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The Nutritional Role of Fats, Oils and the Polyunsaturates

Polyunsaturates form a component part of the fats and oils in our food. Such fats and oils are a combination of substances called fatty acids  and glycerol (also called glycerine). The fatty acids are long chains of carbon and hydrogen atoms, with a mildly acidic group at one end, which is where they join with glycerol to form molecules called triglycerides. Three fatty acids join with one glycerol, hence the name triglyceride.
The various fatty acids which are commonly found in edible oils and fats are classified into three distinct groupings, depending on their actual chemical format.
The saturated fatty acids are characteristic of land animal fats, such as butter, tallow (from beef) or lard ( from pigs), and have a high melting temperature (>20o C) which is why such fats tend to be hard at room temperature. The links between the carbon atoms within the chain are single links, which means that the maximum amount of hydrogen is present. More hydrogen cannot be inserted chemically, hence they are termed to be "saturated".
Certain fatty acids have some of their carbon-carbon links in a different form, called double bonds. If there is only one such double bond present in a molecule, the fatty acid has a somewhat lower melting point, which  usually makes the fatty acid liquid at room temperature. Such fatty acids are called monounsaturated, and are commonly found in certain vegetable oils, e.g. olive oil. Extra hydrogen atoms can be inserted chemically, which is why they are termed "unsaturated".
The presence of more than one such double bond makes the fatty acid polyunsaturated.  From two to six double bonds can be found in the fatty acids of common edible oils, and the presence of two or more such bonds lowers the melting point even more than with monounsaturates. Vegetable oils such as sunflower oil, and the oils from fish commonly contain high levels of polyunsaturates.
Polyunsaturates are further divided into two sub-groups depending on small differences in the way the molecule is made up. The two groups are called omega-6 and omega-3 (sometimes referred to as w-6 or w-3, or even N-6 and N-3).
Both  of these families of polyunsaturates are essential for good health and normal growth. Since they cannot be manufactured by the human body, they must be supplied in the diet. For this reason, they are often referred to as the        " essential fatty acids", or "EFA's"


What we do with oils and fats

The human body uses the oils and fats in the diet for three purposes.

1. As an energy source.

All fatty acids, whether saturated or unsaturated or polyunsaturated contain a high density of carbon and hydrogen atoms. When they are consumed as food, they yield a lot of food energy (37Mj /g, or 9 cals/g) , roughly twice as much as carbohydrates for example. This also means that they are good energy storage materials, providing a lot of energy without taking up too much space. Hence body fat, which is our primary energy reserve, is based on fats rather than carbohydrates such as starches.
Supplying energy is in fact the primary nutritional role of all edible oils and fats, though polyunsaturates tend to be spared this fate, because of their nutritional significance.

2. As a structural component.

The presence of one or more double bonds means that the human body (and of course other animal species) can use these particular fatty acids to do things it can’t do with saturates. This is why the polyunsaturates, and to a lesser extent monounsaturates have an additional role or roles. Both these types of fatty acids also are used by the body to form an integral part of the structure of the membrane, or wall of the cells which go to make up the body itself. The properties of the cell membrane depend on the actual make up of fatty acids within it, and this is determined by several factors, including genetic make-up, but it is also influenced by diet. The way any particular cell responds to the various stimuli which may affect it from time to time will in part depend on the mixture of polyunsaturates and monounsaturates present in the membrane which makes up its wall. Such stimuli might be environmental, physiological, or indeed physical. The cell will only respond in the optimal manner if the membrane surrounding it is composed of the optimal mixture of fatty acids.

3. To make powerful biological regulators

The third role for polyunsaturates is to be the raw material from which the body makes a range of intensely powerful, yet short-lived biological mediators. These influential chemical messengers control many different body processes, ranging from blood clotting, to digestion, from kidney function to the birth process itself.
Both omega-6 and omega-3 polyunsaturates are needed for their structural properties, and as raw materials for the production of biological mediators. As a result of this, the polyunsaturates of the omega-6 and omega-3 families are known as the "essential fatty acids", or EFA's .

Essential Fatty Acids

 It is recognised that there are three "families" of polyunsaturates that are found in human nutrition, though the omega-9 family is of little significance nutritionally. The three families differ in the position of the first double bond counting from the methyl end (or the omega end), of the chain. These three families are known as the omega-3, omega-6 and omega-9 families (also known as the n-3, n-6 and n-9 families). The diagram below shows the three main families of polyunsaturates, and their metabolic interconversions. In this diagram, the arrows represent the actions of enzymes  which convert one polyunsaturate into the next.

The n-9 Family                             The n-6 Family                                  The n-3 Family

 

     18:1 n-9                                            18:2 n-6                                               18:3 n-3

    oleic acid                                      linoleic acid                               alpha-linolenic acid

                                                                                                            

     18:2 n-9                                               18:3 n-6                                        18:4 n-3

                                                   gamma-linolenic acid

                                                                                

                                                                                                   

     20:2 n-9                                         20:3 n-6                                                20:4 n-3

 

                                                                                                    

     20:3 n-9                                         20:4 n-6                                                 20:5 n-3

                                                arachidonic acid                            eicosapentaenoic acid

                                                                                                          

    22:3 n-9                                          22:4 n-6                                                    22:5 n-3

                                                                                                                

                                                           22:5 n-6                                                24:5 n-3

                                                                                                                          

                                                                                                                         24:6 n-3

                                                                                                                          

                                                                                                                         22:6 n-3

                                                                                                            docosahexaenoic acid

The omega-9 family is only of significance when there is an insufficiency of either or both of the other two families, which are essential to human health, and must be supplied in the diet. When adequate amounts of omega-6 polyunsaturates and/or omega-3 polyunsaturates are not available, the body tries to compensate by producing omega-9 polyunsaturates to take the place of the essential omega-3 and/or omega-6 polyunsaturates. Though the omega-9 derivatives can substitute to a certain extent, they are not as effective as the omega-3 or omega-6 derivatives, and health will eventually suffer. The main value of the omega-9 polyunsaturates is as a marker for dietary insufficiency of the essential polyunsaturates.

 


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