To help us understand what exactly trans fats are, and why they are unhelpful for health, Ban Trans Fats website has some very useful information.
The purpose of hydrogenation is to solidify an oil so that it can be made to resemble real foods such as butter. The hydrogenation process imparts desirable features such as spreadability,
texture, "mouth feel," and increased shelf life to naturally liquid vegetable oils. In the hydrogenation process, vegetable oil is reacted under pressure with hydrogen gas at 250 -
400oF for several hours in the presence of a catalyst such as nickel or platinum. However, this industrial process cannot control where the hydrogen atoms are added to the
"unsaturated" double bonds. Randomly adding hydrogen atoms to polyunsaturated fats converts natural food components into many compounds, some of which have never seen before by man until
partially hydrogenated fats were manufactured.
Some of the several dozens of altered compounds created in the manufacture of partially-hydrogenated fats are "trans" fatty acids. Fatty acids are the building blocks of fats, much like amino acids are the building blocks of proteins. Other new compounds accidentally synthesized include fatty acids having double bonds translocated to new and un-natural positions, and various molecular fragments. Many of these altered compounds are detrimental to health.
It is the process of hydrogenation that makes vegetable oils take on the properties of saturated fats, in particular, being solid at room temperature. It was these trans 'saturated' fats that gave saturated fats a bad name.
Here are some of the differences between trans fat and saturated fat.
At first glance these molecules look similar, but a closer look at the middle sections shows that the trans fat is mimicking the saturated fat molecule - it is not idential.
...In the illustration above, (now deleted) the light grey rounded areas are hydrogen atoms and the dark grey areas are carbon atoms. Note the different positioning of the hydrogen atoms in the middle which is caused by partial hydrogenation. The hydrogen atoms in the middle are in a "trans" position which makes this a "trans" fatty acid. The effect is to straighten out the molecules so they can pack together more closely and make the oil less liquid and more solid.
"The problem arises when a large number of the trans fatty acids are consumed from foods and they are deposited in those parts of the cell membranes that are supposed to have either saturated fatty acids or "cis" unsaturated fatty acids; under these circumstances the trans fatty acids essentially foul up the "machinery."
When we compare the biological effects of the trans fatty acids versus the saturated fatty acids, we see that
(1) saturated fatty acids raise HDL cholesterol, the so-called good cholesterol, whereas the trans fatty acids lower HDL cholesterol;
(2) saturated fatty acids lower the blood levels of the atherogenic lipoprotein [a], whereas trans fatty acids raise the blood levels of lipoprotein [a];
(3) saturated fatty acids conserve the good omega-3 fatty acids, whereas trans fatty acids cause the tissues to lose these omega-3 fatty acids;
(4) saturated fatty acids do not inhibit insulin binding, whereas trans fatty acids do inhibit insulin binding (see our page on diabetes);
(5) saturated fatty acids do not increase C-reactive protein, but trans fatty acids do increase C-reactive protein causing arterial inflammation;
(6) saturated fatty acids are the normal fatty acids made by the body, and they do not interfere with enzyme functions such as the delta-6-desaturase, whereas trans fatty acids are not made by the body, and they interfere with many enzyme functions such as delta-6-desaturase; and
(7) some saturated fatty acids are used by the body to fight viruses, bacteria, and protozoa, and they support the immune system, whereas trans fatty acids interfere with the function of the immune system.
I am not suggesting that we go over-board in consuming saturated fats. The diet needs to be balanced with plenty of vegetables and whole grains and quality sources of protein, but it is important to understand that the problems associated with saturated fats are primarily linked to the trans fats.