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| Vitamins and minerals |
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| Vitamins |
Until the beginning of the 20th century, it was believed that the only components of a diet necessary for health were proteins, fats and carbohydrates, and a number of inorganic elements. This view changed when it was found that minute amounts of additional materials were also essential.
These could be extracted from a variety of foods and appeared to be of two types - fat soluble A and water-soluble B. They were later each discovered to contain several active components, or vitamins. The fat-soluble include vitamins A, D, E and K; the water-soluble vitamins include thiamin B1, riboflavin B2, niacin, folate, vitamin B6, vitamin B12, vitamin C, biotin and pantothenic acid.
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| Vitamin A |
Vitamin A is good for skin and colour vision.
Vitamin A, or retinol to give it its proper name, is essential for colour vision and vision in dim light. It is also necessary for the maintenance of healthy skin and surface tissues and is probably involved in the senses of taste and balance.
However, there is an association between very high levels of vitamin A (retinol) consumption during pregnancy and the incidence of some birth defects. As a matter of precaution therefore, women who are or might become pregnant are advised not to take vitamin A supplements except on the advice of their doctor and, as an additional precaution, not to eat liver or liver products since these can be very rich sources of retinol.
Vitamin A is not widely distributed in food. Animals store vitamin A in their livers which are often extremely concentrated, but very variable, sources of retinol. Fish liver oils are also a very rich natural source of vitamin A. Kidneys, dairy produce and eggs contain substantial amounts. Variable amounts of beta-carotene, which can be converted in the body to retinol, are found in carrots and dark green or yellow vegetables, roughly in proportion to the depth of their colour. All margarine and spreads contain about the same amount of vitamin A as butter.
The main sources of vitamin A in the diet are liver, carrots, margarine and spreads, and milk.
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| Vitamin D |
Vitamin D aids calcium absorption into the body.
Vitamin D helps to maintain bone mineralisation by ensuring a plentiful supply of calcium in the blood. This is achieved primarily by enhancing the absorption of dietary calcium from the intestine. But vitamin D may also have a direct positive effect on the depositing of calcium in bone. Some elderly people may suffer from bone softening (osteomalacia), because they absorb too little calcium from a diet which is low in both calcium and vitamin D.
Vitamin D is obtained both from the action of sunlight on a substance in the skin, and from the diet. Sunlight is by far the most important source for most people, who may need little or no extra from food.
But several groups of people should ensure their food contains sufficient vitamin D, including those who receive little exposure to sunlight such as older housebound people.
Few foods contain vitamin D. All those which do so naturally are products of animal origin. Some foods may be fortified including margarine, some breakfast cereals and Complan.
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| Vitamin E |
Vitamin E is a great antioxidant.
A number of related compounds (tocopherols) show vitamin E activity, the most potent being alpha-tocopherol. Its major activity in the body is as an antioxidant. The vitamin is critically important for maintaining the structure of cell membranes.
Vitamin E is found widely in foods, and like other fat-soluble vitamins is stored in the body.
Most foods contain vitamin E, the richest sources being vegetable oil, nuts and seeds, some cereal products and egg yolk. Animal fats, meat, fruit and vegetables contain comparatively little.
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| Vitamin C |
Vitamin C is important for maintaining healthy connective tissues.
Vitamin C is necessary for the maintenance of healthy connective tissue. Humans are among the few animals, along with monkeys and guinea pigs, unable to form their own vitamin C, and must obtain it from food. Requirements are increased during recovery from surgery or severe accidents.
Vitamin C is not widely distributed in foods. Small amounts occur in milk and liver, but virtually all the vitamin C in most diets is derived from vegetables and fruit. Many people do not eat enough vegetables and fruit and vitamin C is readily lost from these foods during storage, preparation and cooking. Citrus fruits and blackcurrants are particularly rich sources. The main sources of vitamin C in the diet are fruit and fruit juices.
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| Thiamin |
Thiamin is necessary for the steady and continuous release of energy from carbohydrate. Thiamin requirements are thus related to the amount of carbohydrate, and more or less to the amount of energy in the diet.
Thiamin is widely distributed in both animal and vegetable foods. Good sources include milk, offal, pork, eggs, vegetables and fruit, wholegrain cereals, bread and fortified breakfast cereals. Cooking, however, may result in considerable losses from these foods.
