on August 31st, 2010 by admin
In reviewing the literature on fats in nutrition, I find it easy to become confused with the terminology. However a review of the chemistry makes the terms become much more realistic.
“Lipo“ means “Lipid” – a general term that refers to biological fats and oils.
Fat cannot dissolve in water or blood, so the body combines the fat or fatty acid with a protein making a compound termed lipoprotein. Lipoproteins can flow easily with the blood.
Blood cholesterol is HDL (High-Density Lipoprotein” and LDL or Low-Density Lipoprotein.
HDL or “good” cholesterol helps lower blood cholesterol because it picks up excess fats and carries them back to the liver where they can be disposed.
LDL is referred to as “bad” cholesterol because it can become so overloaded with fats that the fats are dropped on blood vessel walls rather than carried to the cells where they are needed. (LDL is not all bad as it supplies cells with needed nutrients and transports fat-soluble nutrients like vitamin E to the cells.)
Oxidation of LDL allows the cholesterol to be deposited in plaque on the walls of blood vessels. The chances of cholesterol being oxidized depends on:
1. The quantity (lack of) antioxidants in the blood and
2. The type of fatty acid in the LDL.
Plaque is a deposit of cholesterol on the wall of blood vessels over time. As the build up grows the vessel becomes narrower and increases the pressure of the blood flow.
Linoleic Acid is the fatty acid most likely to be oxidized (Oxidation).
Most fat in the body is formed from fatty acids.
A fatty acid is a long straight chain of carbon atoms with hydrogen atoms with an acid group (carboxylic acid) at one end. This end is water soluble. The rest of the fatty acid is oil soluble and has a methyl group at the opposite end.
Fatty acids in the body usually are
a. not attached to any other molecule (thus termed free fatty acids),
b. attached to glycerol in groups of three (triglycerides), or
c. attached to phosphalidic acid molecules (phospholipids).
Fatty acids are differentiated by
a. the number of carbons in the chain
b. the number of double bonds between the carbon atoms.
Saturated Fatty Acids have only single bonds between the Carbons. Thus the Carbon is “saturated” with as many “Hydrogen” atoms as each Carbon atom can hold.
Mono-unsaturated Fatty Acids have only one double bond.
Poly-unsaturated Fatty Acids have more than one double bond.
Poly-unsaturated fats have been recommended to reduce coronary heart disease
and saturated fats are usually associated with increasing blood cholesterol levels.
However not all saturated fats have the same effect on cholesterol synthesis in the liver.
Only saturated fats of Chain-length 12, 14, & 16, lauric acid, myristic acid, and palmitic acid have been shown to elevate blood cholesterol.
Myristic acid (in coconut and palm oil) elevates the most.
Whereas Stearic acid (18 carbon, saturated fatty acid) has been shown to lower cholesterol and oleic acid (18-carbon -monounsaturated) lowers LDL.
Poly-unsaturated fats although recommended to reduce coronary heart disease can be a problem because carbon-carbon double bonds can lead to free radical formation and reactions with oxygen to form unstable lipid peroxide compounds.
Polyunsaturated “cis” fatty acids can prevent the tight packing of fatty acids in membranes and this makes the membranes more “fluid”. In aging, cell membrane fluidity declines because of cholesterol in the cell and also because of free-radical oxidation. Antioxidants like Vitamin E are believed to be important in opposing membrane oxidation.
Cis and Trans
Fatty acid double bonds may be cis or trans – (cis-carbon chains are on the same side of the double bond) or (trans carbon chains on the opposite sides of the double bonds.
Most of the bonds are cis configuration and these prevent the tight packing of fatty acids and therefore increases membrane fluidity.
A fatty acid with trans double bonds is margarine. By artificially hydrogenating vegetable oils the number of double bonds is reduced and we create trans fatty acids. (However when trans fatty acids get into cell membranes, the fluidity of the membranes is reduced and the cells do not function as well.) We mostly get trans fatty acids from the hydrogenation process – but even some foods, like butter, do contain some trans fatty acids.
The human body creates most of the fat it needs, including cholesterol.
However there are two fatty acids which cannot be manufactured by the body and are necessary for life – termed essential fatty acids. These are Linoleic (omega-6 fatty acid) and alpha-linolenic acid ( omega-3 fatty acid).
Linoleic acid is an 18-carbon chain with 2 double bonds.
Alpha-linoleic acid is an 18 carbon chain with 3 double bonds.
The position of the double bonds in a fatty acid are important to its function – especially if the double bonds are close to the methyl end.
As long-chain fatty acids the human body can not add double bonds near the methyl end.
Carbon atoms in a fatty acid are named by the distance from the carboxylic acid. The Greek alphabet is used and the carbon atom nearest the carboxylic acid is the alpha carbon & the next is the beta, etc. The carbon atom in the methyl group is named the omega, the last letter in the Greek alphabet.
In linoleic acid the closest double bond to the methyl group is 6 carbon atoms from the methyl and therefore linoleic acid is named omega-6.
Omega-3, alpha-linolenic acid, is so named because the double bond closest to the methyl group is only 3 carbons away.
The source of omega-6 fatty acid in the diet is linoleic acid from the fruits of seeds and grains – sunflower, safflower, and corn oil. Avocado is also high in linoleic acid.
Omega-3 fatty acid is alpha-linolenic acid and found in green leaves and flaxseed oil, flaxseed, walnuts, hemp seeds, and soybeans. Fish oil has very little alpha-linolenic acid but is rich in omega-3 derivatures, EPA and DHA.
EPA – eicosapentaenoic acid.
DHA – docosahexaenoic acid.
Tags:
cholesterol. HDL,
dietary fat,
fats in nutrition,
fatty acids,
food fat chemistry,
LDL,
lipids,
lipoproteins,
polu-unsaturated fats,
saturated fates,
unsaturated fats | Posted in
Nutrition
