Fats are organic compounds that are found in both plants and animals. They are a very concentrated source of energy, providing 9 kcal of energy per gram which is more than twice the yield of energy from either protein or carbohydrates (4 kcal of energy/gm). One reason that fats are a very concentrated source of calories are because they do not mix with water, therefore water is unable to dilute fatís calories as it does with protein and carbohydrate (Morrill, 1994). A typical example of this is chicken soup where one can see the fat floating on top of the water portion of the stock. Since fats are composed mainly of carbon and hydrogen they are called hydrocarbons versus carbohydrates which are comprised mainly of carbon and H2O (hydrate=water). Therefore, unlike carbohydrates, fats contain very little oxygen.
Besides providing us with a concentrated source of energy, the body can very readily store that energy as fat if it is not immediately needed. In fact, our bodies convert excess protein and carbohydrates into body fat as well, since protein cannot be stored and carbohydrates are minimally stored. This promotes one of the many functions of fat which is insulation with respect to warmth as well as acting as a cushion for the vital organs. However, when too much fat is stored, obesity can result.
Another important function of fats is that they make it possible for the transport and absorption of the fat soluble vitamins (A, D, E, and K). to occur. Diets very low in fats can cause fat soluble vitamin deficiencies. Fat also provides many desirable properties to foods such as longer satiety (because fats slow gastric emptying), good "mouth feel" or smooth texture, and it carries fat soluble flavors.
Fats are also an essential component of cell membranes and provide them with fluidity and elasticity. This is known as the structural nature of fat and most membrane fats are phospholipids and cholesterol (Bauman and Blazey, 1994). In addition, fats are necessary for the formation of hormones, lipoproteins and bile salts.
There are a variety of fats including triglycerides, phospholipids and sterols. However. triglycerides comprise approximately 95% of the fat in food and is the storage form of fat in the body. Triglycerides are made up of three fatty acids (tri) and one glycerol molecule (glyceride). Glycerol is a short-chained carbohydrate that is water soluble. Glycerol is converted to glucose once triglycerides are metabolized in the body. Fatty acids are responsible for the "greasy" nature of fats and they are what determine the specific properties of a fat. Because glycerol is a constant factor and never changes, it is the fatty acid portion of the triglyceride that accounts for the differences between triglycerides.
A fatty acid consists of a chain of carbon atoms that is water insoluble with an acid group at one end which is water soluble and called a carboxyl group (-COOH). What makes the fatty acids differ from one another is the number of double bonds between the carbons. The shorter the chain, the more water soluble the fatty acid. In fact, acetic acid (vinegar) which is actually a fatty acid, has 2 carbons and is very water soluble. Most dietary fatty acids are usually 16-18 carbon molecules and are attached to hydrogen molecules. Those that donít have any double bonds between the carbons are called saturated fatty acids.
Saturated fatty acids have all the positions for hydrogen filled, thus they are saturated with hydrogen. Most of the saturated fats are found in animal products and are hard at room temperature such as lard and butter. Coconut and palm oils are saturated fats from plants. Because these fatty acids are fully hydrogenated, they cannot be oxidized easily and therefore have a longer shelf life since they donít become rancid (oxidized). Saturated fats increase blood cholesterol levels and blood triglycerides, both of which are associated with a higher coronary risk.
There are two classifications of unsaturated fatty acids, monounsaturated and polyunsaturated (PUFA). These fatty acids are formed when one or more double bonds are inserted between two carbons by removing 2 hydrogen atoms. Unsaturated fatty acids may have from one to six double bonds in their chain and these in turn determine the properties and biological functions of the fatty acids themselves (Erasmus, 1993).
Monounsaturated fats have one double bond and can accept one more pair of hydrogen atoms, thus mono unsaturated. The classic example of a monounsaturated fatty acid is oleic acid which is present in olive oil. Monounsaturated fats are liquid at room temperature but will solidify when cold. They do not produce as much cholesterol in the body as saturated fats, but they do go rancid sooner than the saturated fats (because they can still accept a pair of hydrogen atoms).
Polyunsaturated fatty acids have 2 or more double bonds in their carbon chains where hydrogen can be added. PUFAs are always liquid at room temperature and are the most unstable of the fatty acids meaning that they go rancid more easily. The more double bonds present, the more they can be "attacked" by oxygen. Therefore it is wise to keep polyunsaturated fats away from oxygen, light and heat. One example of a PUFA containing food is fish, which makes sense when one thinks that their fat containing tissues must remain supple in cold water environments (Morrill, 1994). Vegetable oils such as safflower, soybean and corn are also polyunsaturated.
Hydrogenated fats are those that have been altered by food manufacturers who want to prolong the life of unsaturated fats. Via a process of using high pressure, high temperature and a catalyst, hydrogen atoms saturate the carbons at the double bond locations. When this chemical reaction occurs, the physical form of the PUFU is altered to a "trans" form (the natural form is a "cis" form) and the body metabolizes the fat differently. Margarine is the classic example of a transformed fat. It is now known that these trans forms can cause cancers, alter cell membranes, precipitate gallstones and change cholesterol ratios (Bauman and Blazey, 1994).
