Types of Dietary Fat by Jane Thurnell-Read

There are different types of fats occuring naturally in food, and the implications of the different ones varies, so it is good to know about these fats so you can choose the ones you need for a healthy diet. In addition there are trans fatty acids which have been produced by modern food processing and have been shown to be bad for us.

The three types of natural dietary fats:

• Saturated fats (SFA’s).
• Polyunsaturated fats (PUFAs).
• Monounsaturated fats (MFAs).

Most authorities agree that the typical Western diet contains too much fat in total and that the balance between the different fatty acids is not correct. Contrary to what much of the media say, not all fats are bad.

All dietary fats contain a mixture of the three types of fatty acids: saturated, polyunsaturated and monounsaturated, but the proportions vary, so that sources are classified by their predominant fat.

In unsaturated fats not all the chemical bonds are linked to hydrogen atoms. This allows the formation of double bonds within the fat molecule, giving these oils a much more flexible structure.

Saturated Fats (SFAs)

These occur mainly in animal foods and are hard at room temperature. They can be synthesised by the body These include meat, dairy products, coconut and palm oil. These are converted into LDL-cholesterol. Saturated fats increase the stickiness of the blood making thrombosis more likely and interfere with the function of essential fatty acids.

Polyunsaturated Fats (PUFAs)

These include corn oil, evening primrose oil, grape seed oil, safflower oil, sesame oil, soya oil, sunflower oil, wheat germ, fish oil, mixed vegetable oil, flax or linseed oil and walnut oil. They are usually liquid at room temperature, but will solidify in a fridge.

Heating polyunsaturated oil makes it toxic, so these oils should preferably be cold-pressed, stored in a fridge and eaten uncooked. It is easy to add a small amount each day to salads, cooked vegetables or spread on bread. These are high in omega-3 and omega-6 essential fatty acids.

Monounsaturated Fats (MUFAs)

Olive oil, hazelnut oil, peanut oil, rapeseed oil, almond oil are rich in these types of fats. Monounsaturated fats contain oleic acid (omega-9 fatty acids) that helps to keep arteries supple. These fats are more stable with heat and so are preferred for cooking.

Trans Fatty Acids And Hydrogenation

Trans fatty acids are unsaturated fatty acids with an unusual shape, so the body treats them more like saturated fatty acids. Dairy products, lamb and beef contain small amounts of trans fatty acids. When vegetable oils are artificially hardened to produce margarine trans fatty acids are produced. Trans fats are most commonly found in biscuits, cakes, pastries (savoury and sweet), sausages, crackers and take-away food. High intake of trans fatty acids has been linked to heart disease. Because the trans fatty acids are difficult for the body to metabolise, they accumulate in blood vessels causing blockages. Trans fatty acids can also interfere with the metabolism of essential fatty acids. Some studies suggest that trans fats are worse than saturated fats for health.

Food manufacturers want a solid fat that does not go rancid easily and does not have any real taste, but vegetable oil is liquid at room temperature. Hydrogenation gives them this.

Hydrogenation is a high tech process. Vegetable seeds are cleaned and bleached to remove all colour, taste, smells and impurities. The liquid vegetable oil is then heated to high temperatures and a catalyst (commonly nickel, but could be palladium, platinum or rhodium) is added. Hydrogen is bubbled through the liquid. The mixture is then filtered to remove the metal, leaving hydrogenated vegetable oil. Water, whey, salt, vitamins, colourings, flavourings and emulsifiers may then be added to produce hydrogenated margarine.

The advantage of all this is that it gives a uniform product that is solid at room temperature and has a long shelf life. This last characteristic is very important both for manufacturers and consumers. It gives manufacturers increased flexibility, and it means that consumers can buy products and not have to worry so much about 'best before' dates. This seems like a win-win situation for everyone, but there is a potential problem: the hydrogenation process changes some of the fats into trans fatty acids.

It is easy to blame the manufacturer, but as long as we, the consumers, prefer to buy long-shelf life products that always taste and look the same, manufacturers will continue to produce them using hydrogenated fats.

Frying, particularly at high temperatures, also produces trans fatty acids.