It’s difficult to read a newspaper or listen to the evening news without hearing something new about fat and its connection with disease. Diets that are high in fat are strongly associated with an increased prevalence of obesity and an increased risk of developing coronary artery disease, high blood pressure, diabetes mellitus, and certain types of cancer. Health authorities recommend that we reduce our total fat intake to about 30 percent of total calories. They also recommend that we limit our intake of saturated fat (the type of fat most often found in meat and dairy products) to less than 10 percent of our fat calories and try to be sure that the fat we do eat is mostly the monounsaturated or polyunsaturated type. These changes have been shown to decrease our risk for several diseases.
Fat as a Nutrient
Fat is an essential nutrient, because our bodies require small amounts of several fatty acids from foods (the so-called essential fatty acids) to build cell membranes and to make several indispensable hormones, namely, the steroid hormones testosterone, progesterone, and estrogen, and the hormone-like prostaglandins. Dietary fats also permit one group of vitamins, the fat-soluble vitamins (A, D, E, and K), to be absorbed from foods during the process of digestion. Fats help these vitamins to be transported through the blood to their destinations. The fat in our
bodies also provides protective insulation and shock absorption for vital organs. As a macro nutrient, fat is a source of energy (calories). The fat in food supplies about 9 calories per gram, more than twice the number of calories as the same amount of protein or carbohydrate. As a result, high-fat foods are considered “calorie-dense” energy sources. Any dietary fat that is not used by the body for energy is stored in fat cells (adipocytes), the constituents of fat (adipose) tissue . The Dietary Guidelines for Americans recommend that no more than 30 percent of our calories should come from fat, and only a third of that should be saturated fat.
Sorting Out the Fats
Our health is influenced by both the amount and the type of fat that we eat. Fats are molecules; they are classified according to the chemical structures of their component parts. But you don’t need to be a chemist to understand the connection between the various fats in foods and the effect
these fats have on the risk for disease. Some definitions will help. Dietary fats, or triglycerides, are the fats in foods. They are molecules made of fatty acids (chain-like molecules of carbon, hydrogen, and oxygen) linked in groups of three to a backbone called glycerol. When we eat foods that contain fat, the fatty acids are separated from their glycerol backbone
during the process of digestion. Fatty acids are either saturated or unsaturated, terms that refer to the relative number of hydrogen atoms attached to a carbon chain. Fat in the foods that we eat is made up of mixtures of fatty acids—some fats may be mostly unsaturated, whereas others are mostly saturated (see sidebar: A Comparison of Fats Monounsaturated fatty acids are fatty acids that lack one pair of hydrogen atoms on their carbon chain. Foods rich in monounsaturated fatty acids include canola, nut, and olive oils; they are liquid at room temperature. A diet that provides the primary source of fat as monounsaturated fat (frequently in the form of olive oil) and includes only small amounts of animal products has been linked to a lower risk of coronary artery disease. This type of diet is commonly eaten by people who live in the region surrounding the Mediterranean Sea . Polyunsaturated fatty acids lack two or more pairs of hydrogen atoms on their carbon chain. Safflower, sunflower, sesame, corn, and soybean oil are among the sources of polyunsaturated fats (which are also liquid at room temperature). The essential fatty acids, linoleic and linolenic acid, are polyunsaturated fats. Like monounsaturated fats, polyunsaturated fats lower blood cholesterol
levels and are an acceptable substitute for saturated fats in the diet. Saturated fatty acids, or saturated fats, consist of fatty acids that are “saturated” with hydrogen. These fats are found primarily in foods of animal origin—meat, poultry, dairy products, and eggs—and in coconut, palm.