There are three types of fat:
“Saturated” refers to the number of hydrogen atoms in a molecule of fat.
Saturated fats contain as many hydrogen atoms as they can. They are usually solid at room temperature. Saturated fats are present in meats, dairy products, and artificially hydrogenated vegetable oils. The more solid the product, the higher is the proportion of saturated fats. A diet high in saturated fats promotes coronary artery disease.
Unsaturated fats (monounsaturated and polyunsaturated) do not contain as many hydrogen atoms as they could. Monounsaturated fats could contain one more hydrogen atom. They are usually liquid at room temperature but start to solidify in the refrigerator. Olive oil and canola oil are examples.
Polyunsaturated fats could contain more than one additional hydrogen atom. These fats are usually liquid at room and refrigerator temperatures. They tend to become rancid at room temperature. Corn oil is an example. Other polyunsaturated fats include omega-3 fats, contained in deep-sea fatty fish (such as mackerel, salmon, and tuna), and omega-6 fats, contained in vegetable oils.
Trans fats are produced in a process called hydrogenation, in which hydrogen atoms are artificially added to polyunsaturated oils (“Trans” refers to where the hydrogen atoms are added to the fat molecule.). Oils containing trans fats may be used to make food products that do not become rancid and to make solid fat products, such as margarine. Trans fats are particularly common in commercial baked and fried foods, such as cookies, crackers, doughnuts, french fries, and other similar foods.
Trans fats increase low-density lipoprotein (LDL—the bad) cholesterol levels and decrease high-density lipoprotein (HDL—the good) cholesterol levels, and these effects appear to increase the risk of coronary artery disease. Avoiding products that contain trans fats is wise. Trans fats are now listed on food labels. Also, if hydrogenated fat or partially hydrogenated fat is the first fat on the list of ingredients, the product contains trans fats. Some restaurants also provide information on which menu items contain trans fats. Some cities in the United States have barred restaurants from using trans fats in their food, and more cities are likely to follow this trend. The federal Food and Drug Administration has told manufacturers that they should remove trans fats from their products.
The appearance of a margarine or oil can also help identify foods containing these fats—the softer or more liquid, the lower the trans fat content. For example, the trans fat content of tub margarines is lower than that of stick margarines.
Some margarine products contain a plant sterol or stanol, which can lower total and LDL cholesterol levels. Plant sterols and stanols may have this effect because they are not absorbed well in the digestive tract and they interfere with the absorption of cholesterol. These margarine products have been approved as heart healthy foods when they are used as part of a healthy diet. These products are made from unsaturated fat, contain less saturated fat than butter, and do not contain trans fats. However, they are expensive.
The ideal combination of types of fats is unknown. However, a diet high in monounsaturated or omega-3 fats and low in trans fats is probably desirable.