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Fats: What are They?

Dietary Fats

Fats are nutrients that provide the body's most concentrated form of energy: one gram of fat provides 9 calories whereas one gram of carbohydrate provides four to five calories. They are the source of vitamins A, D, E, and K, and certain essential fatty acids which cannot be produced within the body. Fats are necessary for energy storage, cell membrane stability, fat transport, and hormone production

Once in the intestine, dietary fats are broken down by bile into fatty acids and glycerol. These products are further broken down by Lipase, an enzyme produced by the pancreas.

Triglycerides and Other Lipids

There are many different fats (lipids) in the body. Chemically, they can be separated into three groups:
  1. simple lipids (triglycerides),
  2. complex lipids, and
  3. sterol.

Simple lipids or triglycerides are the most common lipids in the body. They are composed of glycerol, which is an oily alcohol, and fatty acids. Triglycerides are formed when a molecule of glycerol combines with one, two, or three fatty acid molecules to form a monoglyceride, diglyceride, or triglyceride, respectively. This ability to form in any one of three combinations earns them the name of triglycerides. They serve as energy storage and function in cell membrane stability.

Complex lipids or phospholipids are a combination of glycerol with two fatty acids and a phosphate group. The difference in chemical structure makes phospholipids more water-soluble and ideal for the double-layered membrane of all cells. They are extremely important in providing cell membrane structure and stability.

Sterols, of which cholesterol is an example, are quite different in structure from the triglycerides and phospholipids. Hormones and vitamins are formed from this group and they too are important constituents of plasma membranes.

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Saturated and Unsaturated Fats

Fatty acids can be saturated or unsaturated. Both have long chains of hydrogen and carbon molecules, with a carboxyl group (COOH) at one end. To your right, is Capric acid (1), which is a saturated fatty acid and is found in coconut oil. Each carbon atom in the CH2 chain forms four single covalent bonds to other atoms: one each to two carbon atoms and one each to two hydrogen atoms. This particular arrangement of the hydrocarbon chain permits the maximum number of hydrogen atoms on the carbon skeleton. Thus, it is described as being "saturated" with hydrogen.

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To your left, is Oleic acid (2), which is an unsaturated fatty acid and is found in olive oil. It has one double bond between two of the carbon atoms. The double bond replaces two hydrogen bonds. This hydrocarbon chain no longer has the maximum possible hydrogen atoms. It is described as "unsaturated".

The unsaturated fatty acids may be monounsaturated, having one double bond, or "polyunsaturated", containing many double bonds. Unsaturated fatty acids have a lower melting point than saturated fatty acids. Stearic acid, which is saturated and is found in lard and peanut oils, melts at 70 degrees centigrade whereas monounsaturated Oleic acid melts at 13 degrees C. The more polyunsaturated, the lower the melting point. Linolenic acid, which is polyunsaturated with three double bonds, melts at 10 degrees C.

Fats with a melting point below room temperature are called oils. By adding more hydrogen to oils, in the presence of a catalyst, the double bonds change to single bonds. The oil then becomes almost hyrogen-saturated or hydrogenated. It is made more solid. Table margarine is prepared by hydrogenation of certain fats and oils to which coloring, flavoring , and vitamins are added.

Lipoproteins

As fat is insoluble in water, lipids must be encased within fat-carrying proteins for transport in the bloodstream. These are called lipoproteins, and are better known by their five different class names:
 1. chylomicron,
 2. very low-density lipoprotein (VLDL),
 3. intermediate-density protein (IDL),
 4. low-density lipoprotein (LDL), and
 5. high-density lipoprotein (HDL).

The "density" classification results from the settling behavior after a test tube of plasma is spun in an ultracentrifuge. The denser particles in plasma, such as large proteins, settle at the bottom of the tube. Less dense particles, such as fats, float closer to the top. The different proportions of fat and protein within each lipoprotein account for the different stratification patterns. Thus, LDL, which contains more lipid than HDL, will be found closer to the top.

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While proportions of fat and protein differ among the different classes of lipoprotein, their structure is basically the same. A central core containing cholesterol and triglycerides is encased in a layer of cholesterol, phospholipids, and one or more proteins. The molecules in this outer layer are precisely arranged to maintain solubility in the watery plasma. The lipoprotein to the right is LDL. It has a central core of cholesterol.

Cholesterol

Cholesterol is necessary for hormones and cell membranes; however, high levels of cholesterol increase the risk of atherosclerosis. The major carrier of cholesterol in the bloodstream is LDL.  Approximately 60-75% of plasma cholesterol is contained in LDL. Thus, LDL is a good indicator of cholesterol levels in most people. HDL is thought to play a role in inhibiting cellular uptake of LDL and in removing some of the cholesterol in tissue cells. Thus, higher levels of HDL are associated with a cardioprotective effect.

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Links of interest:

How Risk Factors Cause Coronary Heart Disease
Elevated Cholesterol
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