Vitamins : History, Nomenclature, Synthesis, And Classification 

Vitamins may be regarded as organic compounds required in the diet in small amounts to perform specific biological functions for the normal maintenance of optimum growth and health of the organism. The bacterium E.coli does not require any vitamins, as it can synthesize all of them. It is believed that during the course of evolution, the ability to synthesize vitamins was lost. Hence, the higher organisms have to obtain them from the diet. The vitamins are required in small amounts since their degradation is relatively slow. 

History and nomenclature of Vitamins

At the beginning of 20th century, it was clearly understood that diets containing purified carbohydrates, protein, fat, and minerals were not adequate to maintain the growth and health of experimental rats, which natural foods (such as milk) could do. 

Hopkins coined the term accessory factors to the unknown and essential nutrients present in natural foods. Funk (1913) isolated an active principle (an amine) from rice polishings and, later in yeast, which could cure beriberi in pigeons. He coined the term vitamin (Greek: vita-life) to the accessory factors with a belief that all of them were amines. It was later realized that only a few of them are amines. The term vitamin, however, is continued without the final letter ‘e’.  The usage of A, B, and C to vitamins was introduced in 1915 by McCollum and Davis. They first felt there were only two vitamins— fat soluble A and water-soluble B (anti-beriberi factor). Soon another water-soluble anti-scurvy factor named vitamin C was described. Vitamin A was later found to possess two components- one that prevents night blindness (vitamin A) and another anti-ricket factor named vitamin D. A fat-soluble factor called vitamin E, in the absence of which rats failed to reproduce properly, was discovered. Yet another fat-soluble vitamin concerned with coagulation was discovered in the mid-1930s. It was named as vitamin K. In the sequence of alphabets it should have been F, but K was preferred to reflect its function (coagulation). 

As regards the water-soluble factors, vitamin C was identified as a pure substance and named as ascorbic acid. Vitamin B was found to be a complex mixture and nomenclature also became complex. B1 was clearly identified as an anti-Beri-Beri factor. Many investigators carried out intensive research between 1920 and 1930 and went on naming them as the water-soluble vitamins B2, B3, B4, B5, B6, B7, B8, B9, B10, B11, and B12. Some of them were found to be mixtures of already known vitamins. And for this reason, a few members (numbers!) of the B- complex series disappeared from the scene. Except for B1, B2, B6, and B12, names are more commonly used for other B-complex vitamins. 

Classification of vitamins 

There are about 15 vitamins, essential for humans. They are classified as fat-soluble (A, D, E, and K) and water-soluble (C and B-group) vitamins. The B-complex vitamins may be sub-divided into energy-releasing (B1, B2, B6, biotin, etc.) and hematopoietic (folic acid and B12). Most of the water-soluble vitamins exert their functions through their respective coenzymes while only one fat-soluble vitamin (K) has been identified to function as a coenzyme. 

Synthesis of vitamins by intestinal bacteria 

Vitamins, as per the definition, are not synthesized in the body. However, the bacteria of the gut can produce some of the vitamins, required by man and animals. The bacteria mainly live and synthesize vitamins in the colon region, where the absorption is relatively poor. Some animals (e.g. rats, deer, etc.) eat their own feces, a phenomenon known as coprophagy. 

As far as humans are concerned, it is believed that the normal intestinal bacterial synthesis and absorption of vitamin K and biotin may be sufficient to meet the body's requirements. For other B-complex vitamins, the synthesis and absorption are relatively less. Administration of antibiotics often kills the vitamin synthesizing bacteria present in the gut, hence additional consumption of vitamins is recommended. 

Fat-soluble vitamins— 

The four vitamins, namely vitamin A, D, E, and K are known as fat or lipid soluble. Their availability in the diet, absorption, and transport are associated with fat. They are soluble in fats and oils and also fat solvents (alcohol, acetone, etc.). Fat-soluble vitamins can be stored in the liver and adipose tissue. They are not readily excreted in the urine. Excess consumption of these vitamins (particularly A and D) leads to their accumulation and toxic effects. 

All the fat-soluble vitamins are isoprenoid compounds since they are made up of one or more of five carbon units namely isoprene units ( CH C.CH3 CH CH ). Fat-soluble vitamins perform diverse functions. Vitamin K has a specific coenzyme function. 

Water soluble vitamins— 

The water-soluble vitamins are a heterogeneous group of compounds since they differ chemically from each other. The only common characteristic shared by them is their solubility in water. Most of these vitamins are readily excreted in urine and they are not toxic to the body. Water soluble vitamins are not stored in the body in large quantities (except B12). For this reason, they must be continuously supplied in the diet. Generally, vitamin deficiencies are multiple rather than individual with overlapping symptoms. It is often difficult to pinpoint the exact biochemical basis for the symptoms. 

The water-soluble vitamins form coenzymes that participate in a variety of biochemical reactions, related to either energy generation or hematopoiesis. It may be due to this reason that the deficiency of vitamins results in a number of overlapping symptoms. The common symptoms of the deficiency of one or more vitamins involved in energy metabolism include dermatitis, glossitis (red and swollen tongue), cheilitis (rupture at the corners of lips), diarrhea, mental confusion, depression, and malaise. 

Deficiency of vitamins B1, B6 and B12 is more closely associated with neurological manifestations. 

Vitamers