A diet that contains the right balance of foods for optimum health
Variation in diet among people living in different countries is closely associated with national patterns of disease. For example, the high-calorie, high-fat diet common in the UK is linked to the high national rates of obesity, heart disease (see Coronary artery disease), cancer, and stroke. In contrast, the traditional Mediterranean diet, which includes plenty of fresh vegetables, olive oil, and fish and is therefore low in saturated fat, seems to be related to the low incidence of heart disease in Mediterranean countries. In Japan, the typical diet is high in fibre, and the incidence of colorectal cancer is low. In many countries, an excessive amount of salt in the diet has been associated with people having high blood pressure (see Hypertension).
Just as an excess of certain elements in the diet can lead to health problems, deficiencies in essential nutrients can result in poor health (see Nutritional deficiencies). Such deficiencies, however, are rare in developed countries such as the UK.
The knowledge gained by nutritionists from studying the effects of diet on health in countries around the world has led to the development of guidelines to ensure a healthy, balanced diet that can reduce the risk of ill health.
There are five essential components of food: carbohydrates, protein, fats, vitamins, and minerals. Each one of these elements makes an important contribution to health, but it is important that they are consumed in the correct proportions, with carbohydrates forming the bulk of the diet and fats only a small proportion (see Healthy eating). By eating plenty of fruit and vegetables, you can ensure you are getting enough vitamins and minerals. You should also include plenty of water in your diet. Drinks that contain sugar, caffeine, or alcohol (see Alcohol and health) should be consumed in moderation. Sugar is known to contribute to tooth decay. Carbonated drinks, even those that are low in sugar, can damage the teeth because they are acidic. Excess caffeine from drinks such as coffee and cola can cause palpitations (a feeling that the heart is beating rapidly) and insomnia, and caffeine may worsen symptoms of anxiety.
Other important health factors are the amount of salt you have in your diet and the types of fat you eat.
These should be the body’s main energy source. However, if you eat more carbohydrate than your body needs, the excess will be stored as fat. There are two main types of carbohydrate in food: simple and complex.
Simple carbohydrates are made up of sugars. They provide your body with a quick energy boost but can be harmful to the teeth. Foods such as biscuits, cakes, and sweets contain large quantities of simple carbohydrates.
Complex carbohydrates have a more complicated structure than simple carbohydrates and are made up of starches and dietary fibre. Starches are digested slowly and therefore provide sustained energy. Pasta, bread, vegetables such as potatoes, and rice contain high levels of starch. Dietary fibre consists of the fibrous parts of plants that are not completely broken down during digestion. Fibre can be divided into two kinds: soluble fibre and insoluble fibre.
Insoluble fibre adds bulk to faeces, aiding the passage of material through the intestines. People who have plenty of this type of fibre in their diet seem to have a reduced risk of colorectal cancer. Major sources of insoluble fibre include unrefined carbohydrate foods such as wholemeal bread, cereals, brown pasta, brown rice, fruits, pulses, vegetables, seeds, and whole grains.
Soluble fibre may lower blood cholesterol levels and has been associated with a reduced risk of heart disease and stroke. Good sources of soluble fibre include fruits, oats, beans and pulses, and vegetables.
Complex carbohydrates should form the major part of your daily diet. Simple carbohydrates should be kept to a minimum because, although they provide energy, they are low in vitamins and minerals and contain little fibre.
Protein is essential for building and repairing cells in the body. Insufficient protein in the diet causes serious health problems, but such deficiency is largely restricted to developing countries where the availability of food is limited. A more common problem in the UK is too much protein in the diet, particularly animal protein. Excess protein is converted into fat in the body. Many protein-rich foods are also high in calories and saturated fat. A high-protein diet may therefore lead to obesity. Foods that are high in protein include meat, fish, cheese, and nuts. For a healthy diet, about a sixth of your total calorie intake should ideally be obtained from protein.
