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Fats and oils - are all fats the same?

Key points from this exercise:

There is a need for fat in the diet because when fat provides less than about 10% of energy intake it is difficult to consume a great enough bulk of food to meet energy requirements.

Vitamins A, D, E and K, and carotenes, are absorbed dissolved in dietary fat.

Two families of polyunsaturated fatty acids (n-3 and n-6) cannot be synthesised in the body, and are required for synthesis of prostaglandins and related compounds, so are dietary essentials.

Intakes of fat above about 40% of energy intake are associated with increased risk of chronic non-communicable diseases, and it is considered desirable to reduce average intakes of fat to about 30% of energy intake. Thus should be at the expense of saturated fat, with a corresponding increase in carbohydrate intake.

Triacylglycerols consist of three fatty acids esterified to glycerol, and provide 30 - 45% of average energy intake.

Phospholipids consist of two fatty acid esterified to glycerol, with a hydrophilic group esterified to carbon-3 by a phosphodiester bond. Phospholipids are major constituents of cell membranes, and also have an important role on lipid digestion and absorption.

The main difference between oils, which are liquid at room temperature, and fats, which are solid at room temperature, is the degree of unsaturation of the fatty acids esterified to glycerol.

Saturated fatty acids have no carbon-carbon double binds; mono-unsaturated fatty acids have one carbon-carbon double bond; polyunsaturated fatty acids have two or more carbon-carbon double bonds.

Most of the naturally occurring unsaturated fatty acids are in the cis-configuration; trans-fatty acids arise as a result of isomerisation during catalytic hydrogenation of oils to produce solid fats for food manufacture.

Cis-polyunsaturated fatty acids in membrane phospholipids permit greater fluidity of the membrane than do saturated fatty acids. Trans-fatty acids behave more like saturated fatty acids, and it is considered desirable that intake should not exceed 1% of energy intake.

There are desaturases in human tissues that can introduce carbon-carbon double bonds between an existing double bond and the carboxyl group, but not between an existing double bond and the terminal methyl group. Therefore numbering of carbons in unsaturated fatty acids is from the methyl group, not, as would be chemically correct, from the carboxyl group.

The shorthand nomenclature for fatty acids shows C followed by the number of carbon atoms, then a colon and the number of carbon-carbon double bonds. For unsaturated fatty acids this is followed by n- (or omega-) and the carbon atom at which the double bond starts, counting from the terminal methyl group.

Compared with mono-unsaturated fatty acids:

 

End of this exercise