Classification of Vitamins

Classification of Vitamins

  • It is difficult to define vitamins precisely. 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 vitamin, 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.


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


  • 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 as shown in the Table.
  • 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.


  • 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 of the animals (e.g. rats, deer, etc.) eat their own feces, a phenomenon is known as coprophagy.


  • The four vitamins, namely vitamin A, D, E, and K are known as fat or lipid-soluble. Their is 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 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 has a specific coenzyme function.


  • Water-soluble vitamins-general The water-soluble vitamins heterogeneous group of compounds since they differ chemically from each other.
  • The only common character 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 pre to pinpoint the exact biochemical basis for the symptoms.


In the following pages, the individual members of the fat-soluble and water-soluble vitamins are discussed with regard to the chemistry, biochemical functions, recommended dietary/daily allowances (RDA), dietary sources, deficiency manifestations, etc.


  • The fat-soluble vitamin A, as such is present only in foods of animal origin.
  • However, its provitamins carotenes are found in plants. It is recorded in history that Hippocrates (about 500 B.C.) cured night blindness.
  • He prescribed to the patient’s ox liver (in honey), which is now known to contain a high quantity of vitamin A.


  • Animal sources contain (preformed) vitamins The best sources are liver, kidney, egg yolk, milk, cheese, butter.
  • Fish (cod or shark) liver oils are very rich in vitamin A.Vegetable sources contain the provitamin A-carotenes.
  • The Yellow and dark green vegetables and fruits are good sources of carotenes e.g. carrots, spinach, amaranths, pumpkins, mango, papaya, etc.


  • The deficiency symptoms of vitamin A are not immediate, since the hepatic stores can meet the body requirements for quite some time (2-4 months).
  • The deficiency manifestations are related to the eyes, skin, and growth. Deficiency manifestations of the eyes: Night blindness (nyctalopia) is one of the earliest


  • The individuals have difficulty seeing in dim light since the dark adaptation time is increased.
  • Prolonged deficiency irreversibly damages a number of visual cells.


  • Vitamin C is a water-soluble versatile vitamin. It plays an important role in human health and disease.
  • Vitamin C has become the most controversial vitamin in recent years.
  • This is because of the claims and counter-claims on the use of vitamin C in megadoses to cure everything from the common cold to cancer.

Vitamin D

  • These are fat-soluble vitamins produced by the response of sunlight.
  1. promoting healthy bones and teeth
  2. supporting immune, brain, and nervous system health
  3. regulating insulins levels, and supporting diabetes management.


  • Vitamin D is absorbed in the small intestine for which bile is essential.
  • Through lymph, vitamin D enters the circulation bound to plasma Cl2-globulin and is distributed throughout the body. The liver and other tissues store small amounts of vitamin D.


  • Vitamins D2 and D3, as such, are not biologically active.
  • They are metabolized identically in the body and converted to active forms of vitamin D.
  • The metabolism and biochemical functions of vitamin D are depicted


  • Vitamin E (tocopherol) is a naturally occurring antioxidant.
  • It is essential for normal reproduction in many animals, hence known as an anti-sterility vitamin.
  • Vitamin E is described as a ‘vitamin in search of a disease.
  • This is due to the lack of any specific vitamin E deficiency disease in humans.
  • Evans and his associates (1936) isolated the compounds of vitamin E activity and named them as tocopherols.


  • Vitamin K is the only fat-soluble vitamin with a specific coenzyme function.
  • It is required for the production of blood clotting factors, essential for coagulation (in German-Coagulation; hence the name K for this vitamin).


  • Thiamine (anti-beriberi antineuritic vitamin) is water-soluble.
  • It has a specific coenzyme, thiamine pyrophosphate (TPP) which is mostly associated with carbohydrate metabolism.


  • Riboflavin through its coenzymes takes part in a variety of cellular oxidation-reduction reactions.


  • Vitamin B6 is used to collectively represent the three compounds namely pyridoxine, pyridoxal, and pyridoxine (the vitamins of B6).


  • Pantothenic acid formerly known as chick anti-dermatitis factor (or filtrate factor) is widely distributed in nature.
  • Its metabolic role as coenzyme A is also widespread.


  • Folic acid or folacin (Latin:folium-leaf) is abundantly found in green leafy vegetables.
  • It is important for one-carbon metabolism and is required for the synthesis of certain amino acids, purines, and pyrimidine-thymine.


  • Vitamin B12 is also known as anti-pernicious anemia vitamin.
  • It is a unique vitamin, synthesized by only microorganisms and not by animals and plants. It was the last vitamin to be discovered.

Vitamins: Overview