ISBN 0070349223
Fats - or lipids, the chemical name - are the most concentrated source of energy in the diet. There are three classes of lipids: triglycerides, phospholipids, and sterols.
When oxidized, fats furnish more than twice the number of calories per gram than carbohydrates or proteins. One gram of fat yields approximately 9 calories to the body. Fats give about 60% of the body's energy requirement during rest. The function of fat to the body is vital, but too much can be a problem. The diet must be designed to include the essential amounts and avoid any excess that may lead to future health problems.
Over the course of time, nature has ensured that a constant reserve of energy is made available for our needs. One of these provisions is fat that is stored in the liver and muscles in a form called glycogen. When needed, enzymes will break down the glycogen to glucose, which is then ready to work for the body. Another provision is fat that is stored under the layers of the skin and throughout the body and issued when the liver's stores are depleted. Unlike the liver, the body is able to accumulate an unlimited amount of fat that can serve as energy to all the body's cells for as long as the reserve allows.
The substances that give fats their different flavors, textures, and melting points are known as fatty acids. When energy-yielding nutrients store their fats, they become fatty acids. They are the most obvious of fats in both the diet and the body. They are found in meats as skin on poultry and the white part of red meat, and on top of pizza and soups. The fat that accumulates in the body to make us look chubby is made up of triglycerides (three fatty acids attached to a glycerol). There are many combinations of triglycerides, and each animal species carries its own characteristic kinds. Animals that are raised for food can be fed different kinds of feed that dictate how hard or how soft their fat will be.
Fatty acids differ in two ways: in chain length and in saturation. Chain length has to do with absorption, and saturation refers to saturated or unsaturated fatty acids. In fish and most plant sources, there are points in the chemical attachment that are missing and the fatty acids are then called unsaturated and are noted on food labels as UFA. When only one point is missing, it is monounsaturated and is noted on food labels as MFA, and if two are missing, the label is polyunsaturated or PUFA.
Unsaturated or soft (short chain) fatty acids, including monounsaturates and polyunsaturates, are usually liquid at room temperature, and come from the oils of vegetable, nut, or seed sources such as corn, canola, safflower, sunflower, and olive. Saturated fatty acids (long chain) are those that are usually hard at room temperature and, except for coconut or palm oil, come from primarily animal sources. Vegetable shortenings and margarines have undergone a process called hydrogenation in which unsaturated oils have converted to a more solid form of fat. These fats are then called "trans" fatty acids. Other sources of saturated fat are milk products, eggs, and cheese.
There are three "essential" fatty acids. The human body can synthesize all the fatty acids it needs from carbohydrates, fats, and proteins from the diet except for linoleic, linolenic, and arachidonic acids. These polyunsaturated fatty acids cannot be made from the breakdown of other substances in the body; they must be supplied by the diet and are therefore called essential. They are necessary for normal growth, and for healthy blood, arteries, and nerves. Arachidonic acid can be synthesized by linoleic acid when it is supplied to the body in the diet.
Linoleic acid belongs to the family of polyunsaturated fatty acids known as the omega 3 fatty acids, found primarily in fish and flax seed oils. The omega 3's have researchers studying growth and development, visual function, cancer development, blood clotting, arthritis, heart disease, and hypertension. The omega 3's keep the skin and other tissues youthful and healthy by preventing dryness and scaliness. Darker-fleshed fish are good sources, as is flax oil.
Linoleic acid is an omega 6 fatty acid and is found in land sources such as in seeds of plants and in oils produced by the seeds. Discoveries of prostaglandins (harmful agents) in the omega 6's has researchers concerned about too much consumption of them. Vegetable fats such as corn, safflower, and soybean oil are high in linoleic acid. Researchers have long known of the importance of the omega 6 fatty acid family. These essential fatty acids are necessary for the transport and breakdown of cholesterol. Both of the omegas should be taken in combination, as a balance is essential. Canola oil has the best of both.
If supplements of one omega is taken, a deficiency of the other may be created. People are rarely deficient in these fatty acids, which are readily available in common foods, both plant and fish.