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| Riboflavin |
Riboflavin (vitamin B2) is essential for the utilisation of energy from food and to maintain healthy eyes and skin.
Although riboflavin is widely distributed in foods, especially those of animal origin, about 40 per cent of the average intake is derived from one source alone - milk and milk products. The main sources of riboflavin in the diet are milk, meat, eggs and fortified breakfast cereals.
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| Niacin |
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Niacin is involved in the utilisation of food energy.
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| Vitamin B6 |
Vitamin B6, is involved in the metabolism of proteins, and is necessary in the formation of haemoglobin in the blood.
Vitamin B6 occurs widely in food, especially in meat and fish, eggs, wholegrain cereals and some vegetables. Microorganisms in the intestine synthesise the vitamin, some of which may be available to the body.
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| Folic acid |
Folate (folic acid and various derivatives) has several functions, including its action with vitamin B12 in rapidly-dividing cells. Deficiency leads to a characteristic form of anaemia which must be distinguished from that caused by a deficiency of vitamin B12. Folate deficiency can result not only from a poor diet, but also from increased requirements arising from certain medical conditions in the elderly and when there is decreased absorption of folate in gastro-intestinal disease.
Women who increase their folic acid or folate intake before pregnancy and during its early stages can help to reduce their risk of having a baby with certain birth defects.
Folate occurs in small amounts in many foods. Rich sources are generally also rich in other B-vitamins. They include offal, yeast extract, green leafy vegetables and fortified foods such as many breakfast cereals. Most fruits, meat and dairy produce contain comparatively little. Folate is readily destroyed in cooking, much being lost in the water used for cooking vegetables.
The main sources of folate in the diet are potatoes, fortified breakfast cereals, bread and fresh vegetables.
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| Vitamin B12 |
Vitamin B12 is needed for healthy bones.
Vitamin B12, the last vitamin to be isolated in 1948, is a mixture of several related compounds, all of which contain the trace element cobalt. Together with folate, it is needed by rapidly-dividing cells, such as those in bone marrow which form blood cells. Deficiency leads to a characteristic anaemia and the degeneration of nerve cells. Because vitamin B12 does not occur in vegetable foods, deficiency may occur in vegans who do not consume meat, milk, eggs, or any special supplement.
Vitamin B12 occurs only in animal products and in microorganisms including yeast. Liver is the richest source, but useful amounts also occur in eggs, cheese, milk, meat and fish, and in fortified breakfast cereals. The main sources of vitamin B12 in the diet are offals, other meat and meat products, and milk.
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| Biotin |
Biotin is essential for the metabolism of fat. Very small amounts are required and sufficient may be made by bacteria normally inhabiting the large intestine. It is therefore probable that no additional biotin need be provided in the diet, except in the very unusual situation when large quantities of raw eggs are consumed. Raw, but not cooked, egg white contains a substance called avidin which combines with biotin making it unavailable to the body.
Rich sources of biotin include offal and egg yolk. Smaller amounts are obtained from milk and dairy products, cereals, fish, fruit and vegetables.
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| Pantothenic acid |
Pantothenic acid is necessary for the release of energy from fat and carbohydrate. Dietary deficiencies of this vitamin are rare in humans because it is so widespread in food.
Animal products, cereals and legumes are especially rich sources.
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| Minerals |
About 15 minerals are known to be essential for the maintenance of good health and must be derived from food.
Minerals have three main functions:
- As constituents of bones and teeth. These include calcium, phosphorus and magnesium.
- As soluble salts which help to control the composition of body fluids and cells. These include sodium, chloride, potassium, magnesium and phosphorus.
- As essential to many enzymes and other proteins such as haemoglobin, which are necessary for the release and utilisation of energy. These include iron, phosphorus and zinc amongst others.
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| Calcium |
Calcium is the most abundant mineral in the body. All but about 1 per cent of it occurs in the bones and teeth, together with more than three-quarters of the body's phosphorus.
In addition to giving strength to bones, these minerals act as a reserve supply for other needs and the calcium is constantly withdrawn and replaced from the blood at carefully controlled rates. The remaining 5-10g of calcium is essential for the contraction of muscles, including heart muscle, for nerve function, for the activity of several enzymes, and for normal clotting of the blood.
