Introduction
Fats provide a concentrated source of energy in the diet. The building blocks of fats are called fatty acids. These can be either saturated, monounsaturated or poly-unsaturated. Foods rich in saturated fats are usually of animal origin. Vegetable fats are generally unsaturated.
Saturated fat raises the level of cholesterol in the blood. Cholesterol is present in animal foods but not plant foods. It is essential for metabolism but is not needed in the diet as our bodies can produce all that is needed. Raised blood cholesterol is associated with an increased risk of heart disease.
Fats and oils are essentially the same. Fats tend to be solid at room temperature whilst oils are liquid. The term lipids include both fats and oils.
Structure & Functions
Fats consist of fatty acids and glycerol. Nearly all the fats in our bodies and in foods are triglycerides, being made up of three fatty acid molecules to one glycerol molecule. There are about 16 different fatty acids commonly present in foods. The nature of fat depends on its constituent fatty acids.
Fats can be classed as either saturated, monounsaturated or polyunsaturated. This depends on the type of chemical bonds present in the fatty acid. If a fatty acid has all the hydrogen atoms it can hold it is termed saturated. However, if some of the hydrogen atoms are absent and the usual single bond between carbon atoms has been replaced by a double bond, then it is unsaturated. If there is just one double bond then it is monounsaturated. If there is more than one then it is polyunsaturated. Most fats contain a proportion of each of these three basic types of fatty acid but are generally described according to which type predominates.
Saturated fats tend to be animal fats and are solid at room temperature. Butter, lard, suet and meat fat are saturated fats. Unsaturated fats are liquid at room temperature. They are usually of plant origin, though fish oils may also be high in polyunsaturated fatty acids. Plant oils may be hardened by the addition of hydrogen atoms, converting double bonds to single bonds. This process is known as hydrogenation. Hydrogenated vegetable oils are often present in margarine and other processed foods.
Fats have a number of important functions in the body. As well as being a concentrated source of energy, fats act as carriers for fat-soluble vitamins A, D, E and K. Fats are also essential for the structure of cell membranes and are precursors of many hormones.
Essential Fatty Acids
Two fatty acids are termed essential fatty acids. These are linoleic acid and a-linolenic acid. These must be present in the diet as the body is unable to make them itself. They are widely present in plant oils such as sunflower, rapeseed and soyabean oils.
Linoleic acid is converted into the body to arachidonic acid from which prostoglandins and other vital compounds are made. Because of this conversion, arachidonic acid is not an essential fatty acid as was once believed. a-Linolenic acid is converted to eicosapentanoic acid (EPA) which is important in proper nerve function. EPA is present in fish oils and is claimed to be beneficial in reducing the symptoms of arthritis and the risk of heart disease. For this reason, fish oils are sometimes used therapeutically. Plant oils containing large amounts of a-linolenic acid can be used as an alternative by vegetarians. Linseeds and linseed oil are particularly rich sources of a-linolenic acid.
Cholesterol
Cholesterol belongs to the sterol group of fats. It is present in all animal tissues but is absent from plants. Cholesterol is essential as a component of cell membranes and a precursor of bile acids and certain hormones. The body can make its own cholesterol and so a dietary source is not required.
Cholesterol is transported in to various proteins. These complex molecules are called lipoproteins. There are four main types of lipoprotein involved in cholesterol transport. The most commonly refer red to are low density lipoprotein (LDL) and high density lipoprotein (HDL).
Cholesterol may form plaques on artery walls if levels in the blood are too high. This can lead to atherosclerosis. Because of this high blood cholesterol is linked with heart disease. It is the LDL cholesterol which has been linked to heart disease. HDL cholesterol may help protect against the risk of heart disease.
The amount of dietary cholesterol is not clearly linked to levels of cholesterol in the blood. Blood cholesterol is more closely related to the amount of saturated fat in the diet, saturated fat raising blood cholesterol. Unsaturated fats are not thought to raise blood cholesterol and may indeed lower levels.
Trans Fatty Acids
Unsaturated fatty acids can exist in two different geometric forms. These are called the cis and trans forms. Unsaturated fatty acids exist naturally in the cis form. During food manufacturing processes these cis fatty acids may be changed to trans fatty acids. Hydrogenation of margarine causes this to occur. It has been suggested that trans fatty acids can increase the risk of heart disease.
Free Radicals
Free radicals are highly reactive molecules which have been linked to both heart disease and cancer. A number of factors, including alcohol, stress and environmental pollutants can increase the generation of free radicals in the body. Polyunsaturated fats can also generate free radicals, especially when exposed to heat or sunlight. Because of this it is suggested that vegetable oils should be stored out of direct sunlight. Mono-unsaturated olive oil is less vulnerable to free radical generation and so is a better choice for frying.
Anti-oxidants such as vitamins A, C and E offer protection against free radicals. Fresh fruit and vegetables are rich in these anti-oxidants.
Dietary Sources
Saturated fats are nearly always from animal foods. Meat, eggs and dairy products all contain saturated fats. Lard and suet are saturated fats. Coconut oil and palm oil are vegetable sources of saturated fats. Olive oil is a monounsaturated fat. Polyunsaturated fats are usually from plant sources.
The ratio of polyunsaturated to saturated fats in the diet is often called the P:S ratio.
Cholesterol is present in all animal foods but not plant foods. Egg yolks and high-fat dairy products are high in cholesterol.
Required Intakes
Currently it is believed that around 42% of energy in the typical British diet is from fat. Dietary advice is to reduce this. The COMA (Committee on Medical Aspects of Food Policy) report advocated that no more than 35% of daily energy requirement should come from fat whilst the NACNE (National Advisory Committee on Nutritional Education) paper recommends a reduction to no more than 30%. Special emphasis is placed on reducing the amount of saturated fat in the diet.
Vegetarian diets tend to be lower in fat than omnivore diets. However, vegetarians consuming dairy products and processed foods high in fat may still be consuming too much. Advice to vegetarians is to keep fat intake to a minimum, avoid high fat dairy products and processed foods containing dairy fats and hydrogenated vegetable fats, and to use olive oil for cooking purposes.