diet

Range of foods eaten by an animal each day; it is also a particular selection of food, or the total amount and choice of food for a specific person or group of people. Most animals require seven kinds of food in their diet: proteins, carbohydrates, fats, vitamins, minerals, water, and roughage. A diet that contains all of these things in the correct amounts and proportions is termed a balanced diet. The amounts and proportions required varies with different animals, according to their size, age, and lifestyle. The digestive systems of animals have evolved to meet particular needs; they have also adapted to cope with the foods available in the surroundings in which they live. The necessity of finding and processing an appropriate diet is a very basic drive in animal evolution. Dietetics is the science of feeding individuals or groups; a dietician is a specialist in this science.

Dietary requirements may vary over the lifespan of an animal, according to whether it is growing, reproducing, highly active, or approaching death. For instance, increased carbohydrate for additional energy, or increased minerals, may be necessary during periods of growth.

An adequate diet for humans is one that supplies the body's daily nutritional needs (see nutrition), and provides sufficient energy to meet individual levels of activity. The average daily requirement for men is 2,500 calories, but this will vary with age, occupation, and weight; in general, women need fewer calories than men. The energy requirements of active children increase steadily with age, reaching a peak in the late teens. At present, about 450 million people in the world – mainly living in famine or poverty-stricken areas, especially in countries of the developing world – subsist on fewer than 1,500 calories per day. The average daily intake in developed countries is 3,300 calories.

The strong link between health and food has long been recognized, and a well-balanced diet is essential to ensure the body's peak performance. There are many factors that determine an individual's energy requirements – such as age, sex, occupation, and general lifestyle – and it is important that diets provide the right amount of energy to match individual needs. In addition to energy-producing carbohydrates, however, the body has many other nutritional requirements, and so the emphasis of a healthy, well-balanced diet is not just a matter of calorie-counting, but ensuring that all the components that together comprise a well-balanced diet are present in adequate and correct amounts. Each person needs minerals, proteins, vitamins, and certain fats to build and maintain tissues and to regulate body functions. A diet that lacks any of these food elements may cause certain deficiency diseases.

Carbohydrates and fibre The carbohydrates include starches and sugars, which are the major source of metabolic energy. Chemically they are made up of carbon, hydrogen, and oxygen. The basic carbohydrates are the simple sugars, or monosaccharides, and nutritionally glucose, fructose, and galactose are the most important of these. The complex sugars, or disaccharides, include sucrose and maltose. Both monosaccharides and disaccharides are water-soluble. Cellulose and starches, which are found in plant cells, are molecules of simple sugars that have combined to produce polysaccharides. Unlike the sugars they are insoluble. Starches and sugars are present in grains, fruits, pulses (peas and beans), nuts, vegetables, and milk. In a balanced diet, starch-rich food, such as whole grains, root vegetables, pulses, and bananas, should provide half the calorie intake. The fibre content of these foods helps to keep the bowel healthy and protects from high cholesterol levels, some cancers, gallstones, and obesity.

Proteins Proteins are composed of long chains of amino acids of which there are about 20 different kinds. In addition to carbon, hydrogen, and oxygen, they also contain nitrogen and small amounts of sulphur. Some proteins, such as haemoglobin – the red pigment in blood – are soluble; others, such as keratin, found in hair and fingernails, are insoluble. Unlike carbohydrates, proteins are not normally used to provide energy. They are used by the body to build and repair cells and to regulate metabolism. Proteins are also needed by the white cells in the blood to produce antibodies; these assist the body's immune system to ward off attacks by bacteria and viruses. All fruits and vegetables contain some protein; good sources include peas, beans, lentils, grains, nuts, seeds, and potatoes. These proteins have a low biological value (LBV). Animal proteins include milk, cheese, meat, eggs, and fish, and these proteins are said to have a high biological value (HBV).

Fats Fats are composed of glycerol and fatty acids and, like carbohydrates, contain carbon, hydrogen, and oxygen atoms. There are two types of fatty acids: saturated and unsaturated. Unsaturated fatty acids are found in fish oils and vegetable oils – coconut oil and palm oil are the only saturated vegetable oils. Most saturated fats are animal fats, and are solid, such as lard and butter. Margarine is saturated by the process of hydrogenation which forces hydrogen gas through vegetable oil. Fats provide the most concentrated form of energy; in other words, when they are burned in the body, they supply more than twice the number of calories per gram available from carbohydrates. They are also high in cholesterol. Therefore it is necessary to control fat intake where obesity and cholesterol levels present a health problem. Heart disease has been linked to the consumption of hydrogenated fats. However, fats are a necessary part of any well-balanced diet. They provide insulation, build cells, and facilitate metabolism. Unsaturated fatty acids are essential for healthy skin, circulation, bone, brain, and nerves.

Most fatty acids can be synthesized by the body, with the exception of three, the essential fatty acids (EFAs): linoleic acid, linolenic acid, and arachidonic acid. These have to be supplied from food; vegetable oils, particularly if they are unrefined and cold pressed, are the best sources of EFAs. Sunflower and safflower oils are among the richest sources, containing up to 90%. EFAs are vital for the maintenance of good health. Among their many uses, they help to prevent atherosclerosis (coronary heart disease) and the formation of blood clots in arteries, and they regulate such diverse reactions as stomach secretions, hormone release, and pancreatic function.

