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Tuesday, March 31, 2009

HISTORICAL RESUME OF VITAMINS

HISTORICAL RESUME

Vitamins have a back-dated history. The dreaded disease scurvy was one of the prevalent diseases in Europe during 15th and 16th centuries. The disease is said to have afflicted the crusadors. Scurvy was reported by Vasco de Gama during his sea voyages and Jacques Cartier in 1535 had reported death of about 25% of his sailing crew due to scurvy. On the recommendation of Sir James Lancaster, an English privateer, the ships of East India Company in 1601 carried oranges and lemons to prevent scurvy. In 1757, James Lind,
a British naval surgeon, stated that fresh fruits and vegetables alone are effective to protect the body from various maladies and urged the inclusion of lemon juice in the diet of sailors to prevent scurvy. And some 40 years later, the Admiralty took his advice. After limes were substituted for lemons in 1865, British sailors began to be known as “limeys.” Similarly, rickets was also attributed to faulty diets and Guérin (1838) produced it experimentally in puppies to prove the dietary connection. In 1887, Admiral Takaki, Director-General of the Medical Services in Japan, demonstrated that another scourge beriberi could be prevented by enriching the diet with meat, vegetables and milk and at the same time decreasing the amount of milled rice in the case of Japanese sailors. Later, Eijkman (1897), a Dutch physician, found that experimental beriberi could be induced in hens when fed with polished rice without bran. Such hens could be cured by giving them the rice polishings. Eijkmann, for a time, believed that the rice polishings contained something that neutralized the beriberi toxin in the polished rice. In 1906, however, Frederick Gowland Hopkins ascribed the diseases such as scurvy and rickets to the lack of some ‘dietary factors’. Six years later (i.e., in 1912), Hopkins in collaboration with Casimer Funk of Poland, who was working at the Lister Institute of London, suggested the vitamin theory which postulates that ‘specific diseases such as beriberi, scurvy and rickets are each caused by the absence from the diet of a particular nutritional factor.’ Funk, for the first time, also isolated the dietary factor from rice polishings which acted as antiberiberi substance. Since this factor was an amine and necessary to life, Funk at the suggestion of Dr. Max Nierenstein introduced the term ‘vitamine’ (vitaL = life) to denote it. Since subsequent studies showed that not all these substances are amines, the terminal letter ‘e’ was dropped from its spelling at the suggestion of Sir. J.C. Drummond (1919), who also proposed their alphabetical nomenclature. In fact, the various vitamins have no structural resemblance to each other, but because of a similar general function in metabolism, they are studied together. Although these molecules serve nearly the same roles in all forms of life, but higher animals have lost the capacity to synthesize them.

DEFINITION and GENERAL CHARACTERISTICS

DEFINITION
The vitamin concept has undergone extensive revisions during the history of biochemistry. However, Franz Holfmeister’s (LT, 1850-1922) definition— ‘vitamins are substances which are indispensable for the growth and maintenance of the animal organism, which occur both in animals and plants and are present in only small amounts in food’— still holds good, although it has been interpreted in various ways. The term ‘vitamin’, in its modern sense, usually refers to the substances distinct from major components of food, required in minute quantities (i.e., oligodynamic in nature) and whose absence causes specific deficiency diseases. As the living organisms cannot synthesize most of these compounds, a steady supply of them is sine quo non for life. Their ultimate source is the plant or bacterial world.
Differences with hormones— Indeed, for some time, it was believed that the distinction between vitamins and hormones was no longer tenable. But there exists a fundamental difference between these two classes of active substances : the hormones are regulatory substances whereas the vitamins are accessory nutrients. The vitamins also differ from the hormones in that they are supplied to the body from some external source (i.e., chiefly from the food ingested) whereas the hormones are synthesized within some part of the body of an organism. Moreover, most vitamins and some hormones are involved either directly or indirectly in enzyme systems in order to carry out biochemical functions. Some vitamins are also known to be coenzymes.
A comparison of enzymes, hormones and vitamins is presented in above Table

