Chemical compounds that enable the body to carry out essential functions
Vitamins are complex chemicals that are essential for the normal growth, development, and functioning of the body. The principal source of most vitamins, except vitamin D and vitamin K, is a balanced diet (see Good sources of vitamins and minerals).
Most healthy people do not need vitamin supplements because a healthy, balanced diet provides enough of all the vitamins needed. However, supplements may be prescribed for people who have conditions such as alcohol dependence that deplete the body’s supply of certain vitamins. Supplements may also be prescribed for people who need larger than normal quantities of certain vitamins because they are recovering from serious injuries or they are taking medications that affect the action or absorption of particular vitamins.
Many types of vitamin are necessary for the maintenance of good health. They are broadly categorized into two main groups: fat-soluble and water-soluble. Fat-soluble vitamins, such as A, D, E, and K, are not readily excreted in urine. They are stored in fatty body tissues, such as the liver, for long periods and do not normally need to be supplemented by a daily intake. Water-soluble vitamins, which include B, C, and folic acid, are rapidly excreted in urine. The body cannot hold long-term reserves of these vitamins, and they must be replaced on a daily basis through food sources to prevent deficiencies.
This vitamin is required for growth, healthy skin and surface tissues, and good eyesight and night vision. Vitamin A is also necessary for fertility in both sexes. Supplements may be prescribed for people with conditions that can cause deficiency of this vitamin, such as certain intestinal disorders. Diets that are too low in fat may also lead to vitamin A deficiency.
Taking excessive doses of vitamin A, however, can cause dry skin, nose-bleeds, and hair loss. Excess vitamin A, especially in the form of retinol, has also been associated with a higher risk of bone fractures. You should not take vitamin A supplements if you are either pregnant or planning to become pregnant because taking too much of this vitamin may cause fetal abnormalities; you should also avoid eating liver and liver products because they contain high levels of vitamin A.
This vitamin is necessary for the normal functioning of the brain and peripheral nerves, the heart, and the muscles. Thiamine is present in unprocessed food, and most people should be able to obtain enough by eating a balanced diet. A severe lack of thiamine can cause certain deficiency diseases, such as beriberi, which affects the nervous system. Your doctor may prescribe supplements for conditions in which severe thiamine deficiency can occur, such as alcohol dependence or alcohol-related liver disease.
The risk of developing side effects from thiamine supplements is very low. However, when given intravenously, this vitamin may sometimes cause serious allergic reactions.
This vitamin, also known as nicotinic acid, plays a vital role in the activities of many enzymes involved in energy metabolism. Severe niacin deficiency can result in the skin disorder pellagra, which primarily occurs in rural areas of poor countries. You may be prescribed niacin supplements if you have alcohol-related liver disease, in which niacin deficiency can occur, or a bowel disorder that results in poor absorption of food, a condition known as malabsorption.
Large doses of niacin inhibit the body’s synthesis of some fats and are used to treat high levels of cholesterol and triglycerides (see Lipid-lowering drugs). Niacin may cause some side effects, including itching, flushing, and headaches. If excessive amounts of niacin are taken, liver damage and gout can occur as a result.
Vitamin B6 aids the activities of enzymes and hormones involved in the body’s processing of carbohydrates, proteins, and fats, the manufacture of red blood cells and antibodies, the functioning of the digestive and nervous systems, and the maintenance of healthy skin. Supplements may be required by people with malabsorption or severe alcohol dependence, those taking certain drugs (such as penicillamine and isoniazid), and elderly people who have a poor diet. Low-dose vitamin B6 may also sometimes be given to help relieve the symptoms of premenstrual syndrome. Long-term use of high-dose pyridoxine supplements has been associated with nerve damage, which may result in symptoms such as numbness and impaired physical coordination.
This vitamin plays a vital role in the activities of various enzymes in the body and is important in the production of genetic material (and therefore in growth and development) and red blood cells, in the utilization of dietary folic acid, and in the functioning of the nervous system. The most common cause of B12 deficiency is pernicious anaemia (see Megaloblastic anaemia), in which a substance necessary for the absorption of the vitamin is not produced by the body. Less commonly, deficiency may also result from a gastrectomy (the removal of all or part of the stomach), malabsorption, or a vegan diet (because B12 occurs naturally only in animal products). The effects of B12 deficiency include megaloblastic anaemia, sore mouth and tongue, and symptoms resulting from damage to the spinal cord, such as numbness and tingling in the limbs; there may also be depression and memory problems.
