Vitamins are complex organic compounds that occur naturally in plants and animals. People and other animals need these compounds in order to maintain life functions and prevent diseases. About 15 different vitamins are necessary for the nutritional needs of humans. Only minute amounts are required to achieve their purpose, yet without them life cannot be maintained. Some vitamins, including vitamins A, D, E, and K, are fat soluble and are found in the fatty parts of food and body tissue. As such they can be stored in the body. Others, the most notable of which are vitamin C and all the B-complex vitamins, are water soluble. These vitamins are found in the watery parts of food and body tissue and cannot be stored by the body. They are excreted in urine and must be consumed on a daily basis.


Vitamins were not discovered until early in the twentieth century. Yet it was common knowledge long before that time that substances in certain foods were necessary for good health. Information about which foods were necessary developed by trial and error—with no understanding of why they promoted health. Scurvy, for example, had long been a dreaded disease of sailors. They often spent months at sea and, due to limited ways of preserving food without refrigeration, their diet consisted of dried foods and salted meats. In 1746, English naval captain and surgeon James Lind (1716–1794) observed that 80 out of his 350 seamen came down with scurvy during a 10-week cruise. He demonstrated that this disease could be prevented by eating fresh fruits and vegetables during these long periods at sea. Because they lasted a long time, limes became the fruit of choice. In 1795, lemon juice was officially ordered as part of the seaman's diet. The vitamin C found in both limes and lemons prevented scurvy in sailors.

In 1897, Dutch physician Christian Eijkman (1858–1930) found that something in the hulls of rice (thiamine, a B vitamin) not present in the polished grains prevented the disease beriberi (a disease that affects the nerves, the digestive system, and the heart). Soon after, British biochemist Frederick G. Hopkins (1861–1947) fed a synthetic diet of fats, carbohydrates, proteins, and minerals (but no vitamins) to experimental rats. The rats showed poor growth and became ill, leading Hopkins to conclude that there were some "accessory food factors" necessary in the diet. Eijkman and Hopkins shared the 1929 Nobel Prize in physiology and medicine for their important work on vitamins.

Words to Know

Antioxidant: A substance that can counteract the effects of oxidation.

Beriberi: A disease caused by a deficiency of thiamine and characterized by nerve and gastrointestinal disorders.

Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.

Deficiency diseases: Diseases caused by inadequate amounts in the diet of some substance necessary for good health and the prevention of disease.

Fat-soluble vitamins: Vitamins such as A, D, E, and K that are soluble in the fatty parts of plants and animals.

Pellagra: A disease caused by a deficiency of niacin and characterized by severe skin problems and diarrhea.

Proteins: Large molecules that are essential to the structure and functioning of all living cells.

Scurvy: A disease caused by a deficiency of vitamin C, which causes a weakening of connective tissue in bone and muscle.

Water-soluble vitamins: Vitamins such as C and the B-complex vitamins that are soluble in the watery parts of plant and animal tissues.

Finally, in 1912, Polish American biochemist Casimir Funk (1884–1967) published a paper on vitamin-deficiency diseases. He coined the word vitamine from the Latin vita, for "life," and amine, because he thought that all of these substances belonged to a group of chemicals known as amines. The e was later dropped when it was found that not all vitamins contained an amine group. Funk identified four vitamins (B 1 or thiamine, B 2 , C, and D) as substances necessary for good health and the prevention of disease.

Since that time additional vitamins have been isolated from foods, and their relationship to specific diseases have been identified. All of these accessory food factors have been successfully synthesized in the laboratory. There is no difference in the chemical nature of the natural vitamins and those that are made synthetically, even though advertisements sometimes try to promote natural sources (such as vitamin C from rose hips) as having special properties not present in the synthesized form.

Nature of vitamins

Vitamins belong to a group of organic compounds required in the diets of humans and other animals in order to maintain good health: normal growth, sustenance, reproduction, and disease prevention. In spite of their importance to life, they are necessary in only very small quantities. The total amount of vitamins required for one day weigh about one-fifth of a gram. Vitamins have no caloric value and are not a source of energy.

