Fats are present in most foods. They add taste and texture to meals, and create a feeling of fullness and satisfaction. The relationship of fats to health is often misunderstood – many people believe that all fats are to be avoided because they are detrimental to health. Although it is true that too much of the wrong type of fat can lead to obesity and an increased risk of heart disease and cancer, there are also ‘healthy fats’ that are essential to the body.
Fats affect the functioning of the body in several ways; some helpful and some harm-ful, depending on their composition. A wide range of foods contain fats – seeds, nuts and fish are sources of healthy fats, whereas meat and dairy products contain less healthy fats. It is possible to adapt your fat consumption to ensure that your body has enough healthy fats to function properly and that unhealthy fats are kept to a minimum.
THE ROLE AND COMPOSITION OF FATS
Fats in our diet are made up of triglycerides, which are compounds of glycerol and fatty acids; the fatty acids can be either saturated or unsaturated. ‘Saturated’ refers to the fact that the carbon atoms in the fatty acids are bonded to a maximum number of hydrogen atoms. In general, harder fats (such as butter) contain more saturated fatty acids, and soft fats (such as vegetable and fish oils) contain more unsaturated fatty acids. Cholesterol is a substance related to fats and commonly found in foods that contain saturated fats. Another group – trans fats -occurs naturally in some foods of animal origin, but is also created during a food process known as hydrogenation. The role of fat in the body – helpful or harmful – is determined by its chemical structure.
Fats rich in saturated fatty acids are found mainly in animal fats such as butter, hard cheese, hard margarines, lard, meat, poultry and eggs. Two vegetable oils – coconut and palm – are rich in saturated fatty acids, but their different chemical structure means that they are not considered to be as harmful as animal fats. Saturated fats do not play an important role in a healthy diet because the body can make its own supply from carbohydrates and protein. A balanced diet should contain no more than 6 per cent of calories from saturated fats.
The term ‘cholesterol’ is used to describe a waxy substance that is related to fat. Nutritionists talk about two kinds of cholesterol: ‘blood cholesterol’ and ‘dietary cholesterol’. Blood cholesterol is needed to make cell membranes and hormones; it is not derived from the diet because the body is able to manufacture its own. Nutritionists also make a distinction between good blood cholesterol (known as ‘high density lipoprotein’, or HDL) and bad blood cholesterol (known as ‘low density lipoprotein’, or LDL). An excess of LDL encourages cholesterol to accumulate in the arteries, increasing the risk of heart disease.
Dietary cholesterol is found in all foods of animal origin, including eggs and dairy products, but is virtually absent in plant foods. Most foods that contain saturated fat also contain dietary cholesterol, but there are some low-fat foods that contain high levels of cholesterol, such as prawns and other shellfish.
It was once thought that a high intake of dietary cholesterol led to high levels of cholesterol in the blood, but research has shown this to be untrue. High-cholesterol foods are no longer blamed for increasing the risk of heart disease in healthy people. What does cause an unhealthy rise in bad blood cholesterol levels (and an increased risk of heart disease) is an excess of saturated fats, trans fats and oxidants (caused by frying, for example) in the diet. These promote the build-up of fatty deposits in the arteries.
Foods rich in unsaturated fatty acids are the major dietary source of vitamin E, which can help to protect against degenerative diseases. Unsaturated fats can be subdivided into mono-unsaturated and polyunsaturated fats.
Also referred to as omega-9 fats, mono-unsaturated fats are rich in a fatty acid called oleic acid. Oils high in mono-unsaturated fatty acids include olive, hazelnut, avocado and almond. Good food sources include avocados, almonds and sardines. As with saturated fats, mono-unsaturated fats are not strictly required in the diet, because the body can make them from carbohydrates and protein. Nevertheless, nutritionists recommend that mono-unsaturated fats make up 7 per cent of total fat intake. These fats are not harmful unless eaten to excess.
Because the body cannot make its own polyunsaturated fatty acids, they need to come from the diet. Polyunsaturate-rich foods include vegetable oils (especially safflower, sunflower and corn), fish oils, sunflower seeds and sesame seeds.
There are two families of ‘essential’ poly-unsaturated fatty acids (EFAs). The first family of EFAs consists of the omega-6 fatty acids, which are based on linoleic acid. Good food sources are seeds and their oils, including corn, safflower, sunflower, hemp, pumpkin, soya bean, wheatgerm and walnuts. Omega-6 fatty acids are important for growth and development, as well as the production of hormone-like substances that help to control a range of functions, including regulating blood pressure, improving the functioning of the immune system and balancing blood-sugar levels. One or two tablespoons of seed oil or two to three tablespoons of ground seeds each day provides a healthy intake of omega-6 fats.
The second family of EFAs consists of the omega-3 fatty acids, which are based on alpha-linolenic acid. It is thought that most people in the West do not include enough omega-3 fatty acids in their diet. Good food sources include seed oils from rapeseed, flax (also known as linseed), hemp and pumpkin,
Good food sources of omega-6 fatty acids include: sunflower seeds, pumpkin seeds, soya beans and walnuts. These foods are also good sources of the antioxidant vitamin E. A diet that is high in polyunsaturated fats requires the individual to eat more vitamin E-rich foods to protect the body against excess oxidation and free radicals.
And fresh oily fish such as sardines, herring, kippers, mackerel and salmon. A healthy intake of omega-3 fats would be provided, for example, by daily consumption of four tablespoons of pumpkin seeds (which would also provide omega-6 fats) or one tablespoon of flax seed oil. Fish oils are the major dietary source of omega-3 fatty acids and they appear to lower the risk of developing heart disease and, certain inflammatory illnesses, such as rheumatoid arthritis. Medical experts believe that the Inuit people of Greenland have a low incidence of heart disease because their diet is rich in fish oils.
For some time, saturated fats have been regarded as the ‘bad’ fats in food, whereas unsaturated fats have been perceived as the ‘good’ ones. However, recent research shows that the true picture is not so black and white. Trans fats – damaged or partially ‘hydrogenated’ polyunsaturated fats – are possibly the new villains in the fat family. Although small amounts occur naturally in some foods of animal origin and are no cause for concern, others are created during an industrial food process known as ‘hydrogenation’. This process hardens polyunsaturated oils, turning them into semi-solid or solid fats. Because the polyunsaturated fatty acids become damaged during this process-twisting out of their normal molecular shape – they are transformed into trans fatty acids. These artificially altered fats are thought to behave in the same way as saturated fats, and may therefore raise blood cholesterol levels and increase the risk of heart disease. Hydrogenated fats and oils are widely used in manufactured foods; when shopping for food, look for the term ‘hydrogenated vegetable oil’ on nutrient content labels and try to minimize your consumption of these products. Some people now prefer to eat butter instead of margarine to avoid the risk of consuming too many trans fats.