Minerals These are inorganic substances, such as compounds of calcium, phosphorus, and iron, and although only very small amounts are needed, they play an important part in a balanced diet. Minerals build and maintain bones and teeth, control the composition of body fluids and cells, and release energy. For instance, calcium, which is found in milk, cheese, and bread, is necessary for healthy bones and teeth. A deficiency of this mineral can result in brittle bones and teeth. Similarly, phosphorus, which is contained in milk, is also needed in the formation of bones and teeth. Iron, which is present in liver and egg yolk, is used by the body in the manufacture of haemoglobin; a deficiency of this mineral can result in various forms of anaemia.

Vitamins These are organic substances which, like minerals, are used by the body in very small amounts but are vital for normal body chemistry. They are all obtained from food, but vitamin D is also produced by the action of daylight on the skin, and vitamin K is produced by micro-organisms in the bowel. Again, like minerals, an insufficient intake of one or more vitamins can result in a wide range of deficiency diseases. For instance, vitamin A, which can be found in eggs, milk, dairy products, fish liver oil, and animal liver, helps to maintain the cells lining the respiratory system and the mucous membranes of the eyes, ears, nose, throat, and bladder; it helps to fight colds, and a deficiency of the vitamin can lead to respiratory infections.

Some 16 different B vitamins have been isolated, and as they usually occur together, they are known as vitamin B complex. They can be found in vegetables and animal foods, such as organ meats (particularly liver), wholemeal bread, yeast extract, and brown rice, and are vital for converting carbohydrates to glucose and food into energy. When B vitamins are lacking in the body, carbohydrates are not fully utilized and this can result in stress, nervousness, constipation, fatigue, and indigestion. Most of the complex are concerned with various processes in the liver, eyes, skin, and hair, and have a wide range of effects, from alleviating stress to preventing atherosclerosis.

Vitamin C, which is also known as ascorbic acid, is a very unstable, water-soluble vitamin, which is easily lost in food preparation – not only in cooking, but also in peeling, stoning, and soaking fruits and vegetables. Among its best natural sources are citrus fruits, peppers, broccoli, tomatoes, cabbage, green leafy vegetables, and potatoes. Vitamin C has many functions in the body, among the best known of which are the prevention of scurvy, a skin condition, and fighting the symptoms of the common cold. In addition, it helps to form collagen (a sub-skin ‘cement’), increases immune responses to infectious diseases, makes connective tissue, aids the absorption of iron, assists in the formation of bones and teeth, and has been found to lower the risk of cancers of the mouth, oesophagus, lung, stomach, colon, cervix, and breast. It has also been shown by several studies to lower cholesterol levels. Human beings are one of very few mammals that cannot synthesize vitamin C, and a regular daily intake from a food source is necessary.

Vitamin D is a fat-soluble vitamin which is supplied from both food, especially milk and dairy foods, and exposure to the sun. It is stored mainly in the liver, but also in smaller quantities in the skin, brain, and bones. Vitamin D promotes absorption of calcium and phosphorus which are both vital for strong teeth and bones, and for the prevention of rickets in children. It also helps to maintain a healthy nervous system, normal heartbeat, and efficient blood clotting. Since vitamin D is scarce in vegetables, people who do not drink milk may need to supplement their diet with cod liver oil and fish such as sardines, herring, salmon, and tuna. Overdoses of the vitamin can lead to toxicity symptoms such as diarrhoea, nausea, excessive urination, and kidney damage.

Special diets There are a number of reasons why a special diet may be adopted or advised. It may be medically recommended in order to balance, increase, or limit certain nutrients. Increasing numbers of people in developed countries, particularly children, are developing allergies or intolerance to various foods, which usually necessitates the elimination of the allergenic foods from the diet. Special diets may be undertaken to lose weight, by a reduction in calorie intake or selection of specific foods. It has been estimated that in the UK, over £80 million a year is spent on slimming products and a further £5.5 million on slimming magazines. In the USA, the slimming industry totals some $40 billion a year. Special diets may also be observed on religious or moral grounds – such as a vegetarian or vegan diet. However, it should be remembered that a healthy diet is a balanced diet, and no one should undertake any form of restricted diet without medical advice and supervision. Unfortunately, there is often controversy among experts regarding the best dietary approach to controlling disease, and this must be taken into account when special diets are adopted.

Malnutrition and poor nutrition It has been estimated that in the USA three out of four people die from diet-related diseases. Obesity, heart attacks, hypertension (high blood pressure), strokes, diabetes, gallstones, diverticular disease, and bowel and breast cancers can all be linked with diet – it is thought that at least 35% of cancers could be prevented by improved diet. There is little danger of people suffering from obesity in the countries of the developing world – although unfortunately deficiency diseases in those nations are widespread. In many areas the combined effects of drought and civil war mean that harvests do not provide sufficient food to feed the people and many die from starvation. Even where there is enough food to keep people alive, they may suffer from malnutrition.

One common form of malnutrition is kwashiorkor, which is caused by a lack of protein in the diet. It is commonest in infants after breast feeding has finished, and is usually caused because there is no high-protein food available. The staple foods of many of the poorest countries are carbohydrate foods such as rice.

It is a paradox that malnutrition – or nutritional deficiencies – are also common in the overfed populations of the affluent countries. The over consumption of commercially-processed ‘fast’ or ‘junk’ foods has resulted in poor and unbalanced diets, and nutritional deprivation; this is becoming an increasing problem among younger people. However, unlike the undernourished populations of the poorer nations, the nutritionally-deprived people of the wealthier countries have, as individuals, ready access to an immediate remedy for a large proportion of their ills – namely, they have both the means and opportunity to choose a nutritionally-balanced diet.

diet

Meeting or convention of the princes and other dignitaries of the Holy Roman (German) Empire, for example, the Diet of Worms of 1521 which met to consider the question of Luther's doctrines and the governance of the empire under Charles V.