Fats and oils are mixtures of fatty acids. Each fat or oil is designated "saturated," "monounsaturated" or "polyunsaturated," depending on what type of fatty acid predominates.

• 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 and turn rancid. Saturated fatty acids raise blood cholesterol, which raises the risk of coronary heart disease and stroke.
  
   
• 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 when refrigerated but is clear at room temperature. Monounsaturated fatty acids seem to lower blood cholesterol when substituted for saturated fats.
  
   
• Polyunsaturated fatty acids have more than one unsaturated bond. 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 lower total blood cholesterol when substituted for saturated fats.

In addition, dietary cholesterol found in animal fats also raises total blood cholesterol and LDL ("bad") 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. This means that 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 can 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. French fries, donuts, cookies, chips and other snack foods are high in trans fatty acids. In fact, nearly all fried or baked goods have some trans fats.

How does hydrogenation create trans fatty acids?

To help foods 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.

How are trans fatty acids harmful?