Lipids are organic substances consisting mostly of carbons and hydrogen atoms . They are hydrophobic, which means that they have little or no affinity to water. All lipids are soluble (or dissolvable) in nonpolar solvents, such as ether, alcohol, and gasoline. There are three families of lipids: (1) fats, (2) phospholipids, and (3) steroids.
Fatty acids and glycerol make up the larger molecule of fats. A fatty acid consists of a long carbon skeleton of 16 or 18 carbon atoms, though some are even longer. The carbonyl group, which is a carbon atom double-bonded to an oxygen atom and single-bonded to an oxygen attached to a hydrogen (OH-C=O), is the acidic group of the fatty acids. The acidic property is determined by the ability of the hydrogen to dissociate, or break away, from the oxygen atom. The carbonyl group is followed by a long chain of carbon atoms bonded to hydrogen, which is referred to as the hydrocarbon "tail." The long hydrocarbon tail gives fatty acids their hydrophobic, or "water-fearing" property. Fats cannot be dissolved in water because fats are nonpolar (an equal distribution of electrons) and water is polar (an unequal distribution of electrons). The polarity of water is unable to form bonds and break down the nonpolar fatty acid molecule.
There are different types of fatty acids, which vary in length and the number of bonds. Saturated fatty acids have single bonds between the carbon atoms that make up the tail. The carbon atoms are "full" or saturated, and therefore cannot take up any more hydrogen. Most animal fat, such as butter, milk, cheese, and coconut oil, are saturated. Unsaturated fatty acids have one or more double bonds between carbon atoms. A double bond is the sharing of four electrons between atoms, while a single bond is the sharing of two electrons. The double bond has the ability to lend its extra two electrons to another atom, thereby forming another bond. Monounsaturated fatty acids contain only one double bond, such that each of the carbon atoms of the double bond can bond with a hydrogen atom. An example of monounsaturated fatty acids is oleic acid, which is found in olive oil. Polyunsaturated fatty acids contain two or more double bonds, such that four or more carbon atoms can bond with hydrogen atoms. Most vegetable fats are polyunsaturated fatty acids. The double bonds change the structure of the fatty acid, in that there is a slight bend where the double bond is located.
Foods high in saturated fatty acids include whole milk, cream, cheese, egg yolk, fatty meats (e.g., beef, lamb, pork, ham), coconut oil, regular margarine, and chocolate. Foods high in polyunsaturated fatty acids include vegetable oils (e.g., safflower, corn, cottonseed, soybean, sesame, sunflower), salad dressing made from vegetable oils, and fish such as salmon, tuna, and herring.
Triglycerides are the basic unit of fat and are composed of three ("tri-") fatty acids individually bonded to each of the three carbons of glycerol. Fatty acids rarely exist in a free form in nature because they are highly reactive, and therefore make bonds spontaneously.
Fat Function, Metabolism, and Storage
Fats and lipids play critical roles in the overall functioning of the body, such as in digestion and energy metabolism. Usually, 95 percent of the fat in food is digested and absorbed into adipose, or fatty, tissue. Fats are the body's energy provider and energy reserve, which helps the body maintain a constant temperature. Fats and lipids are also involved in the production and regulation of steroid hormones , which are hydrophobic (or "water-fearing") molecules made from cholesterol in the smooth endoplasmic reticulum, a compartment within a cell in which lipids, hormones, and proteins are made. Steroid hormones are essential in regulating sexuality, reproduction, and development of the human sex organs, as well as in regulating the water balance in the body. Steroid hormones can also freely flow in and out of cells, and they modify the transcription process, which is the first step in protein synthesis, where segments of the cell's DNA , or the genetic code, is copied.
Fats and lipids also have important structural roles in maintaining nerve impulse transmission, memory storage, and tissue structure. Lipids are the major component of cell membranes. The three most common lipids in the membranes of eukaryots, or nucleus-containing cells, are phospholipids, glycolipids, and cholesterol. A phospholipid has two parts: (1) the hydrophilic ("water-loving") head, which consists of choline, phosphate, and glycerol, and (2) the hydrophobic ("water-fearing") fatty acid tail, which consists of carbon and hydrogen. The hydrophilic head is the part of the phospholipids that is in contact with water, since it shares similar chemical properties with water molecules. The hydrophobic tail of the phospholipids faces inward, and therefore is able to avoid any contact with water. In this particular arrangement, the phospholipids arrange themselves in a bilayer (double layer) alignment in aqueous solution.
Fats are metabolized primarily in the small intestines because the enzymes of the stomach cannot break down fat molecules due to their hydrophobicity. In the small intestines, fat molecules stimulate the release of cholecystokinin (CCK), a small-intestine hormone, into the bloodstream. The CCK in the blood triggers the pancreas to release digestive enzymes that can break down lipids. The gallbladder is also stimulated to secrete bile into the small intestines. Bile acids coat the fat molecules, which results in the formation of small fat globules, which are called micelles. The coating prevents the small fat globules from fusing together to form larger fat molecules, and therefore the small fat globules are more easily absorbed. The pancreatic enzymes can also break down triglycerides into monoglycerides and fatty acids. Once this occurs, the broken-down fat molecules are able to diffuse into the intestinal cells, in which they are converted back to triglycerides, and finally into chylomicrons.
