Classification and Biochemistry
Fats, like carbohydrates, are composed of carbon, hydrogen, and oxygen; however, some phospholipids also contain nitrogen and phosphorus. The common property of all fats is that they are insoluble in water and dissolve only in the fat solvents. Tissue lipids (found in normal tissues such as liver or muscle and surrounding or covering the abdominal organs) make up about 10–15 percent of the average adult’s weight; total body fat consists of all the tissue lipids plus the stored fat (this subcutaneous fat and brown fat can vary with weight and diet). Though we should keep our fat levels down, dietary fat is a high-energy food source and gives food a special flavor to which many people are attracted. The lipids include the triglycerides, the phospholipids, and the sterols.

Triglycerides and Fatty Acids
Triglycerides comprise about 95 percent of the lipids in food and in our bodies. They are the storage form of fat when we eat calories in excess of our energy needs. Burning up the stored fat allows us to live without food for periods of time, as I have done during my many fasts.

All triglycerides have a similar structure, being composed of three fatty acids attached to a glycerol molecule. Glycerol is a short-chained carbohydrate molecule that is soluble in water, and when triglycerides are metabolized, the glycerol can be converted to glucose. Fatty acids may differ in their length and their degree of saturation. They are commonly composed of a series of 16–18 carbon molecules attached to hydrogen molecules. The number of hydrogen molecules is what determines the saturation of the fat. When each carbon has its maximum number of hydrogens attached, the fat is said to be saturated—that is, filled to capacity with hydrogen.

Saturated fats, which are commonly found in animals, are hard at room temperature. Lard, suet, and butter are common saturated animal fats; coconut and palm oil are two saturated vegetable oils. Saturated fats are generally more stable than the unsaturated fats and go rancid (undergo an oxidative change in molecular structure) less easily.

Unsaturated fats are of two varieties—monounsaturated and polyunsaturated. If two adjoining carbon atoms are attached by a double bond, there is room for one more pair of hydrogen atoms, and the fatty acid is said to be monounsaturated. Oleic acid, present in olive oil, is a monounsaturated fat. When more than one area of the carbon chain can accept additional hydrogen atoms, the fat is said to be polyunsaturated. Linoleic acid, an essential fatty acid found in safflower oil, soybean oil, and other vegetable oils, is an example of a polyunsaturated fat. Other oils of this category include peanut, corn, and cottonseed oils.

Unsaturated fats are unstable at room temperature and sensitive to interaction with oxygen, light, and heat. This is why storage in dark glass or cans and/or under refrigeration is ideal. There are two other ways besides refrigeration to prevent rancidification (spoiling) of an oil. The first is to use antioxidants. (Points of unsaturation, the weak spots in the fatty acid, can be “attacked” by oxygen; antioxidants protect the molecules from oxidation.) Vitamin E is a common antioxidant; beta-carotene and the chemicals BHA and BHT are others.

Hydrogenation is another way of dealing with the spoilage problem of unsaturated oils. With chemically induced hydrogen saturation of the carbon bonds, the structure of the unsaturated oils is changed. This alters the way the body metabolizes these fats and often changes the physical form, as with margarines. Many manufacturers hydrogenate oils to make margarine and other spreads. These hydrogenated products are consumed in large amounts in the American culture and there is some question as to their carcinogenicity. Also, they no longer provide their once-available polyunsaturated fats and, since they are now saturated, tend to raise the level of blood cholesterol rather than lower it, increasing risk of cardiovascular disease.