Fats usually occur in the form of triglycerides. They are lipid structures containing a glycerol backbone with three fatty acids attached by ester bonds.
Fats have many roles in the body:
- Structural fats form part of cell membranes and help to protect the vital organs from knocks.
- Storage fats act as a long-term fuel reserve.
- Metabolic fats are used by the body for the production of energy and specific metabolic pathways, (e.g. hormone production, inflammatory reactions). These fats also help to transport molecules and fat-soluble vitamins around the body.
Fats in foods
In the diet, fats are mainly found in the form of triglycerides which are E shaped molecules made with a backbone of glycerol with three fatty acids attached, one connected to each arm of the ‘E’. This is significant, as the combination of fatty acids joined to the glycerol backbone will give that particular fat its own characteristics and its own function in the body.
Every food that we eat will contain a wide variety of different fatty acids. These fatty acids have been allotted to various groups, depending on their chemical structure and the way they work in nature.
For years scientists have used four main fat groups; ‘saturated’, ‘monounsaturated’, ‘polyunsaturated’ and ‘trans’ fatty acids. These fat groupings are often seen on food labels and it is useful to know more about them.
Saturated fatty acids
A saturated fat is completely surrounded by hydrogen atoms – it is ‘saturated’ with hydrogen molecules, making it solid at room temperature – examples are animal fat, butter, hard margarine, lard, palm ‘oil’, coconut ‘oil’, suet, hydrogenated vegetable oil.
Saturated fats are commonly found in biscuits, cakes, pastry, puddings, paté, fried foods and many savoury ‘convenience’ foods. Manufacturers are attracted to saturated fats because they have a longer shelf life than polyunsaturated ones which tend to go rancid unless protected by antioxidants. Saturated fat also produces a popular ‘short’ crumbly texture in baked goods.
Saturated fats, whether natural or hydrogenated, tend to increase the quantity of undesirable cholesterol in the blood. High levels of this type of cholesterol are linked with heart disease so the amount of saturated fat in the Western diet should be reduced.
Polyunsaturated fatty acids
Polyunsaturated fats are like saturated fats, but they are different because two or more hydrogen atoms are missing – they are ‘poly’ (meaning many) ‘unsaturated’. This makes the fat more fluid by nature.
Polyunsaturated fatty acids are classed into two groups, according to their chemical structures, including the n-6 (omega 6) family of polyunsaturates and the n-3 (omega 3) group.
Nutritionally important polyunsaturated n−3 fatty acids include alpha linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).
The body is ingenious in that it can manufacture or modify the majority of fatty acids to ensure that the whole spectrum required for the body are available. However, the body cannot make the polyunsaturated fatty acids linoleic acid or alpha linolenic acid, relying on a daily dietary supply to keep the body healthy. For this reason they are called ‘essential fatty acids’.
Omega 3 fatty acids
The main sources of n-3 polyunsaturates in the diet are:
- vegetables (25%)
- meat and meat products (19%)
- cereal products (19%)
- fat spreads (19%)
- oily fish acts as a very rich source of n-3 fatty acids in some diets
Alpha linolenic acid is the parent fatty acid for a whole range of n-3 fatty acids, especially abundant in flax oil and fish oils. The human body cannot synthesize n−3 fatty acids but it can form “long chain” 20-carbon unsaturated n−3 fatty acids (like EPA) and 22-carbon unsaturated n−3 fatty acids (like DHA) from the “short chain” eighteen-carbon n−3 fatty acid alpha-linolenic acid which in turn can be converted into the longer chain EPA and DHA.
Omega 6 fatty acids
Good dietary sources of the n-6 family of polyunsaturates include:
Vegetables, fruit and nuts which account for 26% of total intake in Britain
Cereal products (22%)
Fat spreads (21%).
Rich sources of linoleic and alpha linolenic acids are plant seed oils.
Linoleic acid is the parent fatty acid for the ‘omega 6’ or ‘n minus 6’ (n-6) group of fatty acids found in foods such as sunflower seeds, evening primrose oil and sesame seeds.
Monounsaturated fatty acids
In a monounsaturated fatty acid, the structure has two hydrogen atoms ‘missing’ and creates one double bond to fill the gap. The quantity of monounsaturates in the diet should be increased. Foods rich in monounsaturates include olive oil, rape seed oil, peanut oil and avocados.
Trans fatty acids
During margarine manufacture hydrogen gas is bubbled through refined liquid oils to add hydrogen atoms to the fats. This makes them less fluid and more solid – much better for food manufacturing processing.
This ‘hydrogenation’ process affects the shape of some of the polyunsaturated fatty acids, their natural characteristics are altered and they transform from a ‘cis’ form, as they are found in nature, to an unnatural, or chemically made ‘trans’ form.
Studies show that trans fatty acids behave like saturated fats in the body and high intakes have been strongly linked to increased risk of heart disease.