VITAMIN

any of various organic compounds that are required in minute quantities in the diet of most animals for the maintenance of normal metabolic functions. Vitamins act especially as coenzymes and precursors of coenzymes in the regulation of metabolic processes. Unlike the macronutrients (proteins, carbohydrates, and fats), they do not provide energy or serve as building units. The value of certain foods in maintaining health was recognized long before the first vitamins were actually identified. In the 18th century, for example, it had been demonstrated that the addition of citrus fruits to the diet would prevent the development of scurvy. In the 19th century it was shown that substituting unpolished for polished rice in a rice-based diet would prevent the development of beriberi. Despite these observations, however, it was not until the early 20th century that the existence of vitamins was actually established. In 1906 the British biochemist Sir Frederick Hopkins demonstrated that foods contained necessary accessory factors in addition to proteins, carbohydrates, fats, minerals, and water. Then, in 1912, the chemist Casimir Funk identified that the antiberiberi substance in unpolished rice was an amine (a type of nitrogen-containing compound), so Funk proposed that it be named vitaminei.e., vital amine. This term soon came to be applied to the accessory factors in general, all of which were originally thought to be closely related. It was later discovered that different vitamins have different chemical properties and different functions and that many of them do not contain amines at all. Because of its wide currency, Funk's term continued to be used, but the final e was dropped. Flow diagram showing the flow of events resulting from vitamin deficiencies. In 1912 Hopkins and Funk advanced the vitamin hypothesis of deficiency, a theory that postulates that the absence of sufficient amounts of a particular vitamin in a system may lead to certain diseases, such as scurvy or beriberi (seeFigure). The letters which were assigned to vitamins in the early days of vitamin research categorize them according to their function. As further research revealed the chemical structure of these substances, they received scientific names, although they are still popularly known by their letter classifications (e.g., vitamin B2 for riboflavin). Although the need for vitamins has been demonstrated in a wide range of animalsand even in heterotrophic bacteria and yeastsa substance that is a vitamin for one species is not necessarily a vitamin for another. Moreover, a substance that can be manufactured by an organism may still be considered a vitamin for that organism if the synthesis of the substance is not always sufficient to meet the organism's metabolic needs. The human nutrient vitamin D, which is essential for bone growth, is an example of such a substance. A form of vitamin D is synthesized in the skin when that tissue is exposed to sunlight. If this tissue synthesis of vitamin D is not sufficient to meet a person's metabolic needs (as is often the case in children, whose rapid skeletal growth requires relatively large amounts of vitamin D), the substance must be provided from the diet. A somewhat similar situation exists with the human nutrient vitamin K. This substance cannot be made by the human body, but it is readily synthesized by the bacteria that normally flourish in the human colon. This bacterial product is absorbed through the colon wall, thereby providing the metabolic needs of the human. It is only when the normal bacterial flora of the colon has been destroyedfor example, by large doses of antibioticsthat vitamin K must be provided in the diet. In regard to human nutrition, the vitamins can be divided into two categories: water-soluble vitamins (the B vitamins and vitamin C) and fat-soluble vitamins (A, D, E, and K). Water-soluble vitamins are absorbed by the intestine and carried by the circulatory system to the specific tissues in which they will be put into use. They are distinguished from each other by the degree to which they are soluble in water, a factor that influences their route inside the body. When intake of water-soluble vitamins exceeds a person's need for them, they are stored to a limited extent in body tissue, but most of the excess is excreted in urine. In their free state, B vitamins are inactive. They must go through several chemical processes to perform their function within the body. Only with the addition of other substances or parts of other molecules do they reach their functional, or coenzyme, form. The coenzyme is a compound that unites with a protein component called an apoenzyme to form an active enzyme. The enzyme then acts as a catalyst in various metabolic and regulatory processes. The physiological functions of vitamin C are well known, but its precise metabolic mechanisms are less certain. It is not known whether vitamin C acts as a coenzyme. Vitamin C is essential for the growth of bones and teeth, for the maintenance of the subcutaneous tissues and the walls of the blood vessels, and for the healing of wounds. A highly debated medical hypothesis which suggests that the intake of high quantities of vitamin C can help prevent or cure some diseases (such as the common cold and some infectious and malignant diseases) continues to be investigated but with no clear scientific data to support the claims. The intestine absorbs fat-soluble vitamins with the help of bile salts. The lymph system carries these vitamins to the different parts of the body. The body can store larger amounts of fat-soluble vitamins than of water-soluble ones. The liver provides the chief storage tissue for vitamins A and D, while vitamin E is stored in body fat and to a lesser extent in reproductive organs. Relatively little vitamin K is stored. The fat-soluble vitamins perform different functions. Vitamin A combines with proteins in the retina of the eye to aid in night vision. It may have other functions that are as yet uncertain. Vitamin D is essential to the growth of the organism, especially in calcium metabolism for bone growth. Vitamin E also facilitates animal growth: deficiencies in the substance cause infertility in some species. Vitamin K is necessary in the enzymatic processes in blood clotting. any of several organic substances that usually are separated into water-soluble (e.g., the B vitamins, vitamin C) and fat-soluble (e.g., vitamins A, D, E, K) groups; small quantities (from 0.00002 percent to 0.005 percent of a diet) are necessary for normal health and growth in higher forms of animal life. The substances commonly known as vitamins are diverse in chemical structure and function. Originally defined as organic compounds obtainable in a normal diet and capable of maintaining life and promoting growth, vitamins are distinct from carbohydrates, fats, and proteins in function, as well as in the quantities in which organisms require them. A number of compounds (e.g., choline, carnitine) once grouped with vitamins no longer are considered vitamins (see below Vitamin-like substances). If a vitamin is absent from the diet or is not properly absorbed by an organism, a specific deficiency disease may develop. The term vitamin originated from vitamine, a word first used in 1911 to designate a group of compounds considered vital for life; each was thought to have a nitrogen-containing component known as an amine. The final e of vitamine was dropped when it was discovered that not all of the vitamins contain nitrogen, and, therefore, not all are amines. The term accessory food factor sometimes is used instead of vitamin to refer to these substances. Since they generally cannot be synthesized by an animal (or, if synthesized, the amounts are insufficient to meet body needs) and must be obtained from the diet or from some synthetic source, vitamins are called essential nutrients. The requirements for some of the B vitamins may be met in part by bacterial synthesis in the intestines of some mammals. The amino acid tryptophan can be converted to nicotinic acid (or niacin, a water-soluble vitamin) and thus can serve as a source for part of the nicotinic acid required by an animal. Vitamin C (also a water-soluble vitamin) can be synthesized by some organisms in sufficient amounts so that the dietary requirement is eliminated; vitamin C usually is considered a vitamin, however, because it must be included in the diet of man. Vitamins are distinct from many other compounds, which, although indispensable for proper animal functions, can be synthesized in adequate quantities.