GENERAL CHARACTERISTICS

The vitamins are characterized for some general facts, which are listed below :
  1. Vitamins are of widespread occurrence in nature, both in plant and animal worlds.
  2. All common foodstuffs contain more than one vitamin.
  3. The plants can synthesize all the vitamins whereas only a few vitamins are synthesized
    in the animals.
  4. Human body can synthesize some vitamins, e.g., vitamin A is synthesized from its
    precursor carotene and vitamin D from ultraviolet irradiation of ergosterol and 7-
    dehydrocholesterol. Some members of the vitamin B complex are synthesized by
    microorganisms present in the intestinal tract. Vitamin C is also synthesized in some
    animals such as rat.
  5. Most of vitamins have been artificially synthesized.
  6. All the cells of the body store vitamins to some extent.
  7. Vitamins are partly destroyed and are partly excreted.
  8. Vitamins are nonantigenic.
  9. Vitamins carry out functions in very low concentrations. Hence, the total daily requirement is very small.
  10. Vitamins are effective when taken orally.
  11. Synthetically-made vitamins are just as nutritionally good as natural vitamins.
  12. Old people need about the same amounts of vitamins as young people.

CLASSIFICATION AND STORAGE OF VITAMINS IN THE BODY

CLASSIFICATION
In 1913, McCollum and Davis described a lipid-soluble essential food factor in butter fat and egg yolk. In 1915, a water-soluble factor in wheat germ necessary for the growth of young rats was also described. Since then, two categories of vitamins are usually recognized : fat-soluble and water-soluble. These two groups discharge rather different functions.
A. Fat-soluble vitamins: These are oily substances, not readily soluble in water and their biochemical functions are not well understood. They contain only carbon, hydrogen and oxygen. Their examples are vitamins A, D, E and K. They, however, play more specialized roles in certain group of animals and in particular type of activities. For instance, they function in the formation of a visual pigment (vitamin A), in the absorption of calcium and phosphorus from the vertebrate intestine (vitamin D), in protecting mitochondrial system from inactivation (vitamin E) or in the formation of a blood clotting factor in vertebrates (vitamin K). The individual fat-soluble vitamins bear a closer resemblance to each other chemically. In fact, the 4 fat-soluble vitamins can be regarded as lipids. Vitamins A, E and K are terpenoids, and vitamin D is a steroid. All four areisoprenoid compounds, since they are synthesized biologically from units of isoprene, a building block of many naturally-occurring oily, greasy or rubbery substances of plant origin. Unlike the water- soluble vitamins (B and C), fat-soluble vitamins can be strored in the body, e.g., an adult's liver can store enough vitamin A to last several months or longer. However, because fat-soluble vitamins are storable, their excessive intakes can result in toxic conditions (hypervitaminoses).

B. Water-soluble vitamins. Most of these are universally vitamins since they perform the
same general functions wherever they occur. Besides C, H and O, they also contain nitrogen. They are catalytic factors and as such form vital links in the chains of biochemical reactions characteristic of all living objects. For instance, thiamine is required whenever sugars are oxidized aerobically to release energy. The individual water-soluble vitamins bear no closer resemblance to each other chemically. The biochemical or coenzyme function of nearly all of these is known. The common water-soluble vitamins are vitamins of B complex such as B1 through B12 (vitamins B4, B8, B10and B11, however, do not exist) and the vitamin C. Choline, inositol, p-aminobenzoic acid, bioflavonoids and α-lipoic acid are frequently included in this category. Many nutritionists, however, do not consider them as true vitamins, although their dietary deficiencies in animals lead to the development of characteristic symptoms. Moreover, none of them except α-lipoic acid is a part of the coenzyme system. The B-series of vitamins, being water-soluble and excretable, are required daily in meagre amounts (in milligrams or even less) for the normal growth and good health of humans and many other organisms. It is virtually impossible to ‘overdose’ on them.