Treatment for pernicious anaemia and B12 deficiency due to a gastrectomy involves regular injections of the vitamin. Side effects are very uncommon but can include itching, flushing, and nausea. If deficiency results from a vegan diet or malabsorption, oral supplements may be required.
This vitamin is needed for nervous system function and for the formation of red blood cells. Dietary sources include green leafy vegetables, liver, and nuts. Supplements may be advised if you are taking certain drugs, including some antimalarials or anticonvulsant drugs, which deplete folic acid. Folic acid also lowers levels of the amino acid homocysteine, high levels of which may increase the risk of coronary artery disease.
Women are advised to take the recommended dose of folic acid supplements before conception and during the first 12 weeks of pregnancy to reduce the risk of neural tube defects, such as spina bifida, in the fetus; a higher dose may be recommended if there is a family history of neural tube defects.
This vitamin is needed for the formation of bones, teeth, ligaments, and blood vessels. It is found in fresh fruit and vegetables, and therefore severe deficiency is uncommon. If it does occur, a severe deficiency causes a disorder called scurvy, which is rare in developed countries. However, mild deficiency is common among people who eat a poor diet, and particularly among elderly people living alone.
Supplements of vitamin C are rarely necessary except to treat scurvy. In some people who are prescribed iron supplements, supplementary vitamin C may also be prescribed because the vitamin improves the efficiency of iron absorption by the body. There is no evidence that vitamin C supplements can protect against or alleviate the symptoms of colds, or promote wound healing.
This vitamin helps to regulate the amount of calcium in the body. The body’s requirements for the vitamin are usually met by normal diet and exposure to sunlight, which the body needs to make vitamin D. However, some people may not receive sufficient sun exposure, especially in winter, to make enough vitamin D and are therefore at risk of deficiency. Those most at risk are people with dark skin who live in northern latitudes. Vitamin D deficiency can cause a disorder in which the bones become weak and soft (see Osteomalacia and rickets).
Vitamin D supplements may be given to increase calcium levels in people with hypoparathyroidism; in this disorder the body produces too little of a hormone called parathyroid hormone, which controls levels of calcium in the blood. Supplements are sometimes given to elderly people and to premature infants, who may not obtain enough dietary vitamin D or sun exposure. They may also be recommended for some dark-skinned people at risk of deficiency. Supplements may also be given in combination with calcium to prevent or treat osteoporosis.
Taking high doses of vitamin D may increase levels of calcium in the body, which may lead to calcium deposits in soft tissues, impaired kidney function, or impaired growth in children.
Although usually referred to as a single substance, vitamin E is actually the collective term for a group of substances that are essential for normal cell structure, for maintenance of the activities of certain enzymes, and for the formation of red blood cells. Vitamin E also protects the lungs and other tissues from damage by pollutants and is believed to slow cell aging. Dietary deficiency of vitamin E is rare; it is most common in people with malabsorption or certain liver disorders, and in premature infants. Deficiency leads to destruction of red blood cells, which eventually results in anaemia. In infants, it causes irritability and oedema (accumulation of fluid in tissues). Supplementary vitamin E is prescribed only to treat established deficiency. Prolonged excessive intake of the vitamin may cause abdominal pain, nausea, and diarrhoea. It may also reduce intestinal absorption of vitamins A, D, and K.
This vitamin is essential for the formation of blood clotting factors, substances that are necessary for blood to clot and seal off damaged blood vessels. Most of the required amount is produced by bacteria in the intestines, but the body also obtains some vitamin K from dietary sources, such as green leafy vegetables, eggs, and liver.
Newborn babies lack the intestinal bacteria that produce vitamin K and, as a result, are at risk of developing a condition called haemorrhagic disease of the newborn, which results in easy bruising and internal bleeding. For this reason, newborn babies are given vitamin K routinely. Supplements may be given to older children and adults taking antibiotics for long periods because these drugs destroy the intestinal bacteria that produce vitamin K. In addition, people who are unable to absorb nutrients, or those who experience abnormal bleeding as a side effect of oral anticoagulant drugs (see Drugs that prevent blood clotting), may be given supplements of this vitamin.
Preparations containing a combination of vitamins are used to treat people who have nutritional deficiencies, alcoholism, and other conditions in which dietary vitamin intake is insufficient. Some of these multivitamin preparations contain iron and other minerals. Multivitamins can be purchased over the counter, usually in the form of oral liquid, tablets, or capsules.
Vitamin A may harm a developing fetus. Do not take supplements if you are pregnant or planning to conceive, except on medical advice.
Do not exceed the prescribed dose of vitamin D. Excessive intake can cause a dangerous rise in the level of calcium in the blood.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.