Fats, or lipids, are the third main class of the macronutrients needed in human nutrition. The lipids are found primarily in meats and dairy foods, at least, these are the most visible sources, but most foods contain some fat. Some of the richer vegetable sources of dietary fat are nuts and seeds, soybeans, olives, peanuts, and avocados, and these contain the needed or essential fatty acids (EFAs). Fats are an important component of our diet, and at least a minimum intake is essential. However, many problems are associated with excessive intake of dietary fat, including obesity, cardiovascular disease, and some forms of cancer.
Primarily a form of energy reserves and insulation in the body, fats can be burned to make energy when we need energy and are not getting enough from our diet. Fats are important in transporting other nutrients, such as vitamins A, D, E, and K—the “fat-soluble vitamins.” Fats are also an essential component of the cell membrane, and internal fatty tissues protect the vital organs from trauma and temperature change by providing padding and insulation. Fatty tissue, in fact, even helps regulate body temperature.
An important component of lipids is the fatty acids. Three essential fatty acids are needed biochemically by our bodies and are available to us only from our diet: linoleic acid (LA), arachidonic acid, and linolenic acid (LNA). All are commonly contained in plant oils. Some sources describe linoleic acid as the only true essential fatty acid, as the others may be made from it. With more recent research it appears that this is true in plants but not in humans. We can make arachidonic acid from linoleic acid but not the important linolenic acid. Actually humans need higher amounts of alpha-linolenic acid than the other fatty acids, and luckily, these three fatty acids are commonly found together in food sources anyway. Most of our dietary intake of fat is in the form of triglycerides, which are composed of three fatty acids and a glycerol molecule. A less prevalent form of dietary fat is the phospholipids, such as lecithin, which is important to cell membranes and the brain and nerves. Cholesterol, a member of the sterol family, is both found in foods and manufactured by the body. It is essential to many body functions but has been implicated as a primary factor in heart and blood vessel disease.
Levels of fat intake are highly correlated with weight. High consumption of dietary fat is associated with both increased body fat and obesity. Fats are the most concentrated source of food energy, supplying nine calories per gram, more than double the calorie content of the proteins and carbohydrates. They provide about 42 percent of the calories in the average American diet. A diet that derives closer to 20–25 percent of total calories from fat is probably healthier. A range of 10–20 percent is also acceptable and may be helpful in reducing the incidence and progression of cardiovascular disease by lowering blood levels of triglycerides and cholesterol. Reducing fat intake to this level means cutting down greatly on consumption of red meats and dairy products such as milk, cheese, and butter. Restricting dietary fat (which usually reduces calorie intake as well) while maintaining adequate protein and complex carbohydrate intake is probably the best long-range approach to weight loss, maintenance of optimal body weight, and general good health.
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.
The essential fatty acids (EFA) include linoleic, linolenic, and arachidonic acids, collectively termed vitamin F. They are all polyunsaturated fatty acids that cannot ordinarily be synthesized in the body; although, if sufficient quantities of linoleic acid (omega-6) are present, arachidonic acid can be made. Alpha-linolenic, an omega-3 fatty acid that is found in special oils such as linseed (flax), rapeseed (canola), and soybean, is also essential and is the precursor of other important omega-3 oils EPA and DHA commonly found in fish. Ideally we need more linolenic, about a 2:1 ratio, than linoleic. The essential fatty acids are important for normal growth, especially of the blood vessels and nerves, and to keep the skin and other tissues youthful and supple through their lubricating quality.
Linoleic acid is necessary for synthesis of prostaglandins in the E1 and E2 series; linolenic acid is the precursor of the E3 series and other omega-3 fatty acids. Prostaglandins, hormonelike substances, have various effects on smooth muscle and inflammatory processes. The E3 (PGE3) series seems to reduce cholesterol levels as well as platelet aggregation and thrombosis and are generally anti-inflammatory. Prostaglandin E2, related to arachidonic acid, tends to promote platelet aggregation, is more inflammatory, and may even be related to high blood pressure and cancer. PGE1 is mildly inflammatory as it prevents release of arachidonic acid from the cells. Safflower oil is particularly high in linoleic acid, as are sunflower and corn oils; other vegetable oils, nuts, and seeds are also good sources. As stated, soybean, flaxseed, canola, as well as pumpkin and walnut are the best sources of alpha-linolenic acid. (See Vitamin F, Chapter 5, Vitamins.)
Another fatty acid recently shown to be beneficial is eicosapentaenoic acid (EPA). It is a polyunsaturated, omega-3 fatty acid found in high concentrations in cold-water fish. Where it is consumed in large amounts, as among Greenland Eskimos or in fishing villages in Japan, there is reduced cardiovascular disease. EPA seems to reduce serum triglycerides, raise HDL (good) cholesterol, and prolong bleeding time by reducing platelet aggregation, thus preventing thrombosis. Fish such as mackerel, sardines, and salmon are high in eicosapentaenoic acid. Consumption of cold-water fish once or twice a week seems to have a positive effect on cholesterol levels. When taken as a supplement such as MaxEPA (there are many products now available), which contains EPA and another omega-3 fatty acid, decosahexaenoic acid (DHA), may decrease the risk of vascular thrombosis and cardiovascular disease. (See more on EPA in Chapter 7, Accessory Nutrients.)
Phospholipids, of which the most common is lecithin, are important in the structure of all membranes. Their structure is similar to that of triglycerides, but they contain only two fatty acids (both polyunsaturated). The third molecule attached to the glycerol is a phosphatidylcholine molecule (choline is one of the B vitamins). Certain phospholipids also contain inositol (another B vitamin) as phosphatidylinositol, as well as phosphatidylethanolamine, another phospholipid that has several functions, such as being a precursor to choline and acetylcholine. Lecithin is found in highest concentration in soybeans and egg yolks. Recently, egg lecithin has been used in the treatment of acquired immune deficiency syndrome (AIDS). There is some question as to whether supplemental lecithin helps to lower cholesterol levels. It seems to have a mild influence, perhaps due to its polyunsaturated nature. Because of their carbohydrate-fat construction, phospholipids move well in fats as well as in water and thus move easily