The average intake of calcium from food consumed at home is around 800mg per day, which is adequate for the needs of most people. Too little calcium in adults may show as osteomalacia. However, the primary deficiency in this instance is of vitamin D, which causes too little calcium to be absorbed.
As people age, their bone density decreases through losses of all components, not only calcium. This decline is particularly rapid in women about the time of the menopause. Large losses of bone lead to a condition, most common in elderly women, known as osteoporosis in which the bones are brittle and easily fractured. The basic cause of osteoporosis is unknown. However, it is wise for young people to obtain plenty of dietary calcium and to take plenty of exercise to aid maximum bone development.
Few foods apart from milk, yogurt, cheese and most bread, through fortification, contain significant sources of calcium. It is therefore important that these foods are included in the diet.
The main sources of calcium in the diet are milk, cheese, bread and vegetables. For some people, hard water and the bones in canned sardines and salmon can be important.
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| Phosphorus |
Phosphorus is the second most abundant mineral in the body and, in the form of various phosphates, has a wide variety of essential functions. Calcium phosphates provide the strength of the bones and teeth, and inorganic phosphates are a major constituent of all cells. Phosphates play an essential role in the liberation and utilisation of energy from food. Some B-vitamins are combined in the body with phosphates before they can be active.
Phosphorus is present in nearly all foods. The main sources of phosphorus in the diet are milk and milk products, and meat and meat products.
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| Iron |
The healthy adult body contains 3-4g of iron, more than half of which is in the form of haemoglobin, the red pigment of blood. Anaemia due to lack of iron in the diet is one of the commonest nutritional disorders, particularly likely to affect women. Iron is also present in the muscle protein myoglobin, and is stored to some extent in organs such as the liver.
Iron plays a major part in the body's use of oxygen since haemoglobin transports oxygen from the lungs to the tissues, and other iron-containing substances utilise the oxygen within the cells.
The average adult daily iron intake is around 14mg for men and 12mg for women. These values are above the recommended intake for men but below that for women, many of whom have increased iron needs due to menstrual blood losses.
If food provides insufficient iron to replace the body's losses, the stores are gradually depleted and eventually anaemia results.
The extent to which iron from food is absorbed depends on whether it is in the form of haem or non-haem iron. The absorption of non-haem iron is influenced by the body's iron status and by the combination of foods ingested. Haem iron, which is the form found in the red pigments of meat and offals, is relatively well absorbed. The non-haem iron found in cereals, pulses, vegetables, fruits, eggs and dairy products is generally less well absorbed. But absorption increases when the body's stores are depleted and when needs are greatest. Its absorption also depends on other factors in the diet such as the presence of meat or vitamin C which increase absorption or tannins in tea and some forms of fibre which decrease absorption of iron.
Anaemia due to blood loss is likely after surgery and accidents. An iron rich diet should be given during convalescence.
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| Magnesium |
Most of the magnesium in the body is present in the bones, but it is also an essential constituent of all cells and is necessary for the functioning of some of the enzymes which are involved in energy utilisation.
Magnesium is widespread in foods, especially those of vegetable origin, because it is an essential constituent of chlorophyll.
The main sources of magnesium are milk, bread and other cereal products, potatoes and other vegetables.
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| Zinc |
Zinc helps with the healing of wounds, and is also associated with the activity of a wide variety of enzymes. About one third of the zinc in the body is present in the bones.
Zinc is present in a wide range of foods, particularly in association with protein, and meat and dairy products are excellent sources.
About one third of the zinc in the diet as a whole is absorbed, but this is reduced if large amounts of wholegrain cereals rich in dietary fibre and phytic acid are eaten, although the amount of zinc in wholegrain cereals is enough to offset this.
Average adult intakes are between 9 and 12 mg per day.
The main sources of zinc in the diet are meat and meat products, milk, bread, and other cereal products.
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| Iodine |
Iodine is an essential constituent of hormones produced by the thyroid gland in the neck, and most of the iodine in the body is in this gland. It is needed for wound healing, and taste and flavour perception.
The richest source of iodine is seafood. Because of the widespread use of iodine in animal feed, milk and dairy products are the main sources of iodine in the diet, and meat and eggs are also important. The average adult intake is about 200µg per day.
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