You see the words on the labels of processed foods all the time: hydrogenated oil, partially hydrogenated vegetable shortening, partially hydrogenated oil. But, what exactly are the processes of hydrogenation and partial hydrogenation?

When fat or oil is subjected to hydrogenation, the molecular structure of the fat is actually changed. The changed molecules cause the oil to harden, which makes it easier to handle and increases its shelf life. It also has a high melting temperature and an appealing creamy texture. For these reasons, food manufacturers like working with the hydrogenated oils.

So, how are the molecules changed? In the hydrogenation process, vegetable oil – which is actually fairly healthy polyunsaturated or monounsaturated fat – is heated and placed under pressure along with hydrogen gas. A metal catalyst – often nickel – is also present in the hydrogenation chamber. The oil, gas and metal are left to interact for several hours. The hydrogen and carbon atoms combine, causing the fat to harden. The fat is then filtered and bleached. The resulting compound is very different from the original oil. It is much closer to plastic in nature than the original oil it started as. In addition, the hydrogenation process destroys Omega 3 essential fatty acids and Omega 6 essential fatty acids – the healthiest parts of some oils.

Why are these changed molecules so different in the body? While the digestive system treats hydrogenated fats as food, the bloodstream cannot use it. The cells simply try to store the trans fats and deposits them on the artery walls.

Ironically, fat that has been completely hydrogenated does not actually contain trans fat; only the partially hydrogenated oil has been shown to cause arterial buildup. However, full hydrogenation makes the oil so hard that it is not usable on its own. Some new products are using fully hydrogenated oil blended with un-hydrogenated vegetable oils to make a workable fat.

The part that makes it even more difficult for consumers to learn what they are eating is that the terms partially hydrogenated and hydrogenated are often both used to mean partially hydrogenated. Unless a product says it is "fully hydrogenated," it may actually be partially hydrogenated and contain trans fatty acids.

The new labeling system, requiring packaged food products to disclose their trans fat content will be a great first step toward helping consumers make better choices about the fats they eat. It has also encouraged many food manufacturers to seek healthier alternatives to trans fats in their foods.