In modern societies, fat is consumed in much larger amounts than in countries where people are eating whole grains as their principal food.
For example, in the United States, about 42 percent of the ordinary diet is composed of fat, while in rural Mexico among the Tarahumara, a native people renowned for their health and longevity, the amount is only 12 percent. About 15 percent of the standard macrobiotic diet consists of fat.
Lipids are the family name for fats, oils, and fatlike substances including fatty acids, cholesterol, and lipoproteins. Fats are solid at room temperature, while oils are fluid. Solid lipids tend to contain more saturated fatty acids. Fatty acids are long chains of carbon and hydrogen atoms including an oxygen molecule at one end.
Saturated fatty acids are bonded or saturated to hydrogen atoms.
Unsaturated fatty acids lack at least one pair of hydrogen atoms.
Polyunsaturated fatty acids are those in which more than one pair is missing.
Fatty acids are the building blocks of fats, just as simple sugars are the fundamental units of carbohydrates. In order to help digest fats, which are insoluble in water and form large globules, the liver secretes bile, a yellowish liquid stored in the gallbladder. In the intestine, bile serves to emulsify fats and enables them to be broken down into fatty acids and glycerol by digestive enzymes.
Lipids are essential to digestion but can be harmful to the body, especially saturated acids like stearic acid, found in animal tissues, which coats the red blood cells, blocks the capillaries, and deprives the heart of oxygen. One of the main constituents of lipids is cholesterol, a naturally occurring substance in the body which contributes to the maintenance of cell walls, serves as a precursor of bile acids and vitamin D and also a precursor of some hormones. Cholesterol is not found in plants foods but is contained in all animal products, especially meat, egg yolks, and dairy products. Since cholesterol is insoluble in the blood, it attaches itself to a protein that is soluble in order to be transported through the body. This combination is called a lipoprotein. However, excess cholesterol in the bloodstream tends to be deposited in artery walls and as plaque eventually causes constriction of the arteries, reduces the flow of blood, and can lead to a heart attack, stroke, or peripheral artery disease. Normally, fat is absorbed by the lymph and enters the bloodstream near the heart. However, if excess lipids accumulate in the body, eventually some will become deposited in the liver. Such stored fat, primarily from meat, poultry, eggs, and dairy products, is usually the chief source of liver malfunctions. Excess fat, especially saturated fat, is also stored in and around vital organs, such as the kidneys, the spleen, the pancreas, and the reproductive organs and is a leading cause of cancer in these sites.
Because of the increased public awareness of the connection among cholesterol, saturated fat, and heart disease and cancer, many people have switched to unsaturated fats and oils, including vegetable cooking oils, mayonnaise, margarine, salad dressings, and artificial creamers and spreads. Today, these make up the large single source of fat in the American diet. However, unsaturated fats, especially those of a refined quality, serve to redistribute cholesterol from the blood to the tissues and combine with oxygen to form free radicals. These are unstable and highly reactive substances that can interact with proteins and cause the loss of elasticity in tissue and general weakening of cells.
Hydrogenated fats, moreover, such as margarine, are specially treated to remain solid at room temperature, a process that converts their unsaturated fatty acids into saturated fatty acids to a significant degree. Hydrogenated fats are also known as trans fatty acids.
Whole grains, beans, seeds and nuts contain polyunsaturated fats and oils, but these are naturally balanced by the right proportion of vitamin E and selenium, which are usually lost in the refining process. Similarly, unrefined polyunsaturated cooking oils (in which the vitamin E remains) such as dark sesame oil are a balanced product and, if used moderately, will contribute to proper metabolism, including more flexible motion and thinking.