The fats in our food are important because they are carriers of the fat-soluble vitamins: A, D, E, and K. In addition, dietary fat includes and supplies essential fatty acids, particularly linoleic acid.
All animal life requires polyunsaturated fatty acids, and linoleic acid is of primary importance to humans. The adult requirement is low, and easily met by a well-rounded diet, but children have a greater need for linoleic acid for growth. The essential fatty acids are important in preventing drying and flaking of the skin and have several metabolic roles: maintaining cell membranes, regulating cholesterol metabolism, and helping to create hormone-like substances needed for many body processes. But what, in fact, are these fatty acids? The fats in our body contain the same substances as the fats in our food: fatty acids and f atlike compounds called phospholipids (such as lecithin) and cholesterol.
Fats are made up of the same three elements as carbohydrates: carbon, hydrogen, and oxygen. Fat differs from carbohydrate in that the former is a more concentrated form of fuel and contains proportionately more carbon and less oxygen than does carbohydrate. This difference causes fat to supply 9 calories per gram, whereas carbohydrate supplies only 4. When one molecule of fat is broken down, it becomes three molecules of fatty acids and one molecule of glycerol. These molecules are known as triglycerides. You may have heard of “serum triglycerides,” a medical term used to describe the level of fat molecules in the blood.
Fatty acids are straight chains of carbon atoms ranging in number from 2 to 20. Each of these carbon atoms may be linked to hydrogen atoms, also in varying numbers from 0 to 3. The number of carbon and hydrogen atoms in a fatty acid, as well as the different combinations of fatty acids found in a particular fat, will determine what type of fat it will be and how it will taste. Some of the many fatty acids may be familiar to you from the labels on food packages: acetic, butyric, myristic, palmitic, stearic, and linoleic.
Fatty acids are either saturated or unsaturated – terms that refer to the number of hydrogen atoms attached to the carbon atoms. When all the carbon atoms are linked to hydrogen atoms on both sides, the fatty acid is considered saturated, or filled with hydrogen. If any of the carbon atoms are free, that is, not linked to a pair of hydrogen atoms, the fatty acid is unsaturated. Monounsaturated fatty acids lack only one pair of hydrogen atoms, while polyunsaturated fatty acids lack many such pairs.