When unsaturated vegetable fats are hydrogenated, the process produces yet another type of fatty acid as a side product – a trans fatty acid.
These contain double carbon–carbon bonds - and hence are unsaturated – but the configuration of the carbon chain is altered so as to make the fatty acid behave as though it were saturated.
Trans fatty acids already exist in small amounts in animal fats. It is the process of hydrogenation which produces these fatty acids in vegetable oils which do not naturally contain trans fatty acids.
Food manufacturers, alerted to the fact that saturated fats are potentially harmful, started adding partially hydrogenated vegetable oils to processed foods. This means that a typical Western diet now contains relatively large amounts of trans fatty acids and it is only fairly recently that people started to realise the risks associated with these fats.
Why are trans fatty acids bad for you?
We know that saturated fats are harmful. They are often found in foods which also contain cholesterol and they raise the levels of LDL.
Now, not only do trans fatty acids raise LDL levels, and so the total levels of cholesterol, but they also lower the levels of HDL – known to protect against coronary artery disease.
So, now you not only have to avoid saturated fats, but you also have to try to avoid trans fatty acids.
Food containing trans fatty acids
Sources of trans fatty acids in a typical Western diet include:
Margarines – the solid ones are the worst, the more liquid the best
Doughnuts, cakes, biscuits which are high fat (there are reduced-fat biscuits on the market. Read the labels carefully)
Many processed foods, including "slap" chips, potato crisps, many corn crisps and many types of cheese crackers
Read food labels and try to avoid those which state that the product contains "partially hydrogenated vegetable oils".
(The Heart and Stroke Foundation/Health24)