trans fat raises the risks of coronary heart disease and arteriosclerosis. Some researchers also point out that trans fat is absorbed together with normal cis fat into the cell membranes and the brain lipid, which lowers their functions and eventually to cause various diseases. Trans fat is hazardous material with no health benefit.
In fact, trans fat is found in nature, too. One of them is vaccenic acid, which exists in the cow. This vaccenic acid is present along with conjugated linoleic acid (CLA) that has an anticancer effect. And vaccenic acid is converted into CLA in the cow's body or in the human body who ingested the acid, according to a recent research. Therefore, we should not regard naturally occurring trans fat and artificial trans fat such as elaidic acid as the same substances.
It is said that human beings have lived on dairy fats as milk, butter, and cheese for thousands of years. So have been other ruminant fats. Taking this fact into consideration, it is unlikely that these ruminant fats do some harm to the human body. In contrast, artificial trans fat occurs in a peculiar condition and has not found in any organisms on the earth. How could a human digest the new substance properly in the body? From this point of view, too, we should recognize that we can ingest naturally occurring trans fat, whereas should not artificial trans fat.
Oils are somewhat hardened through hydrogenation to become margarine or shortening. These hydrogenated oils are used in bread, cakes, fries, and other processed foods. The food can keep its volume, shape and shortness well after it is manufactured since the hydrogenated oils in it remain semi-solid at room temperature.
And, hydrogenated oil is hard to be oxidized and so it has a prolonged shelf life. This is because their original unsaturated double bonds, cause of rotting, reduce through hydrogenation process. Sardine oil and other fish oils are rich in highly unsaturated fatty acids such as EPA and DHA, which make them vulnerable to oxidization. Some of the fish oils are, therefore, hydrogenated to become shortenings.
However, oil is used as shortening without hydrogenation if the oil has a high melting point due to its saturated fat. Butter, lard, and palm oil are among them with high percentages of saturated fats and are semi-solid at room temperature. Therefore, some processed foods contain these fats without hydrogenation. But, a majority of margarines and shortenings seems to contain hydrogenated oils; possibly because it is less costly to use hydrogenated soybean oil or something, or to tune the oil's melting point.
Incidentally, the word "shortening" means fragile or crisp. But, the word is better recognized as "life-shortening"; because trans fat in shortening raises the heart disease risk, that is to say, the fat does shorten people's life expectancies. Hence, we should understand, as a precaution, that margarine, shortening, and processed oil on food labels are "life shortening fats".