Preparation

If you are making a presentation to a group then you will need to prepare:

• a hand-out information sheet.
• audio visual illustrations (blackboard or OHP or data projection).
• a short script for yourself.

Include within your presentation:

• the similarities in structure between fats and edible oils
• the differences between fats and edible oils
• differences in behaviour between saturated and unsaturated fatty acids
• the biological importance of unsaturated fats

Fats and oils

Fats, also called lipids, are the third main class of food type needed in the human diet, the others being proteins and carbohydrates. Oils are simply fats that are liquid at room temperature. In foods derived from animals, the main sources of fat are dairy produce and meat although most foods contain some fat. Some of the richer vegetable sources of dietary fat are nuts and seeds, soyabeans, olives and peanuts. Fats are an important component of our diet and at least a minimum intake is required. However, many health problems, particularly in the western world, are associated with an excessive intake of fat.

The main functions of fat in the body are as an energy reserve and for insulation. Fats can be burned to release energy when we need it and are not getting enough from the carbohydrates in our diet. Fatty tissue around internal organs help to protect them from trauma and temperature change by providing padding and insulation. They also have other uses. They are important in transporting other nutrients such as the vitamins A, D, E and K which are not water-soluble. Fats also form an essential part of the cell membrane. Finally, they are also a source of essential fatty acids.

Structure of Fats

Triglycerides are by far the most common type of fat and make up about 95% of the lipids in food and in our bodies. All triglycerides have a similar structure (Figure 8). They are esters formed from the condensation of fatty acids with glycerol (propane-1,2,3-triol). Fatty acids are long, straight chain carboxylic acids which contain an even number of carbon atoms, typically between 12 and 20 atoms.

Figure 8 - Structure of a saturated fat

The three fatty acids present in a triglyceride are generally different, giving rise to a large number of possible combinations.

Fats can be classified as saturated or unsaturated according to the nature of the fatty acids present. The fatty acids in turn can be classified as saturated, monounsaturated or polyunsaturated depending on the presence and number of CC bonds (Table 4).

Table 4 - Examples of fatty acids

Physical properties

Saturated fats have higher melting points than unsaturated fats. This can be explained in terms of differences in the packing of the molecules in the solid state. The long hydrocarbon chains in saturated fat molecules can be packed quite closely together which maximises the van der Waals forces (Figure 8).

Figure 9 - Structure of a cis-unsaturated fat

Unsaturated fats (oils) contain one or more CC bond. Almost invariably the CC bonds have a cis configuration. This introduces a 'kink' into the hydrocarbon chain since there is restricted rotation around this bond. This makes close packing much more difficult (Figure 9). The intermolecular forces are weaker and so unsaturated fats have lower melting points than saturated fats.

Figure 10 - Structure of a trans-unsaturated fat

It is worth pointing out that triglycerides made from trans unsaturated fatty acids can pack together much more closely (Figure 10) and so have higher melting points than their cis equivalents.

Essential fatty acids

About 40 different fatty acids occur naturally. Table 4 shows some of the most common. The most important essential fatty acids are linoleic, linolenic and arachidonic acids, collectively known as vitamin F (Figure 11). These are all polyunsaturated fatty acids that are needed by the body. However, they cannot be synthesised from other compounds in the human body, although if there is sufficient linoleic acid it can be converted to arachidonic acid. They must be obtained from vegetable oils in the diet. (The term 'essential' is also used for those amino acids which must be obtained in the diet). Linoleic acid is found in most foods while linolenic acid is common in oily fish.

Figure 11 - Essential fatty acids

These fatty acids are important for normal growth, especially of the blood vessels and nerves, and to keep the skin and other tissues youthful and supple through their lubricating properties.

They are also essential precursors in the biosynthesis of a group of compounds known as prostaglandins. These are hormone-like substances present in small amounts in all body tissues and fluids. They are all C₂₀ carboxylic acids that contain a five-membered ring with two long side chains.

Figure 12 - Prostaglandin

The basic structural similarity between arachidonic acid and the prostaglandin shown in Figure 12 is obvious. One step in the conversion involves the formation of a five membered ring between carbons 8 and 12 in the fatty acid chain. If the configuration of either of the two central CC bonds in arachidonic acid was trans, such a transformation would be impossible. Prostaglandins have an extraordinarily wide range of biological effects, including controlling blood pressure and controlling inflammation. They are also involved in blood clotting, kidney function and the reproductive system and are the subject of a great deal of research to produce new drugs.

Diet

Many health problems are associated with an excessive intake of fat in the diet. Levels of fat intake are strongly linked to body weight and therefore obesity. Fats provide about 42% of the calories in the average American diet. A diet in which about 25% or less of total calories is derived from fat would be healthier and help to reduce blood cholesterol levels and risk of blood and heart disease. The type of fat in the diet is also important. Increasing the proportion of unsaturates and polyunsaturates in the diet has been shown to produce a significant reduction in blood cholesterol levels.

Current suggestions for a healthy diet include:

• reduce the total amount of fat in the diet
• specifically reduce the saturated fat intake by reducing consumption of red meats and dairy produce
• reduce intake of hydrogenated fat found in cooking oils and margarines
• raise the levels of polyunsaturated fats found in vegetable oils such as corn oil and sunflower oil as well as in most seeds and nuts
• eat more fish, particularly oily fish like herring and mackerel rather than red meat
• and of course, keep blood pressure under control, don't smoke and exercise regularly.

Summary exercise

Using information given above, complete the table by dragging and dropping items from the word bank.