Vitamins cannot be synthesized by the cells of an animal but are vital for normal cell function. Certain plants manufacture these substances, and they are passed on when the plants are eaten as food. Not all vitamins are required in the diets of all animals. For example, vitamin C is necessary for humans, monkeys, and guinea pigs but not for animals able to produce it in their cells from other chemical substances. Nevertheless, all higher animals require vitamin C, and its function within organisms is always the same.

Vitamins were originally classified into two broad categories according to their solubilities in water or in fat. As more vitamins were discovered, they were named after letters of the alphabet. Some substances once thought to be vitamins were later removed from the category when it was found that they were unnecessary or that they could be produced by an animal. Four of the better known vitamins, A, D, E, and K, are fat soluble. Other vitamins such as vitamin C and the B-complex vitamins are water soluble.

This difference in solubility is extremely important to the way the vitamins function within an organism and in the way they are consumed. Fat-soluble vitamins lodge in the fatty tissues of the body and can be stored there. It is, therefore, not necessary to include them in the diet every day. Because these vitamins can be stored, it also is possible (when consumed in excess) for them to build up to dangerous levels in the tissues and cause poisoning.

The Food and Drug Administration publishes a set of nutritional recommendations called the U.S. Recommended Dietary Allowances (USRDA) patterned on the needs of the average adult. These recommendations are based on the best information available but are less than perfect; most of the research upon which they are determined is done on experimental animals. Because the amounts of vitamins required are so small, precise work is very difficult.

A person who eats a balanced diet with plenty of fresh fruits and vegetables should receive adequate amounts of all the vitamins. Many vitamins, however, are very sensitive to heat, pressure cooking, cold, and other aspects of food preparation and storage and can be inactivated or destroyed.

A controversy about vitamins exists among experts regarding the dose needed to fight off some common diseases. According to these experts, the USRDA are really minimum requirements, and higher doses will keep a person healthier. Thus, many people worldwide take vitamin supplements as insurance that they are getting all they need. Overdosage on vitamins, especially the fat-soluble ones, can cause serious side effects, however, and in some cases they even interfere with the proper function of other nutrients.

Vitamin A

Vitamin A is present in animal tissue, mainly in liver, fish oil, egg yolks, butter, and cheese. Plants do not contain vitamin A, but they do contain beta carotene, which is converted to vitamin A in the intestine and then absorbed by the body. Beta carotene occurs most commonly in dark green leafy vegetables and in yellowish fruits and vegetables such as carrots, sweet potatoes, cantaloupe, corn, and peaches. The bodies of healthy adults who have an adequate diet can store several years' supply of this vitamin. But young children, who have not had time to build up such a large reserve, suffer from deprivation more quickly if they do not consume enough of the vitamin.

Vitamin A is necessary for proper growth of bones and teeth, for the maintenance and functioning of skin and mucous membranes, and for the ability to see in dim light. There is some evidence that it can help prevent cataracts and cardiovascular disease and, when taken at the onset of a cold, can ward it off and fight its symptoms. One of the first signs of a deficiency of this vitamin is night blindness, in which the rods of the eye (necessary for night vision) fail to function normally. Extreme cases of vitamin A deficiency can lead to total blindness. Other symptoms include dry and scaly skin, problems with the mucous linings of the digestive tract and urinary system, and abnormal growth of teeth and bones.

Vitamin A is stored in the fatty tissues of the body and is toxic in high doses. As early as 1596, Arctic explorers experienced vitamin A poisoning. In this region of extreme conditions, the polar bear was a major source of their food supply, and a quarter pound of polar bear liver contains about 450 times the recommended daily dose of vitamin A. Excessive amounts of vitamin A cause chronic liver disease, peeling of the skin of the entire body, bone thickening, and painful joints. However, it is nearly impossible to ingest beta carotene in toxic amounts since the body will not convert excess amounts to toxic levels of vitamin A.

Vitamin D

Vitamin D is often called the sunshine vitamin. It is produced when compounds that occur naturally in animal bodies are exposed to sunlight. Thus, it is difficult to suffer a vitamin D deficiency if one gets enough sunshine. One form of vitamin D is often added to milk as an additive. Storage and food preparation do not seem to affect this vitamin.

