What is Fat?
Types of fat
There are two chief categories of fat or fatty acids - saturated and unsaturated. Within these categories, there are at least 40 different types of fatty acids. Broadly speaking, unsaturated fatty acids are 'good' fatty acids and saturated fatty acids are 'bad' fatty acids.
Saturated fats and trans fats are "bad" fats as they can raise cholesterol (Mozaffarian D 2006, Mensink RP 2003). Unsaturated (good) fats are better for you, as consuming foods containing "polyunsaturated" and "monounsaturated" fats does not increase cholesterol (Keys A 1965). Furthermore, unsaturated fatty acids have been found to elicit other health benefits, ranging from beneficial effects on insulin sensitivity, haemostasis and inflammation. (Vessby B 2001, Dyerberg J 1978, Calder PC 2001)
Unsaturated fat can be found in vegetable oils such as sesame, sunflower, soya and olive; oily fish, such as mackerel, sardines, pilchards and salmon; nuts and seeds; and also spreads.
Simply switching from foods which are high in saturated fats to those containing polyunsaturated fats or monounsaturated fats can play a major part in maintaining a healthy diet.
Saturated fats
If the fatty acid has all the hydrogen atoms it can hold it is said to be saturated (see below)
 |
Key: |
= single bond
= double bondThis type of fat is typically found in large amounts in foods from animals, e.g. meat, butter, cheese and cream. Many baked goods such as cakes, biscuits and pastries are also high in saturated fat. Excessive intake of saturated fat can increase blood cholesterol levels.
Recommended fat intakes are based on energy needs and physical activity levels. Present guidelines suggest that women eat no more than 20 grams (men = 30 grams) of saturated fat a day, but on average we are currently eating about 17% too much.
| RECOMMENDED SATURATED FAT INTAKE | |||
| Energy intake in calories | 11% x EI | Guideline Daily Amount (GDA) | |
| Women | 1,940 kcal | 213 kcals | 20 g/d |
| Men | 2,550 kcal | 280 kcals | 30 g/d |
Unsaturated fats
In unsaturated fats, some of the carbon atoms are joined to others by a double bond and, therefore, could accept more hydrogen atoms. This means that they are not completely saturated with hydrogen, so are called unsaturated fats.
There are two main types of unsaturated fats - monounsaturated (containing one double bond) and polyunsaturated (containing more than one double bond). Most monounsaturated and polyunsaturated fats have good qualities, with one exception - trans-fatty acids. Trans fatty acids are, technically speaking, an unsaturated fat but offer no health benefits.
There is convincing evidence that swapping saturated fats for monounsaturated or polyunsaturated fats will reduce bad cholesterol (Hodson L 2001, Mensink 2003).
Trans-fatty acids
There has been a lot of attention on this type of fatty acid in recent months. They are classed as bad fats because they raise cholesterol, although technically speaking they are unsaturated fats. They are the one exception to the rule that unsaturated fats are good for health.
Double bonds in unsaturated fatty acids can exist on two configurations; the cis or the trans. When the hydrogen atoms are on opposite sides of the carbon chain (see diagram below), the double bond is in the trans configuration and the fat is known as a trans-fatty acid. If the hydrogen atoms are on the same side of the chain, the double bond is in the cis configuration and it is known as a cis-fatty acid. Most of the unsaturated fats in our diets are cis-fatty acids and not harmful to heart health.
 |
Key: |
Trans-fatty acids (TFAs) in our diet come from two sources; i) from ruminant animals and ii) during the process of partial hydrogenation.
Ruminant animals, such as cows and sheep, contain bacteria in their rumen (part of the gastrointestinal tract), which naturally convert cis double bonds to trans double bonds. This means that approximately 4-7% of the fats from the meat and milk of ruminant animals are trans fatty acids.
