Over the last decade, there has been a lot of press about the  negative effects of hydrogenated oils and trans-fats, which recently led  to New York City passing the first law to ban the use of hydrogenated  oils in restaurants. Yet even with all the negative publicity about  hydrogenated oils, few people actively avoid consuming them and only a  very small percentage of people really understand what they are and why  they are harmful.
This article is designed to explain the major  differences between good and bad fats and take the mystery out  hydrogenated oils and trans-fats. It also explains how eliminating  hydrogenated oils from your nutritional program will improve your  long-term health and fat loss. However, before getting to hydrogenated  oils, I will cover some basic scientific information about fats that is  necessary to fully understand why some fats are good and hydrogenated  oils are bad.
The structure of fats: The main part of a fat  molecule is made up of carbon atoms chained together with hydrogen atoms  attached to their sides. When all of the carbon atoms have hydrogen  atoms attached to both sides, they form a saturated fat. If one hydrogen  atom is missing, the fat is monounsaturated and fats with multiple  hydrogen atoms missing are called polyunsaturated.
This by itself  is not too important, but whenever a hydrogen atom is missing  (unsaturated), there will be a double bond between the carbon atoms  instead of the usual single bond. You may be wondering why this is  significant, but understanding the double bonds is the key to  understanding hydrogenated fats and fats in general.
The double  bond between carbon atoms along with the missing hydrogen atom allows  the fat molecule to be more versatile and becomes useable for various  physiological reactions throughout the body. Fats with double bonds have  many beneficial effects, such as improving your immune system, heart  health, mood, skin, energy, nutrient absorption, and much more.
Some  fats are more beneficial than others and generally the more double  bonds the fat contains, the more positively it will affect your body.  Probably the most well known beneficial fats are the Omega-3 oils found  in fatty fish, which have either 5 or 6 double bonds. On the down side,  fats with more double bonds are also more fragile and susceptible to  physical manipulations. The importance of this will become apparent when  you read about hydrogenation.
Saturated fats (no double bonds) on  the other hand can only be used as energy for the body and cannot be  used in the cellular reactions that create the beneficial effects listed  above. In addition any extra saturated not needed for energy will be  stored as fat. There is however one benefit to saturated fats: they are  very durable and are highly resistant to physical alterations.
Hydrogenation  is a manufacturing process used to alter unsaturated fats, generally to  increase the shelf life of packaged products. This change is achieved  by altering unsaturated fats at the point of their double bond(s).  Unsaturated oils can either be completely hydrogenated or partially  hydrogenated and each process affects fats differently. It is also  important to note that saturated fats cannot be hydrogenated, because  they have no double bonds or missing hydrogen atoms.
When an  unsaturated fat is completely hydrogenated, it essentially becomes a  saturated fat. Unfortunately there are also some additional unnatural  compounds created during the process that are not found in natural  saturated fats. After this process, the fat will have almost an  indefinite shelf life, although it loses all any health benefits  associated with the original fat.
Completely hydrogenated fats are  generally only used with fats that are almost completely saturated to  begin with and they are less common than partially hydrogenated fats.  This is unfortunate, because contrary to how the name sounds completely  hydrogenated fats are actually less unhealthy than partially  hydrogenated ones.
As the name implies, partially hydrogenated  fats do not undergo the full hydrogenation process. This means the fat  does not end up resembling a saturated fat and there are even more  unnatural compounds produced during this process than full  hydrogenation. In addition, permanent damage is done to the double bonds  that change their properties from healthy to unhealthy.
Bear with  me for just a little bit more science, because this is what the  previous information has been leading up to. When double bonds are in  their natural occurring state, they are in a "cis" configuration. You  don't have to remember this, but you do need to know that during  hydrogenation the "cis" configuration gets altered into a "trans"  configuration. In other words, hydrogenation turns healthy unsaturated  fats into unhealthy trans-fats.
In the past it was believed that  saturated fats were the unhealthiest fats, but in recent years  trans-fats have taken over that title. It is important to note that  since trans-fats are created by altering unsaturated fats, you will  never find trans-fats in saturated fats or completely hydrogenated fats.  Also, healthier fats with many double bonds are easier to corrupt  during the hydrogenation process.
I hope you feel somewhat  comfortable with the science behind fats, because it helps explain why  trans-fats are so unhealthy. As previously discussed, unsaturated fats  are used in many beneficial chemical reactions throughout the body and  the double bonds are key to these reactions. Trans-fats maintain the  appearance of a healthy unsaturated fat and the body can't tell the  difference between them. Unfortunately, there is a big difference in the  way they function.
When your body tries to use the trans-fats in  necessary physiological reactions, they will not be effective.  Trans-fats essentially stop the beneficial reactions from taking place,  which can affect virtually every important system within your body. Some  of the many negative effects include: impairing heart performance,  weakening your immune system, weakening the protective barrier around  cells, and disrupting the function of essential fats.
Now that you  have read the science, here is some additional practical information to  help you limit your consumption of trans-fats. While it is true that  partially hydrogenated oil is a major source of trans-fat, it is not the  only one. Simply exposing unsaturated oils to high temperatures, such  as when frying food, will alter double bonds and create trans-fats.  Since fats with many double bonds are quite fragile, they can be turned  into trans-fats at much lower temperatures than fats with only 1 double  bond (monounsaturated).