Fat is one of three main classes of nutrients that provide energy to the body. The other two classes are carbohydrates and proteins. Fats are found in animals and plants. An animal fat or plant fat that is liquid at room temperature is called an oil. A processed type of beef fat called tallow and some other fats are hard at room temperature. Such fats as butter, lard, and margarine are soft at room temperature.
Fat has many important uses. It is a concentrated source of food energy for animals and plants. Fat is stored under the surface of the skin of many kinds of animals, including human beings. These fat deposits provide energy reserves and act as insulation against heat loss. Deposits of fat around the eyeballs and other organs of animals serve as cushions against injury. Women need a certain amount of fat in their bodies to have normal menstrual cycles. Most of the fat in plants is stored in seeds, where it provides the first food for young seedlings as they grow. Many industries use animal or plant fats in manufacturing various products.
Structure.
Fats consist primarily of compounds called triglycerides. Triglycerides contain one molecule of an alcohol called glycerol, also called glycerin, which is made up of atoms of carbon, hydrogen, and oxygen. The glycerol combines with three molecules of substances called fatty acids. Each of these fatty acids consists of a long chain of carbon atoms that have hydrogen atoms attached. The fatty acid chains are linked to the glycerol molecule to form a molecule of triglyceride.
Fatty acids occur in two forms called saturated and unsaturated. A saturated fatty acid has a chemical structure in which as many hydrogen atoms as possible are linked to its carbon chain. When all three of the fatty acids in a triglyceride are saturated, it is called a saturated fat. Most fats from animal sources contain a large proportion of saturated fats and are said to be highly saturated. For example, butter contains about 60 percent saturated fat.
An unsaturated fatty acid contains at least two fewer hydrogen atoms than a saturated fatty acid containing the same number of carbons. A triglyceride that contains one or more unsaturated fatty acids is known as an unsaturated fat. A fat with one unsaturated fatty acid is called a monounsaturated fat. A fat that contains more than one is called polyunsaturated. Most—but not all—fats from vegetable sources are unsaturated.
The most highly saturated fats tend to be hardest at room temperature. The hardness of a fat is also affected by the length of the carbon chains in its fatty acids. Most oils are polyunsaturated. The hardness of an unsaturated fat can be increased by hydrogenation, a process that artificially adds hydrogen to the fatty acids. Hydrogenation of such vegetable oils as corn, cottonseed, and soybean produces margarines and shortenings (see Hydrogenation).
Biological and nutritional importance.
The chemical structure of fats gives them certain important biological properties. For example, fats and oils do not dissolve in water, which makes up most of the body. Their insolubility in water enables fats to form membranes that surround all the body’s cells. These membranes help cells maintain an environment within their borders that differs from the environment outside them. Some of the body’s most important processes occur in this environment inside cells.
Fat is a more efficient fuel than either carbohydrates or proteins. Fat can produce about 4,000 calories of energy per pound (9 calories per gram). Carbohydrates and proteins can each produce about 1,800 calories per pound (4 calories per gram), less than half the energy produced by fat. Fat is also the body’s most efficient form of stored fuel. The body converts carbohydrates and proteins into fat for storage and draws on this fat when extra fuel is needed.
Certain fatty acids, called essential fatty acids, are needed for growth and maintenance of the body. The body cannot make essential fatty acids, so they must be obtained from the diet. Essential fatty acids are building blocks for the membranes around cells. They also help form many of the complicated structures inside cells. Essential fatty acids are a main part of the membranes of the retina, the part of the eye that turns light into nerve impulses. Synapses, the junctions between the body’s individual nerve cells, are also rich in essential fatty acids. All essential fatty acids are polyunsaturated.
Fats and disease.
Many scientists believe that limiting the amount and types of fats eaten can help reduce the risk of developing certain diseases, including coronary artery disease. This disease results when deposits of plaque build up on the inner walls of the arteries that nourish the heart. Plaque consists of fat, calcium, and dead cells. The deposits may eventually make artery walls hard, rough, and narrow. Many heart attacks result from a blood clot that blocks such a narrowed artery.
A fatty compound called cholesterol is one of the main substances involved in formation of plaque. Cholesterol is found in many foods, including egg yolks, butter, and red meat. Eating both cholesterol and saturated fats seems to increase the amount of cholesterol in the blood. Most scientists recommend eating a diet low in overall fat, with cholesterol and saturated fats eaten most sparingly of all. Many scientists think that limiting the amount of fat eaten—especially saturated fat—may also help people avoid certain types of cancer.
Eating a high-fat diet can contribute to excess consumption of calories, which can lead to obesity. Obesity is overweight severe enough to pose a medical risk. People with obesity have an increased likelihood of many health problems, including diabetes, coronary artery disease, and a liver disorder called cirrhosis. In the United States, all processed foods must carry a label that states the amount of total fat and the amount of cholesterol and saturated fat in a serving. This information is provided to help people control the amount of fat and calories in their diets.
Industrial uses.
Fats from a wide variety of plants and animals supply many of the raw materials used in manufacturing. Linseed oil is used in making paints that have an oil base. Manufacturers use coconut oil in making such products as hydraulic brake fluid, lipstick, soap, and chocolate coating for ice cream bars. Tallow is an important ingredient in soaps, cosmetics, and lubricants.