Another group of fatty acids are called essential fatty acids or EFAs. These are polyunsaturated fats that cannot be manufactured in the body and are therefore essential. The essential fatty acids are lineoleic or omega 6 and linolenic or omega 3. Omega refers to the location of the first double bond counting from the CH3 end of the fatty acid. Therefore, lineleic acid which is a chain of 18 carbons and 2 double bonds has its first double bond located between the 6th and 7th carbon (omega 6). The body can add double bonds only beyond the omega or first double bond. Therefore, different products are manufactured in the body from omega 3ís versus omega 6ís (Morrill, 1994). That is why hormones made from omega 3ís inhibit blood clotting and inflammation whereas those from omega 6ís promote blood clotting and inflammation.
Many of the functions and products of EFAs are still being discovered. It is known that they serve as the precursors for prostoglandins, thromboxanes and other hormones having to do with clotting and inflammation. They are also the building blocks of the bodyís cell membranes. EFA deficiencies have been shown to cause reduced growth rates, infertility, skin scaliness, kidney abnormalities, abnormal liver function, decreased immune function, decreased myocardial contractility and decreased prostaglandin production (Pinscher and Vergroesan, 1994). They also play a role in brain and eye function but the exact nature of that function is unknown.
Although EFAs are by definition essential to our diet, we only require approximately 2-3% of our total calories from these acids (Heinz, 1991). Food sources high in linoleic acid are safflower, sunflower and corn oils as well as nuts and seeds. Rich sources of linolenic acid are cold water fish, soybeans, flaxseed, canola oil, pumpkin and walnuts.
The fats discussed above are triglycerides, but as previously mentioned, phospholipids and cholesterol are the two other types. Phospholipids are very similar to triglycerides in their structure, but instead of 3 fatty acids they have 2. One of the fatty acids is replaced by a phosphorous containing molecule which can vary just as the fatty acids can vary. Lecithin is the most common phospholipid in the body and in food. Its phosphorous molecule is phosphatidylcholine which contains choline, a B vitamin. Lecithin is known as an emulsifier because it can suspend small particles of fat in a watery medium. The fatty acid portion of the phospholipid serves as the fat soluble portion of lecithin and the phosphorous containing portion is water soluble.
In the body, phospholipids form the bulk of cellular membranes because of the same chemical properties that make them good emulsifiers. Other functions of phospholipids are to enable nerve impulse conduction, attach to enzymes to help them function, and to transport fatty substances across membranes.
The third main class of lipids are sterols which include cholesterol, phytosterols and some steroid hormones. Cholesterol is made mainly in the liver, but all tissues of the body except the brain can manufacture it. It is also found in most animal foods. Cholesterol is the precursor for steroid hormones (sex), the adrenal corticosteroid hormones, and bile salts. It is an essential part of every cell membrane and is found in especially high quantities in the blood and liver, nerve and brain tissues.
Lipoproteins are fat-protein combinations that circulate in the blood and tissues. The important lipoproteins are chylomicrons, VLDLs (very low density lipoproteins), LDLs (low density lipoproteins) and HDLs (high density lipoproteins). Chylomicrons transport triglycerides into the circulation which are then carried to the liver and other organs. VLDLs are manufactured in the liver and intestines to carry fats throughout the body. LDLs made by the liver, are the main transporters of cholesterol in the blood to the organs and cells. HDLs circulate in the blood and carry already used or unused cholesterol back to the liver for recycling. Essentially, the HDLs clean the blood of cholesterol and therefore may help to lower cardiac risk.
The digestion and absorption of ingested fats occurs primarily in the intestines. The presence of fat in the stomach triggers a delayed gastric emptying and also slows the digestion of other foods in the stomach. Once the fats enter the intestine, bile secretion is triggered and the bile salts emulsify the fat so that enzymes can then break the fats apart. These enzymes are called pancreatic lipases and phospholipase which separate the fatty acids from the glycerol.
Once the fats are broken down, they are absorbed according to their chain length. The short chain fatty acids (less than 12 carbons ) are more hydrophilic and are therefore absorbed directly through the cell membranes of the small intestine villi. Once in the villi, they are transported through the bloodstream to the liver.
The longer chained fatty acids with 13 or more carbons as well as the mono- and diglycerides have to be converted back to triglycerides in the intestinal wall. These are then surrounded by a layer of protein and are called chylomicrons which are transported first to the lymph circulation and then to the liver via the bloodstream. If one eats a meal heavy in fats, the chylomicrons will turn the blood a milky color; this will disappear in about 4 hours.
The liver is the primary site of fat metabolism. Once in the liver, the chylomicrons can be converted into other fats, or made into lipoproteins, phospholipids or glycolipids (carbohydrates plus lipids). Individual cells can extract triglycerides from lipoproteins and use the fatty acids for energy. If energy is not needed, the excess fat is stored in the fat cells.
Overall, although fats have earned a bad reputation, they are just as necessary as proteins and carbohydrates for our bodyís effective functioning. Although we have major health concerns related to fat such as obesity, heart disease, etc., these are more a function of excess fat intake (>30%). The ideal fat consumption should be from 10-20%, depending on the energy needs of the individual (people living in colder climates usually need more). The goal of most persons living in countries in which there is abundant food should be to reduce saturated fat intake to less than 10%, consume more fiber and plant foods, eat less animal products and eat organic, whole foods as much as possible.
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