Fats are combinations of fatty acids and glycerol. A mixture of all types of fatty acids is needed for normal health.
These have their carbon chain saturated with hydrogen atoms. These are stable fats and keep well. Palmitic acid and stearic acid form butter, lard, suet and cocoa butter. Myristic acid forms part of butter and coconut oil.
If hydrogen atoms are missing from the carbon chain, the carbon atoms double-bond to each other instead. Mono-unsaturated fats have a single double-bond, but poly-unsaturated fats have more double-bonds. These double bonds are more reactive and the fat easily combines with oxygen from the air to go rancid.
We need a range of fats in our diet.
Oleic acid forms 60% -70% of olive oil and rapeseed oil.
Linoleic acid is found in seed oils such as maize, soya and sunflower.
Linolenic occurs in vegetable oils in small amounts.
Arachidonic acid only occurs in animal fats, but it can be formed from linoleic acid.
The double bonds in natural unsaturated fats are normally in the 'cis' position. This is the natural structure that our body expects. When margarine is being manufactured, some of these bonds are changed into the 'trans' position. These are totally alien to our bodies. These trans fats may increase levels of blood cholesterol and increase the risk of heart disease.