Fats and oils are either saturated or unsaturated, the latter classification being broken down into monounsaturated and polyunsaturated fats. To illustrate the difference between the terms saturated, monounsaturated and polyunsaturated, picture a fat molecule as a train of passenger cars (carbon atoms). If every seat on the train is filled by a "passenger" (hydrogen atom), then this is a saturated fat molecule. If there's one seat open in each car where a hydrogen-atom "passenger" can sit, the molecule is monounsaturated; if there are several seats available, it's polyunsaturated. In general, saturated fats come from animal sources and are solid enough to hold their shape at room temperature (about 70 degrees F). Exceptions to this rule are tropical oils such as coconut oil and palm oil, which, though of plant origin, are semisolid at room temperature and highly saturated. Saturated fats are the nutritional "bad guys" because they're known to be associated with some forms of cancer and to increase cholesterol levels, which can be a contributing factor to heart disease. In addition to the two aforementioned tropical oils, the most commonly commercially used saturated fats are butter, lard, suet and hydrogenated vegetable oils such as margarine and vegetable shortening. Hydrogenated (or partially hydrogenated) oils have been chemically transformed from their normal liquid state (at room temperature) into solids. During the hydrogenation procedure extra hydrogen atoms are pumped into unsaturated fat. This process creates trans fatty acids, converting the mixture into a saturated fat and obliterating any benefits it had as a polyunsaturate. Some researchers believe that hydrogenated oils may actually be more damaging than regular saturated fats for those limiting cholesterol in their diets. Unsaturated fats are derived primarily from plants and are liquid (in the form of an oil) at room temperature. Generally speaking, oils are composed (in varying percentages) of both monounsaturated and polyunsaturated fats. Monounsaturated fats are known to help reduce the levels of LDL (bad) cholesterol. The three most widely used oils that are high in monounsaturates are olive oil, canola oil and peanut oil. Polyunsaturated fats are also considered relatively healthy and include the following (ranked in order, most to least, of polyunsaturates): safflower oil, soybean oil, corn oil and sesame oil.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.