TFAs are also formed during the process of partial hydrogenation, which was used by the food industry to increase the melting point of fats. Partially hydrogenated fats typically contain anywhere between 5% and 30% TFAs. In this process, common unsaturated fatty acids such as oleic acid (found in olive oil) are converted to their trans equivalents, such as elaidic acid, which increases the melting point of the oil from 13°C to 44°C respectively. Fully hydrogenated oils do not contain trans fatty acids and are not believed to have a negative effect on health.
Current scientific evidence suggests that TFAs increase levels of bad cholesterol and decrease the good cholesterol in our bodies. Currently, most of us are eating within the recommended amounts - no more than 5 grams a day for men and 4 grams for women (that's 2% of our daily energy intake) (Henderson L 2001). The Food Standards Agency recently found that our average intakes are around 1% of daily energy intake. A relatively small proportion of people may have a very high intake of TFAs if they eat a lot of meat products, full-fat dairy products, fast foods, some cakes and biscuits. These people need to take steps to cut their intake or their health could be affected.
Margarine is often quoted as a source of trans-fatty acids, but margarine and spreads do not contain partially hydrogenated oils any more and so they have very low levels of trans fats - no more than one percent. By comparison, butter contains between three and six per cent TFAs. (ref http://www.efsa.europa.eu/EFSA/efsa_locale-1178620753812_1178620767491.htm )
Monounsaturated fatty acids
If there is one double bond as below, the fatty acid is known as a monounsaturated fatty acid. This is found in significant amounts in most types of fats of plant origin, such as nuts, avocado pears, rapeseed oil and olive oil. Monounsaturates do not raise blood cholesterol and evidence shows that they reduce blood cholesterol levels if they replace saturated fat in the diet (Hegsted DM 1965, Hodson L 2001, Mensink 2003).
 |
Key: |
Oleic acid is the main monounsaturated fat in our diets and this is sometimes called omega-9 (because the double bond is in position 9 of the fatty acid chain). It is found in significant amounts in most types of nuts, avocado pears, rapeseed oil and olive oil and spreads made from these.
Likewise, a diet which is relatively high in monounsaturated fats may offer improved glycaemic control for diabetics (Louheranta AM 2002, Vessby 2001).
Polyunsaturated fatty acids
If there is more than one double bond, then the fatty acid is known as a polyunsaturated fatty acid. These come mostly from vegetable sources, such as sunflower oil or seeds, but are also found in, nuts, green leafy vegetables and oily fish such as mackerel and sardines.
Polyunsaturates can actively reduce blood cholesterol levels. The polyunsaturates found in oily fish specifically appear to have no effect on blood cholesterol levels, but they do alter the consistency of blood.
There are two 'series' of polyunsaturated fats that are found in our diet, which are also known as essential fatty acids. They are omega 3 and omega 6. Essential fatty acids are so called because the body cannot make them but they are essential to the body's normal functioning, therefore, we need to eat them as part of our daily diet. Humans are unable to make these essential fatty acids, because we do not have the particular destaurase enzymes that insert double bonds in position 3 and 6 of the fatty acid chain.
Omega 3 plays an important role in the maintenance of healthy skin, reproduction, immune function and development of the brain and visual system. There is considerable evidence that omega 3 can have a positive effect on joint health, due to its anti-inflammatory effects. Omega 3 is found in vegetable oils (such as soya, rapeseed and walnut oil), green leafy veg, walnuts, linseeds, spreads and oily fish, such as sardines, mackerel and salmon. Most of us are eating enough omega 3, but some of the health benefits only come from a specific type of omega 3 called 'very long chain' and many of us are not getting enough of this. It is found in fish and some fortified products and about 10% of normal long chain omega 3, found in vegetable oils and spreads, can be converted to very long chain by the body.
People should be eating at least 450mg omega 3 per day. Oily fish is an excellent source of omega-3 and the current advice from the Food Standards Agency is that most of us can eat up to four portions of oily fish a week although girls and women of childbearing age should limit that to a maximum of two portions a week. However, seventy per cent of people do not consume any oily fish per week (Henderson 2002), however, so getting omega 3 form other sources is important.