Vitamin D lets the body utilize calcium and phosphorus in bone and tooth formation. Deficiency of this vitamin causes a bone disease called rickets. This disease is characterized by bone deformities (such as bowlegs, pigeon breast, and knobby bone growths on the ribs where they join the breastbone) and tooth abnormalities. In adults, bones become soft and porous as calcium is lost.

Excessive amounts of vitamin D cause nausea, diarrhea, weight loss, and pain in the bones and joints. Damage to the kidneys and blood vessels can occur as calcium deposits build up in these tissues.

Vitamin E

Vitamin E is present in green leafy vegetables, wheat germ and other plant oils, egg yolks, and meat. The main function of this vitamin is to act as an antioxidant, particularly for fats. (When oxidized, fats form a very reactive substance called peroxide, which is often very damaging to cells. Vitamin E is more reactive than the fatty acid molecule and, therefore, takes its place in the oxidizing process.) Because cell membranes are partially composed of fat molecules, vitamin E is vitally important in maintaining the nervous, circulatory, and reproductive systems and in protecting the kidneys, lungs, and liver.

All of the symptoms of vitamin E deficiency are believed to be due to the loss of the antioxidant protection it offers to cells. This protective effect also keeps vitamin A from oxidizing to an inactive form. And when vitamin E is lacking, vitamin A deficiency also frequently occurs. However, because vitamin E is so prevalent in foods, it is very difficult to suffer from a deficiency of this vitamin unless no fats are consumed in the diet. When it does occur, the symptoms include cramping in the legs, muscular dystrophy, and fibrocystic breast disease.

According to some current theories, many of the effects of aging are caused by the oxidation of fat molecules in cells. If this is true, then consuming extra vitamin E might counteract these effects because of its antioxidant properties.

Vitamin K

Vitamin K is found in many plants (especially green leafy ones like spinach), in liver, and in the bacteria of the intestine. Nearly all higher animals must obtain the vitamin K they need from these sources. Although the exact method by which vitamin K works in the body is not understood, it is known that vitamin K is vital to the formation of prothrombin—one of the chemicals necessary for blood clotting—found in the liver.

When vitamin K deficiency develops, it is rarely due to an incomplete diet. Instead, it results from liver damage and the blood's inability to process the vitamin. The deficiency is characterized by the inability of the blood to clot, and it manifests in unusual bleeding or large bruises under the skin or in the muscles. Newborn infants sometimes suffer from brain hemorrhage due to a deficiency of vitamin K.

Vitamin B

What was once thought to be vitamin B was later found to be only one of many B vitamins. Today, more than a dozen B vitamins are known, and they are frequently referred to as vitamin B-complex. Thiamine was the first of these vitamins to be identified. All of the B vitamins are water soluble.

Each of the B vitamins acts by combining with another molecule to form an organic compound known as a coenzyme. A coenzyme then works with an enzyme to perform vital activities within the cell. The function of enzymes within a cell vary, but all are somehow related to the release of energy. The most common members of this group of vitamins include vitamin B 1 (thiamine), vitamin B 2 (riboflavin), vitamin B 6 (pyridoxine), vitamin B 12 (cobalamin), biotin, folate (also folacin or folic acid), niacin, and pantothenic acid.

Vitamin B 1 is present in whole grains, nuts, legumes, pork, and liver. It helps the body release energy from carbohydrates. More than 4,000 years ago, the Chinese described a disease we know today as beriberi, which is caused by a deficiency of thiamine. The disease affects the nervous and gastrointestinal system and causes nausea, fatigue, and mental confusion. Thiamine is found in the husks or bran of rice and grains. Once the grain is milled and the husks removed, rice is no longer a source of this vitamin. Manufacturers today produce enriched rice and flour by adding thiamine back into the milled products.

