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The word "lipid" is the biochemical term used to denote what is commonly called fat. Lipids are very important chemical substances; they include cholesterol, triglycerides, fatty acids, phospholipids, and sterols. Lipids provide a concentrated source of energy, delay the emptying of the stomach, stimulate the gallbladder to empty its contents, and provide the materials for the absorption of fat-soluble vitamins and other dietary substances. Lipids are by far the most important form of storage fuel we have, and they also act as insulation for the body.

Other functions and processes that lipids participate in are much more complex than those of either proteins or carbohydrates. Lipids provide the structure of the brain and all nerve tissue, and they are the major components of all cell membranes and the membranes of structures inside cells. As fatty acids and cholesterol esters they store energy in adipose tissue. When energy is needed, free fatty acids are released by the action of stress or epinephrine (adrenaline).

The body can make lipids from carbohydrates and amino acids if enough fats are not supplied from the diet. This is usually not the case in the American diet, though.

Adipose tissue is composed of 60 to 90 percent triglycerides. Triglycerides are one major group in the lipid family. The normal serum (blood) level of triglycerides ranges from 10 to 150 mg/ml. Triglycerides and cholesterol, another lipid, are of major clinical significance in the development of heart and vascular disease. Lipoproteins (proteins coupled to lipids) are important in relation to heart disease, also. The following terms are frequently used when discussing the causes of heart disease, colon cancer, and breast cancer: chylomicrons, very low and low-density lipoproteins, and high-density lipoproteins. Chylomicrons are large particles in which cholesterol and other lipids in the diet are transported from the intestine into the blood. Very low density lipoproteins (VLDLs) are proteins in the blood that transport triglycerides made in the body. VLDLs are broken down into low-density lipoproteins (LDLs), which are thought to cause atherosclerosis. High-density lipoproteins (HDLs) contain cholesterol and aid in the chemical reaction that modifies cholesterol. HDL is the most important of these proteins for our discussion because it correlates fairly well with heart disease.

For an analogy of HDL's importance and function, consider that each HDL is a street bus, each blood vessel a street, and each cholesterol molecule a person. Now consider that there is only one bus (one HDL) to pick up one hundred people (one hundred cholesterols) along the street. Well, obviously, not all the people can be transported by the bus, and many people have to stay in the street. In this situation, with only one HDL, the cholesterol molecules that were not picked up get embedded in the blood vessel and start to cause narrowing and hardening of the artery—atherosclerosis is the medical term. If there are two buses (two HDLs), then more people (cholesterol) can be transported and fewer are left behind to stay in the street (get embedded in the artery). If four buses are available, all the people can be transported and none are left behind (all the cholesterol is transported and none embed in the arteries). Cholesterol is crucial to the development and progression of atherosclerosis. The level of HDL is important only if the cholesterol level is high. With very high cholesterol (people), you would want to try to raise the HDL level (buses). The HDL protein can be raised by vigorous exercise, by administering small amounts of alcohol, and by decreasing substantially the total intake of cholesterol in your diet.

Cholesterol exists in all animal cells. Everyone requires cholesterol in correct amounts for good health, but too much can lead to the development of heart and blood vessel disease. It is a precursor substance needed for the manufacture of sex hormones and some adrenal hormones that regulate many phases of salt, sugar, protein, and fat metabolism in the body. Part of the cholesterol in your body comes from food of animal origin, and part is made in your body. Cholesterol content is very high in many foods, including eggs, milk products, and organ meats such as liver. It is moderately high in shellfish.

The normal cholesterol range is 150 to 200 mg/ml. One third of the U.S. male population 25 years old and older has serum cholesterols over 250 mg/ml and is at risk for coronary heart disease and vascular disease. There is no cholesterol in foods of plant origin—fruits, vegetables, grains, cereals, and nuts.

Fatty acids are part of the lipid family also. There are three types. First there are the saturated fatty acids, which consist predominantly of saturated fats hardened at room temperatures. Saturated fats tend to raise the blood cholesterol level and should be restricted or totally eliminated from your diet. Most Americans eat foods that are high in saturated fats and high in cholesterol, and therefore they tend to have high cholesterol levels. People who eat high-cholesterol, high-saturated-fat foods have a greater risk of having heart attacks than people who eat low-fat, low-cholesterol diets. Eating extra cholesterol and saturated fat increases blood cholesterol levels in most people, but there are wide variations among individuals due to inherited tendencies and the way in which the person uses cholesterol. It appears that some people can eat diets high in saturated fats and cholesterol and still maintain normal blood levels of cholesterol, while other people cannot. Unfortunately, some people have high blood cholesterol levels even though they eat low-fat, low-cholesterol foods.

Saturated animal fats are found in beef, lamb, pork, and ham; in butter, cream, and whole milk; and in cheeses made from cream and whole milk. Saturated vegetable fats are found in many hydrogenated and solid shortenings; and in coconut oil, cocoa butter, and palm oil (used in commercially prepared cookies, pie fillings, and nondairy milk and cream substitutes).

The other two groups of fatty acids are the monounsaturated fats, with a single unsaturated site, and the polyunsaturated fats, which have two or more unsaturated sites in the fat chain. An unsaturated site does not have hydrogen in it; if enough hydrogen were added, the fat would then become saturated or hydrogenated. Fats with a high proportion of unsaturated fat sites, such as cod liver oil, olive oil, and whale oil, are usually liquid at room temperatures. There is only one essential fatty acid: arachidonic acid. This can easily be made in the body from linoleic acid, which is widespread in nature. In reality, a deficiency of fatty acids is very rare in humans.

Polyunsaturated fats are liquid oils of vegetable origin. The following oils are high in polyunsaturated fats: corn, cottonseed, safflower, sesame seed, soybean, and sunflower seed. These lower the blood cholesterol level by eliminating newly formed excess cholesterol. Peanut oil and olive oil are vegetable oils and are low in polyunsaturated fats. Polyunsaturated fats can be attacked chemically by free radicals because they have empty or "unsaturated" chemical bonds that can "accept" the extra electron of the free radical. These free radicals can damage cells and lead to the development of cancer. There are, however, agents that neutralize free radicals—beta-carotene, vitamin E, vitamin C, and selenium, for example—that we can take daily to aid in our defense.

All fats, then, whether saturated or unsaturated, should be severely restricted or eliminated to reduce your risk of developing cancer as well as heart disease. The next chapter will discuss how certain nutrients can help build a strong immune system and protect against cancer and other illnesses.