Chylomicrons, which are composed of fat and protein, are macromolecules that travel through the bloodstream into the lymphatic capillaries called lacteals. The lymphatic system is a special system of vessels that carries a clear fluid called lymph, in which lost fluid and proteins are returned to the blood. The lacteals absorb the fat molecules and transport them from the digestive tract to the circulatory system, dumping chylomicrons in the bloodstream. The adipose and liver tissues, which release enzymes called lipoprotein lipase, break down chylomicrons into monoglycerides and fatty acids. These molecules diffuse into the adipose and liver cells, where they are converted back to triglycerides and stored as the body's supply of energy.
The energy value of fats is 9 kcal/gram (kilocalories per gram), which supplies the body with important sources of calories. Calories are units of energy. The breaking of bonds within fat molecules releases energy that the body uses. A kilocalorie is the unit used to measure the energy in foods. It is the equivalent of "calories" listed on Nutrition Facts labels on food packaging.
Some of the foods known to contain large amounts of fat include the obvious examples, such as butter on toast, fried foods, and hamburgers. But many of the foods that people consume on a daily basis have hidden sources of fat that may not be obvious to the person eating them. These foods include cookies and cakes, cheese, ice cream, potato chips, and hot dogs. One way to avoid foods that contain high amounts of fat is to look at the Nutrition Facts label located on the packages of most foods, where the total fat content of the food is listed.
Actual intake of fat can vary from 10 percent to 40 percent of the calories consumed daily, depending on personal or cultural regimens. Limiting one's daily fat intake to less than 30 percent of total calorie intake and increasing consumption of polyunsaturated fatty acids have been shown to be beneficial in maintaining a healthful diet .
Effects of Excess Dietary-Fat Intake
The recommended intake of fats in the American diet is to limit fats to below 30 percent of the total daily caloric intake. One-third of fats should come from saturated fats, with the other two-thirds split evenly between monounsaturated and polyunsaturated fat. It is estimated that in the average American diet (as of 2002), fats make up 42 percent of calories, with saturated fat making up between a third and a half of that amount.
The effects of this excess intake of dietary fat has some well-established implications for the health of overweight Americans. For instance, the consumption of excess amounts of saturated fats has been recognized as the most important dietary factor to increase levels of cholesterol. A high cholesterol level is detrimental to health and leads to a condition known as atherosclerosis. Atherosclerosis is the build-up of cholesterol on the walls of arteries , which may eventually result in the blocking of blood flow. When this occurs in the arteries of the heart, it is called coronary artery disease. When this process occurs in the heart, a myocardial infarction, or heart attack , may occur.
Besides the cholesterol implications due to high fat intake, obesity is a factor in the causation of disease. Being overweight or obese is highly associated with increasing the risk of type II diabetes , gallbladder disease, cardiovascular disease, hypertension , and osteoarthritis .
The purpose of fat-replacement strategies is to reduce the percentage of fat in various foods, without taking away the appealing taste of the food. There are three broad categories of fat-replacement strategies: (1) adding water, starch derivatives, and gums to foods, (2) using protein-derived fat replacements, and (3) using engineered fats.
The addition of water to foods lowers the quantity of fat per serving in the selected food item. When starch derivatives are added to food, they bind
Protein-derived fat replacements are made from egg and milk proteins, which are made into a microscopic globule of protein. They give the sensation of fat in the mouth, although they contain no fatty acids. One such product is Simplesse, which is used mostly in frozen desserts. Because its chemical structure is easily destroyed by cooking or frying, its use is limited in most other foods.
The third fat-replacement strategy includes the use of engineered fats, which are made by putting together various food substances. One popular engineered fat is olestra, which is made by adding fatty acids to regular table sugar molecules (sucrose). This process results in a product that can neither be broken down in the digestive tract nor absorbed. It therefore cannot provide energy, in terms of carbohydrates or fatty acids, to the body. Olestra is the first engineered fat to be used in fried foods. It does have its drawbacks, however. Olestra can cause abdominal cramping, loose stools, and it can bind beneficial substances that are normally absorbed, such as the fat-soluble vitamins (vitamins A, D, E, and K) and carotenoids .
In addition to fat-replacement strategies, there are low-fat or fat-free versions of many foods on the market. Some products made to be low-fat or fat-free include milk, yogurt, some cheeses, and deli meats. As a general rule, products that claim to have reduced amounts of fat should conform to the following stipulations: (1) a product labeled "reduced-fat" must have at least 25 percent less fat than the normal product, (2) a "low-fat" product can have no more than three grams of fat per serving, and (3) a "fat-free" product most have less than 0.5 grams of fat per serving. But one does not always need to look for foods made to contain less fat than normal, as there are plenty of natural foods that contain very little fat, or no fat at all, including most fruits and vegetables. Other foods that fit into the category of low-fat or nonfat foods include egg whites, tuna in water, skinless chicken, and pasta.
Foods that are low in fat are important for a healthful diet. While fats are essential components for bodily function, excess consumption of fats can lead to health problems such as obesity and heart disease . A healthful diet therefore consists of balanced proportions of proteins, fats, and carbohydrates.
Campbell, Neil A., et al. (2000). Biology, 4th edition. San Francisco: Benjamin/Cummings.
Must, A., et al. (1999). "The Disease Burden Associated with Overweight and Obesity." Journal of the American Medical Association 282: 1523.
Robinson, Corinne H.; Weigley, Emma S.; and Mueller, Donna H. (1993). Basic Nutrition and Diet Therapy, 7th edition. New York: Macmillan.
Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. Boston: McGraw-Hill.