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Fats |
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Dietary fats are efficient energy storage units in the body. Fat molecules are combinations of glycerol and fatty acids. Dietary fats are stored for long term use by the body in adipose cells and tissues, called body fat. Body fat also provides organ support and insulation from physical shock and temperature changes. Dietary fats transport fat-soluble (and water-insoluble) vitamins A, D, E and K through the blood to cells. Dietary fats usually come from animals, such as cattle and pigs, or from animal products, such as milk and cheese. Vegetable oils are fats in liquid form at room temperatures. Vegetable oil is liquid fat extracted from plant sources. Although in principle several parts of plants may yield oil, in practice seeds are almost the exclusive sources. Vegetable oils are used as cooking oils and for industrial uses. In the mid-1970s, palm oil was made into a stable fat. It eventually replaced animal fat to give foods a longer shelf life. Critser 14 Fats belong to a subclass of lipids called triglycerides. Animal fats have more saturated fatty acids and less unsaturated fatty acids than vegetable oils. Saturated fats are generally considered unhealthy because of their association with high cholesterol levels in the blood. Animal fat contains some cholesterol, but they also stimulate cholesterol production in humans; therefore, animal fat contributes in two ways to cholesterol levels. Dietary fats belong to class of molecules called lipids (In the diagram, the hydrophilic end attracts water; the hydrophobic end avoids water,), which are hydrocarbon-containing organic compounds that are insoluble in water and soluble in nonpolar organic solvents such as ether. Lipids constitute structural components of biological membranes, provide energy reserves, predominantly in the form of triglycerides, and act as vitamins and hormones. Wiki n.p. Lipid Groups: Wiki n.p.
Fatty acids are the basic components of all glycerides. They are polymers with 4 to 24 carbon atoms. At one end of the fatty acid is a carboxyl group and at the other end is a methyl group. Fatty acids and the fats made from them are of 2 significant types: saturated and unsaturated. Saturated fatty acids do not contain any double carbon bonds along the chain. The term "saturated" means that all carbons (apart from the carboxylic acid [-COOH] group) contain as many hydrogens as possible. Saturated fatty acids form straight chains and, as a result, can be packed together very tightly, allowing organisms to store chemical energy densely. Unsaturated fatty acids have one or more carbons double-bonded. When required, the liver converts fatty acids to glucose for celll energy in a chemical reaction called gluconeogenesis. Unsaturated fatty acids are of 2 types: A cis configuration means that the two hydrogen atoms are on the same side of the double carbon bond. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend. The more double bonds the chain has in the cis configuration, the more bent it is. When a chain has many cis bonds, it becomes curved. A trans configuration, by contrast, means that the two hydrogen atoms are on opposite sides of the double carbon bond. As a result, they don't cause the chain to bend much, and their shape is similar to the straight saturated fatty acids. In most naturally occurring unsaturated fatty acids are cis bonds (cis fats). Most fatty acids in the trans configuration (trans fats) are unnatural and the result of food processing. The differences in geometry between these various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in the human body. The majority of fatty acids are acquired in the diet. However, the lipid biosynthetic capacity of the body (fatty acid synthase and other fatty acid modifying enzymes) can supply the body with most fatty acid structures needed. Two exceptions to this are the highly unsaturated fatty acids know as linoleic (omega-3) acid and linolenic (omega-6) acid. These two fatty acids cannot be synthesized in the body, and are thus considered essential fatty acids; that is, they must be provided in the diet. Since plants are capable of synthesizing linoleic and linolenic acid, humans can aquire these fats by consuming a variety of plants or else by eating the meat of animals that have consumed these plant fats. Glycerol is an important component of triglycerides (i.e. fats and oils) and of phospholipids, which form a part of cell membranes. Glycerol is a three-carbon alcohol that combines with fatty acids by a chemical reaction called hydrolysis to form triglycerides. When the body uses stored fat as a source of energy, glycerol and fatty acids are released into the bloodstream. When required for cell energy, glycerol is converted to glucose by the liver in a chemical reaction called gluconeogenesis. Glycerides are formed from glycerol and fatty acids. Glycerol has three hydroxyl functional groups with one, two or three fatty acids to form monoglycerides, diglycerides and triglycerides. Vegetable oils and animal fats contain mostly triglycerides, but are broken down by natural enzymes called lipases into mono- and diglycerides and free fatty acids in the process of digestion. A monoglyceride is a glyceride consisting of one fatty acid chain covalently bonded to a glycerol molecule. A diglyceride is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule. A triglyceride is a glyceride in which the glycerol is bonded with three fatty acids. Triglycerides are the main constituent of vegetable oils and animal fats. Triglycerides are important energy sources. They contain more than twice as