The phospholipids are the second class of fats; like the triglycerides, they have a backbone of glycerol. Lecithin is the most well known member of this class. These fatty acids have only two fats attached to them (for lecithin a third is choline), are a major part of the cell membrane, and are essential to life and therefore valuable nutrients (see Choline in Section IV). Lecithin can be made from scratch by the liver and is therefore not an essential nutrient. Lecithin has been found to decrease excessive levels of cholesterol in some individuals.
The third class of fats are the sterols, of which the most famous is cholesterol, but they also include vitamin D and the sex hormone testosterone. The sterols are large, complex molecules consisting of interconnected rings of carbon that are essential for all sexuality. Cholesterol is a fat-related substance that is essential for good health. It is a normal component of most body tissues, especially those of the brain, nervous system, liver, and blood. More than nine-tenths of cholesterol ends up in the cells, where it performs vital structural and metabolic functions. The unused parts may be harmful in some people, and for this reason high-cholesterol foods containing animal fats should be avoided.
Cholesterol foods from other sources like liver and eggs appear to be less harmful but should still be used in moderation, especially for those with family histories of coronary artery disease and/or those who have high LDL cholesterol levels. Each body handles cholesterol in its own manner. Testing of both kinds to find each personal level is important.
Some cholesterol is changed into compounds such as vitamin D, and the rest has three purposes: to aid in digestion and absorption of fat (in the form of bile in the liver); to be circulated for structural cell maintenance; and to be packaged into lipoproteins. There are two kinds of lipoproteins: the good kind (HDL) that help bring cholesterol out of the body, and the bad kind (LDL) that at high levels will clog the arteries and leave the vascular system at risk for a number of diseases and disorders. Like lecithin, cholesterol can be made by the body (2.8 grams daily) whether it is eaten or not, so it is not an essential nutrient. Only 25% of cholesterol in the blood is from food. The raw materials that the liver makes cholesterol from are carbohydrates and fats.
In addition to supplying energy and providing valuable nutrients to the body, fats act as carriers for the fat-soluble vitamins, A, D, E, and K. Removing fat from foods also removes these vitamins. By aiding in the absorption of vitamin D, fats help make calcium available to body tissues and to the bones and teeth. Fats are also important for the conversion of carotene to vitamin A. Fat deposits surround, protect, and hold in place organs such as the heart and liver. Hard deposits under the kidneys protect them from being jarred. The mammary glands in the breast are also protected by fats. A layer of fat insulates the body from environmental temperature changes and preserves body heat. This layer also rounds out the contours of the body. Fats prolong the process of digestion by slowing down the stomach's secretions of hydrochloric acid; thus fats create a longer-lasting satiety or sensation of fullness after a meal.
Fats are beneficial for other reasons as well. Natural oils provide a healthy complexion and nourish the scalp for shiny hair. Our muscles work because the fat that is laced between the fibers carries a constant reserve of energy.
If fats are eaten excessively, abnormal amounts of cholesterol may be stored throughout the body, contributing to atherosclerosis. Many current recommendations suggest that excess saturated fat and cholesterol be avoided. Many ailments are linked to excessive intake of dietary fat or to excess body fat. If more calories are consumed than are needed by the body, weight gain occurs, which contributes to obesity, diabetes, hypertension, cancer, and probably other diseases and disorders. In addition, excessive fat intake will cause abnormally slow digestion and absorption, resulting in indigestion.
Although a fat deficiency rarely occurs in humans, such a deficiency would lead to loss of the fat-soluble vitamins, eczema, or other skin disorders. If fat is extremely deficient, severely retarded growth would result. If a lack of carbohydrates is accompanied by a lack of water in the diet, or if there is a kidney malfunction, fats cannot be completely metabolized and may become toxic to the body.
Fat and fat-containing foods should be stored in covered containers, away from direct light and in a cool place to prevent rancidity caused by oxidation. Some protection from rancidity will be provided by vitamin E, a fat-soluble vitamin that is a natural antioxidant and is present in most fat-containing foods.
Dietary guidelines recommend, that total fat intake not exceed 30% of the day's total energy intake. Saturated fat should not be more than 10%, both the unsaturates and polyunsaturated not less than 10%, and the monounsaturates providing the remaining 10% or so. Guidelines for cholesterol generally suggest an upper limit of 300 milligrams daily; however, this will vary from person to person. Linoleic and linolenic acid food sources should also be included.
to the top of this page.