Fats are a source of energy and are essential for the absorption of certain vitamins. The amount and type of fat in your diet are important in determining your general health. Your intake of fat also affects your risk of developing coronary artery disease or of having a stroke. The link here is cholesterol, a fat-like substance that is essential for normal body functioning but can be a risk to health in excess. The higher the level of cholesterol in your blood (see Hypercholesterolaemia), the greater your risk of developing atherosclerosis, the narrowing of the arteries that may eventually lead to cardiovascular disease.
An individual’s level of blood cholesterol depends partly on genetic factors, but in many cases the main influence is the amount and type of fat in the diet. Fats can be either saturated or unsaturated, depending on their chemical structure; it is the saturated fats found in dairy products and meat that mainly contribute to raised cholesterol levels in the body. It is also advisable to avoid hydrogenated fats, sometimes known as transfats, which are artificially produced and have properties that are similar to saturated fats. Hydrogenated fats are often found in butter substitutes and manufactured food items such as biscuits and cakes. In contrast, unsaturated fats seem to provide some protection against cardiovascular disease, with polyunsaturates having a greater protective effect than monounsaturates.
For a healthier diet, make sure that no more than a third of your daily calories are obtained from fat and choose foods that contain unsaturated rather than saturated fats. You should also try to limit your intake of foods that are high in cholesterol, such as shellfish and eggs. However, the fat intake of young children should not be restricted.
Vitamins and minerals both play vital roles in growth and metabolism (see Good sources of vitamins and minerals). Apart from vitamin K, which is formed by intestinal bacteria, and vitamin D, which is produced in the skin by the action of sunlight, all vitamins and minerals must be obtained from your diet.
The majority of people in the UK obtain adequate amounts of vitamins and minerals from their diet. Some vitamins, including vitamins A, D, E, and K, are harmful if they are consumed in excess. However, some people may need to take vitamin and mineral supplements. For example, women who regularly lose iron through heavy menstrual bleeding may be advised to take iron supplements. Children who are reluctant to eat a normal range of foods may be given vitamins A, C, and D in the form of drops. Elderly people who have a low or restricted intake of food may also need supplements. Older women should make sure they have adequate calcium in their diet to help to prevent the bone disorder osteoporosis. People who eat a vegetarian diet usually have an adequate intake of vitamins and minerals, but vegans, who eat no animal products, are at risk of vitamin B12 deficiency and should also ensure that they get enough calcium in their diet.
Water is essential for life, and makes up almost four-fifths of the body. Water is lost by sweating and passing urine and must be replaced. Inadequate water intake may lead to kidney problems, such as kidney stones, and constipation. Some water is obtained from solid foods, but you should also drink plenty of fluids. Try to drink at least 8 glasses (2 litres/3
The body needs a constant reserve of energy to function properly. Energy from food is measured in units called kilo-joules (kJ) or kilocalories (kcal), which are usually referred to simply as calories.
The number of calories you need to obtain from your diet depends on how much energy your body uses. This depends partly on how efficiently your body cells use energy, which is genetically determined, and partly on your level of physical activity. The rate at which your body uses energy simply to maintain basic processes such as breathing and digestion is called your basal metabolic rate (BMR). Extra calories are needed for all your day-to-day activities. Energetic activities, such as sports, increase the calorie requirement. Most of these extra calories should come from complex carbohydrates, such as bread. However, people who have physically demanding jobs also require extra fat and sugar, both of which are rich sources of energy.
Your stage of life also has an influence on your energy requirements. For example, a growing, active teenager usually needs more calories than an adult. Calorie requirements tend to decrease as you get older because BMR declines with age and activity may be reduced. A pregnant woman needs more calories than a nonpregnant woman.
Fresh foods tend to be healthy, but they often have a short shelf life. Processing techniques are used to increase shelf life. Many processed foods have as many vitamins and minerals as fresh food. Food preservatives prevent the growth of microorganisms, which could cause food poisoning.
Labels on packaged food (see Understanding food labels) provide information about the content, allowing you to compare similar products and make healthy choices. In the UK, regulations are in place concerning how food is labelled and the type of nutritional information that is given. In addition to listing food ingredients and nutritional information, many food labels provide extra information so that people who, by choice or necessity, restrict their diet in some way can make decisions about which foods to select or avoid.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.