much energy (9 Cal/g) as carbohydrates and proteins. In the intestine, triglycerides are split into glycerol and fatty acids by a process called lipolysis, which can then move into blood vessels. The triglycerides are rebuilt in the blood from their fragments and become constituents of lipoproteins, which deliver the fatty acids to and from fat cells. Various tissues can release the free fatty acids and take them up as a source of energy. Fat cells can synthesize and store triglycerides, since they are insoluble in water. When the body requires glycerol as an energy source, the hormone glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. Because the brain cannot use glycerol as an energy source, the glycerol component of triglycerides is converted into glycogen for brain fuel. Wiki n.p. Phospholipids are made from glycerol, two fatty acids, and (in place of the third fatty acid) a phosphate group. The hydrocarbon tails of the fatty acids are hydrophobic, but the phosphate group end of the molecule is hydrophilic because of the oxygens with all of their pairs of unshared electrons. This means that phospholipids are soluble in both water and oil. Cell membranes are made mostly of phospholipids arranged in a double layer with the tails from both layers inside (facing toward each other) and the heads facing outside (toward the watery environment) on both surfaces. UC n.p. Phospholipids, e.g., lecithin, are made in the body, so they are not essential nutrients. Cholesterol is a sterol (a combination steroid and alcohol) and therefore a lipid that is found in cell membranes of all body tissues. It is made mostly in the liver and transported in the blood plasma of all animals by lipoproteins. Cholesterol is used to make cell membranes, nerve sheaths, vitamin D, bile acid, and hormones. Since it is produced sufficiently in the body, it is not an essential component of food in adults. Children, however, must include some their diets. Because many foods contain cholesterol, excessive amounts of cholesterol may be contained in the blood stream of adults to cause atherosclerosis (accumulated blockage ("harding") of arteries) and heart disease. Saturated fats tend to increase cholesterol in the blood. Animal fats, which are mostly saturated, contain high amounts of cholesterol, whereas vegetable oils, which are mostly unsaturated fats (coconut and palm oils are exceptions), contain low amounts of cholesterol. While elevated blood cholesterol levels have been linked to heart disease, there is no necessary relationship between cholesterol intake and blood cholesterol level. A properly functioning liver regulates the blood cholesterol level by storing and releasing as well as producing and excreting cholesterol as appropriate - primarily as bile. No matter what the diet, cholesterol is always present in the blood. Wiki n.p. Brody 61-62 Since cholesterol is synthesized in the body, it is not an essential nutrient, except in children. Lipoproteins in the blood carry insoluble fats, such as cholesterol, through the body. The protein end of the lipoprotein has charged groups that attract water molecules, which makes them soluble in the salt water blood. The triglyceride fats and cholesterol are carried internally, shielded from the water. High density lipoproteins (HDL) carry cholesterol from body tissues and bring it back to the liver; thus, they are beneficial. Low density lipoproteins (LDL) carry cholesterol from the liver to the cells; thus, they are harmful. Very low density lipoproteins (VLDL) carry triglycerides from the liver to the cells for deposit. Thereafter, they become low density (LDL) lipoproteins. Wiki n.p. Saturated fats increase LDL (Low Density Lipoprotein) levels in the blood, whereas unsaturated fat/oil molecules decrease LDL levels. Elevated LDL is a partial cause of atherosclerosis (hardening of the arteries) and heart disease. Therefore, health scientists discourage eating foods containing relatively large amounts of saturated fats. Likewise, the trans isomer has been found to be a contributing cause of high LDL, which can lead to atheroschlerosis and heart disease, whereas the cis isomer appears to be harmless. Hydrogenation is a process that adds hydrogen atoms to a molecule. The reverse process is called dehydrogenation, i.e., removing hydrogen atoms from a molecule. In food processing hydrogenation, hydrogen is added to vegetable oils, which are mostly unsaturated fatty acids and liquids at room temperature, to make many of the double carbon-carbon bonds into single carbon-hydrogen bonds, i.e., make more saturated fatty acids. This change has the effect of raising the oil melting point to make it more solid, e.g., margarine, shortening. Hydrogenation prevents spoiling and thus lengthens the shelf-life of the product. The reverse process, dehydrogenation, removes hydrogen atoms, which increases the molecule carbon-carbon double bonds and decreases the molecule carbon-hydrogen bonds to make the fat softer or turn it into a liquid. Since a typical oil molecule has many double bonds, partial hydrogenation leaves some of the oil molecules with both double carbon-carbon bonds and single carbon-hydrogen bonds. Likewise, partial dehydrogenation leaves some of the fat molecules with both single carbon-hydrogen bonds and double carbon-carbon bonds. A completely saturated fat/oil molecule has no double carbon-carbon bonds, i.e., it has all single hydrogen-carbon bonds. A completely unsaturated fat/oil molecule has all double bonds, i.e., no carbon-hydrogen bonds. A partially saturated (partially polyunsaturated) fat/oil molecule has some double and some single bonds. |
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