STORAGE OF VITAMINS IN THE BODY

The vitamins can be stored in the body to a slight extent. The liver cells are, however, rich in certain fat-soluble vitamins. For instance, the amount of vitamin A contained in the liver is sufficient enough to meet its requirement without any additional intake for about 6 months. Similarly, the quantity of vitamin D stored ordinarily in the liver is sufficient to maintain a person without any additional intake of vitamin D for about 2 months. The storage of vitamin K is, however, relatively slight.

The water-soluble vitamins are stored even in lesser amounts in the cells. Evidently, in cases of deficiency of vitamin B compounds, clinical symptoms appear rather early, that is within a few days. Similarly, absence of vitamin C can induce deficiency symptoms within a few weeks. Vitamin C is stored in the adrenal cortex.

DAILY HUMAN REQUIREMENTS OF VITAMINS and AVITAMINOSES (Deficiencies of Vitamins)

DAILY HUMAN REQUIREMENTS OF VITAMINS

The requirement of vitamins varies considerably depending on the nature of the individual consuming them. Some general facts regarding vitamin requirement may be listed below :
  1. Greater the size of the individual, higher are his vitamin needs.
  2. Younger ones require higher quantities of vitamins than do the elders.
  3. The vitamin requirements increase when a person performs exercise.
  4. During ailments, the vitamin requirements are ordinarily enhanced.
  5. Under certain specific conditions of metabolic disorders when the vitamins cannot be
    properly utilized, the requirement for one or more specific vitamins is at extreme.
  6. Growing children require comparatively high quantities of vitamin D.
  7. During pregnancy and lactation , the vitamin D requirement by the mother is greatly enhanced.
  8. The requirements of vitamin B complex (esp., that of vitamin B1) are increased under
    conditions of greater utilization of carbohydrates.
The daily requirement of any vitamin (refer Table ) for any individual is not a fixed quantity and varies according to the rate of metabolism. In general, in all cases of high metabolic activity (such as heavy muscular work, during pregnancy and lactation and in growing children), the vitamin requirement is proportionately high. Normally, a man doing ordinary work can obtain enough vitamin from his balanced diet.
The intestinal organisms may synthesize vitamins in significant amounts and play a vital role in regulating the quantity of vitamin available to the organism. Most of the vitamins of B complex group (such as thiamine, riboflavin, nicotinic acid, pyridoxine, biotin, folic acid) and vitamain K are some of the vitamins synthesized by the intestinal organisms. These may be absorbed to varying extents and utilized. This fact renders rather ‘inaccurate’ the figures for daily requirement of the various vitamins. Certain organisms, however, destroy vitamins. Supplementing the diet with certain antibiotics and sulfa drugs enhances the growth of these organisms.
In measuring human requirements of vitamins, certain units have been used. In the beginning, these were arbitrarily fixed and were mainly based on the amount necessary to check avitaminosis in animals under standard conditions. With the passage of time, the various vitamins were synthesized and so it became possible to base the unit on the weight of purified preparations.

AVITAMINOSES
(Deficiencies of Vitamins)


A lack of one or more vitamins leads to characteristic deficiency symptoms in man. Multiple deficiencies caused by the lack of more than one vitamin are, however, more common in human beings. Vitamin deficiencies occur rather frequently in certain parts of the world for socioeconomic reasons. Avitaminosis may be of following 2 types :
  • A. primary or direct— This arises due to inadequate intake of vitamins resulting from chronic alcoholism, dietary fads etc.
  • B. secondary or ‘conditioned deficiency’— This arises due to other factors such as malabsorption, increased excretion, allergies, anorexia, gastrointestinal disorders etc.
Vitamin deficiency, whether primary or secondary, leads to:
  1. a gradual decrease in tissue levels of the vitamin(s) deficient,
  2. a biochemical lesion,
  3. an anatomic lesion, and
  4. finally cellular pathology and disease.
This sequence is schematically represented in Fig.

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