The low-down on fat

One of the main problems with the average man's diet is that he eats far too much fat — it provides over 40% of the calories he needs whereas it should, from a health point of view, provide less than 30%. Excess fat consumption can lead to weight gain (it's very high in calories), and too much saturated fat can raise cholesterol levels.

You do need some fat in your diet for good health, however. It helps food taste good, provides energy, is a structural part of every cell membrane, supplies "essential" fatty acids (the body can't make them so they must come from foods), and promotes the absorption of fat-soluble vitamins. It's really only a problem if you overdo it.

There are four main types of fat in foods: polyunsaturated; mono-unsaturated, saturated and trans. Their different chemical structures affect how they act in the body, as well as their hardness at room temperature: for example lard, which is rich in saturated fat, is solid, while sunflower oil, which is rich in polyunsaturates, is liquid. All foods and oils contain a mixture of fats, not just one type.

Polyunsaturated fats

There are two families of polyunsaturated fats each headed by a parent essential fatty acid. These are linoleic acid (omega 6 family) and alpha-linolenic acid (omega 3 family).
Omega 6 and omega 3 fats are needed for growth, the structure of cell membranes and to produce chemical messengers which help regulate functions such as blood clotting (so reducing the risk of heart attack), blood pressure and immunity.
In moderation, omega 6 fats help lower blood cholesterol levels.
The balance between these two families is important for their optimal function. As omega 6 fats are already widespread in processed foods in our diet, it's best to limit these when possible and choose foods rich in omega 3 fats.
Good sources of omega 3 fats include oily fish (mackerel, salmon, kippers, trout, sardines), rapeseed oil, soya oil and spread, walnut oils, pumpkin seeds and linseeds, wholegrains, walnuts and sweet potatoes.
Good sources of omega 6 fats include sunflower, corn and safflower oils and margarines, grapeseed oil, and sunflower and sesame seeds.

Mono-unsaturated fats

In moderation, mono-unsaturated fats have beneficial effects on blood cholesterol levels, especially when eaten in place of saturated fats.
Good sources include olive oil and olive oil-based spreads, rapeseed oil, avocados and most nuts.
The majority of "vegetable oils" that do not specify a fat source are usually rapeseed oils, and therefore a cheap source of mono-unsaturates.

Saturated fats

Eating too many saturates causes the liver to make more bad "LDL" (low-density lipoprotein) cholesterol (high levels are linked to heart disease) and raise cholesterol levels. Saturated fats incorporated into our blood cells can also make them more "sticky", increasing the risk of blood clots (thrombosis).
Saturated fats increase the risk of atherosclerosis — the fatty cholesterol-rich deposits that can cause "furring up" of the arteries. If the arteries supplying blood to the heart become too furred up, the blood supply through them is limited, causing the painful symptoms of angina. The narrowed parts of the arteries alter the blood flow through them, increasing the risk of turbulence and the risk of a clot formation.
Saturated fats are mostly found in animal foods, for example fatty meat, cheese, butter, cream. Hydrogenated fats found in fatty processed and fast foods are also saturated fats.

Trans fats

Trans fats are formed when unsaturated fats are bombarded with hydrogen to make them more saturated (a process called hydrogenation) so they'll be firmer and last longer. As well as creating more saturated fats, hydrogenation forms trans fats.
Like saturated fats, too many trans fats can raise blood cholesterol levels. They are found in hard margarines, fast food, pastries, biscuits and any food which has "hydrogenated vegetable oil/fats" in its ingredient list. Very small amounts occur naturally in butter, full-fat milk and meat.