Vitamin B 2 helps the body release energy from fats, proteins, and carbohydrates. It can be obtained from whole grains, organ meats, and green leafy vegetables. Lack of this vitamin causes severe skin problems. Vitamin B 6 is important in the building of body tissue as well as in protein metabolism and the synthesis of hemoglobin. A deficiency can cause depression, nausea, and vomiting. Vitamin B 12 is necessary for the proper functioning of the nervous system and in the formation of red blood cells. It can be obtained from meat, fish, and dairy products. Anemia, nervousness, fatigue, and even brain degeneration can result from vitamin B 12 deficiency.

Niacin is required to release energy from glucose. It is present in whole grains (but not corn), meat, fish, and dairy products. Inadequate amounts of this vitamin cause a disease called pellagra, which is characterized by skin disorders, weak muscles, diarrhea, and loss of appetite. Pellagra was once common in Spain, Mexico, and the southeastern United States, where a large component of the diet consisted of corn and corn products. The niacin in corn exists as part of a large, fibrous molecule that cannot be absorbed by the blood or used by the body. However, after it was discovered that treating corn with an alkaline solution (such as lime water) releases the niacin from the larger molecule and makes it available for the body to use, pellagra became much less common.

Pantothenic acid helps release energy from fats and carbohydrates and is found in large quantities in egg yolk, liver, eggs, nuts, and whole grains. Deficiency of this vitamin causes anemia. Biotin is involved in the release of energy from carbohydrates and in the formation of fatty acids. It is widely available from grains, legumes (peas or beans), egg yolk, and liver. A lack of biotin causes dermatitis (skin inflammation).

Vitamin C

Because of its association with the common cold, vitamin C (also known as ascorbic acid) is probably the best known of all the vitamins. Most animals can synthesize this vitamin in the liver, where glucose is converted to ascorbic acid. Humans, monkeys, guinea pigs, and the Indian fruit bat are exceptions and must obtain the vitamin from their diets. The vitamin is easily oxidized, and food storage or food processing and preparation frequently destroy its activity. Soaking fruits and vegetables in water for long periods also removes most of the vitamin C. Citrus fruits, berries, and some vegetables like tomatoes and peppers are good sources of vitamin C.

The exact function of vitamin C in the body is still not well understood, but it is believed to be necessary for the formation of collagen, an important protein in skin, cartilage, ligaments, tendons, and bone. It also plays a role in the body's absorption, use, and storage of iron. Vitamin C is an antioxidant and is therefore believed to offer protection to cells much as vitamin E does. An increasing body of evidence suggests that a greatly increased amount of vitamin C in the diet lessens the risk of heart disease and cancer.

Polarized light micrograph of crystals of ascorbic acid, or vitamin C. The vitamin is found almost exclusively in fruits (particularly citrus) and vegetables. (Reproduced by permission of Photo Researchers, Inc.)
Polarized light micrograph of crystals of ascorbic acid, or vitamin C. The vitamin is found almost exclusively in fruits (particularly citrus) and vegetables. (Reproduced by permission of
Photo Researchers, Inc.

A deficiency of vitamin C causes a disease called scurvy. Scurvy weakens the connective tissue in bones and muscles, causing bones to become very porous and brittle and muscles to weaken. As the walls of the circulatory system become affected, sore and bleeding gums and bruises result from internal bleeding. Anemia can occur because iron, which is critical to the transport of oxygen in the blood, cannot be utilized. Vitamin C is metabolized (broken down) very slowly by the body, and deficiency diseases do not usually manifest themselves for several months.

Linus Pauling (1901–1994), the winner of two Nobel Prizes (one for chemistry and one for peace), believed that massive doses of vitamin C could ward off the common cold and offer protection against some forms of cancer. While scientific studies have been unable to confirm this theory, they do suggest that vitamin C can at least reduce the severity of the symptoms of a cold. Some studies also suggest that vitamin C can lessen the incidence of heart disease and cancer. If this is true, it could be that the antioxidant properties of the vitamin help protect cells from weakening and breaking down much as vitamin E does. In fact, vitamins A, C, and E all play similar roles in the body, and it is difficult to distinguish among their effects.

[ See also Malnutrition ; Nutrition ]

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