Saturated Fatty Acids
Saturated fatty acids have all the hydrogen the carbon atoms can hold. Saturated fats are usually solid at room temperature and they are more stable - that is, they do not combine readily with oxygen. Saturated fatty acids are the main dietary culprit in raising blood cholesterol. The main sources of saturated fatty acids in the typical American diet are foods from animals and some plants.

•  Foods from animals that have high amounts of saturated fatty acids include beef, beef fat, veal, lamb, pork, lard, poultry fat, butter, cream, milk, cheeses and other dairy products made from whole milk. These foods also contain dietary cholesterol.

•  Foods from plants that contain high amounts of saturated fatty acids include coconut oil, palm oil and palm kernel oil (often called tropical oils) and cocoa butter.

Trans Fatty Acids
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 of either vegetable or fish oils.

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. Double bonds convert to trans double bonds, and the fatty acids become saturated.

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.

Because there are no standard methods, it's difficult to estimate the trans fatty acid content of food items. It's also difficult to estimate intake, especially long-term intake. On the basis of current data, the American Heart Association recommends that consumers follow these tips:

•  Use naturally occurring, unhydrogenated oil such as canola or olive oil when possible.

•  Look for processed foods made with unhydrogenated oil rather than hydrogenated or saturated fat.

•  Use margarine as a substitute for butter, and choose soft margarines (liquid or tub varieties) over harder, stick forms. Shop for margarine with no more than 2 grams of saturated fat per tablespoon and with liquid vegetable oil as the first ingredient.

•  French fries, donuts, cookies and crackers are examples of foods that are high in trans fatty acids.

Polyunsaturated and Monounsaturated Fatty Acids
Polyunsaturated and monounsaturated fatty acids make up the total of unsaturated fatty acids. Unsaturated fatty acids have at least one unsaturated bond - that is, at least one place that hydrogen can be added to the molecule. They are often found in liquid oils of vegetable origin.

•  Polyunsaturated oils are liquid at room temperature and in the refrigerator. They easily combine with oxygen in the air to become rancid. Common sources of polyunsaturated fatty acids are safflower, sesame and sunflower seeds, corn and soybeans, many nuts and seeds and their oils.

•  Monounsaturated oils are liquid at room temperature but start to become solid at refrigerator temperatures. Canola, olive and peanut oils and avocados are sources of monounsaturated fatty acids.

Polyunsaturated fatty acids tend to help the body get rid of newly formed cholesterol. Thus, they keep the blood cholesterol level down and reduce cholesterol deposits in artery walls. Recent research has shown that monounsaturated fatty acids may also help reduce blood cholesterol as long as the diet is very low in saturated fat.

Both types of unsaturated fatty acids may help lower your blood cholesterol level when used in place of saturated fatty acids in your diet. But you should be moderate in your intake of all types of fat.

Poly- or monounsaturated oils - and margarines and spreads made from these oils - should be used in limited amounts in place of fats with a high saturated fatty acid content, such as butter, lard or hydrogenated shortenings.

Hydrogenated Oils
During food processing, fats may undergo a chemical process known as hydrogenation. Hydrogenate means to add hydrogen, or, in the case of fatty acids, to saturate. The process changes an oil, naturally high in unsaturated fatty acids, to a more solid and more saturated form. The greater the degree of hydrogenation, the more saturated the oil becomes. Many commercial products contain hydrogenated or partially hydrogenated vegetable oils.