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.