Fats and oils are mixtures of fatty acids. Each fat or oil is designated "saturated," "monounsat-urated" or "polyunsaturated," depending on what type of fatty acids predominates in that one.

• Saturated fatty acids have all the hydrogen the carbon atoms can hold. Saturated fats are usually solid at room temperature, and they're more stable - that is, they don't combine readily with oxygen. Saturated fatty acids raise blood cholesterol. And the risk of coronary heart disease rises as blood cholesterol levels increase.
• Unsaturated fatty acids have at least one unsaturated bond - that is, at least one place that hydrogen can be added to the molecule. There are two common types.
• Monounsaturated fatty acids have only one unsaturated bond. Monounsaturated oils are liquid at room temperature but start to solidify at refrigerator temperatures. For example, salad dressing containing olive oil turns cloudy in the refrigerator but is clear when left out for a little while. Monounsaturated fatty acids seem to lower blood cholesterol.
• Polyunsaturated fatty acids have more than one unsaturated bond. And polyunsaturated oils, which contain mostly polyunsaturated fatty acids, are liquid at room temperature and in the refrigerator. They easily combine with oxygen in the air to become rancid. Polyunsaturated fatty acids help to lower total blood cholesterol.

In addition, dietary cholesterol found in animal fats also has a blood cholesterol-raising effect. A food rich in both saturated fat and cholesterol can cause problems for people trying to lower blood cholesterol.

What are trans fatty acids and where do they come from?

A fatty acid molecule consists of a chain of carbon atoms in carbon-carbon double bonds with hydrogen atoms "attached." In nature most unsaturated fatty acids are cis fatty acids, meaning the hydrogen atoms are on the same side of the double carbon bond. In trans fatty acids the two hydrogen atoms are on opposite sides of the double bond.

Trans double bonds also occur in nature as the result of fermentation in grazing animals. People eat them in the form of meat and dairy products. Trans double bonds are also formed during the hydrogenation (hi"dro-jen-A'shun or hi-DROJ'en-a"shun) of either vegetable or fish oils.

How does hydrogenation create trans fatty acids?

To make foods that will stay fresh on the shelf or to get a solid fat product, such as margarine, food manufacturers hydrogenate polyunsaturated oils. Hydrogenate means to add hydrogen. When unsaturated fatty acids are hydrogenated, some of the hydrogen atoms are added on opposite sides of the molecule to the already attached hydrogen. Cis double bonds convert to trans double bonds, and the fatty acids become saturated.

How are trans fatty acids harmful?

In clinical studies, trans fatty acids or hydrogenated fats tend to raise total blood cholesterol levels but not as much as more saturated fatty acids. Trans fatty acids also tend to raise LDL ("bad") cholesterol and lower HDL ("good") cholesterol when used instead of cis fatty acids or natural oils. These changes may increase the risk of heart disease. It's not clear if trans fats that occur naturally have the same effect on cholesterol and heart disease as those produced by hydrogenation of vegetable oils. A detailed explanation of trans fatty acids can be found in the article, The Dangers of Hydrogenated Or Partially Hydrogenated Fats & Oils.