Vitamins are classified as:
(i) fat soluble, e.g. vitamin A, D, E, a-lipoic acid and K; and
(ii) water soluble, e.g. vitamin B complex and C. There are some biological inactive compounds which are quite similar to vitamins in stucture.
They are known as provitamins as they are converted easily into active vitamins in vivo.
For example, β-carotene and ergosterol are the provitamins for the vitamins A and D2 respectively.
1. Vitamin A, Axerophytol or Retinol : Its main sources are cod liver oil, egg, milk, butter, cheese, carrots, spinach, sweet potatoes and other green and yellow vegetables. β-Carotene is important provitamin A. In childhood, lack of vitamin A retards growth and hence like other vitamins, it is also said to be a growth promoting factor. In mild deficiency it leads to night blindness or nyctalopia. Its prolonged deficiency leads to xerophthalmea.
2. Vitamin D (Calciferol) : Vitamin D itself is not widely distributed in nature, in plants it is found in relatively insignificant amounts. The only rich sources of vitamin D are the liver and viscera of fish. Among common foods, egg-yolk (chicken), milk and butter contain vitamin D. It is the only vitamin that is synthesised in the body by the action of light on its provitamins, present in body.
Lack of sunlight or a deficiency of ergocalciferol in the food produces rickets (rachitis) in the growing child and osteomalacia in the adult. In both conditions there is inadequate deposition of calcium salts in newly formed bone matrix.
3. Vitamin E (Tocopherols) : Vitamin E is the antisterility factor which is necessary for fertility of the male and the birth process of the female. Tocopherols are generally found in the free form mostly in the seed oils, eg, cotton seed oil.
4. Vitamin K : Although vitamin K is present mainly in green leafy tissues (eg, alfalfa, spinach). it is also found in cauliflower, cabbage, kale, tomatoes, soyabeans, rice bran, and oat shoots. Vitamin K is indirectly necessary for normal blood coagulation and its deficiency delays the coagulation period of blood and hence vitamins K are known as antihaemorrhagic vitamins or factors.
5. Vitamin B Complex : This complex includes all the water soluble vitamins found in yeast, liver, rice polishings, etc. This group contains a vitamin, known as B1, which is destroyed by heat, and various other compounds, most of which are stable to heat, known as B2 complex. All the members of vitamin B complex form coenzymes or prosthetic groups of different enzymes. Another similarity among the B complex vitamins is the presence of nitrogen.
Vitamin B1 (Thiamine) : It is sulphur containing vitamin (other sulphur containing vitamin is biotin) Its main source is seeds. In cereal grains, it is concentrated in the outer layers (e.g, rice polishing), which are often discarded during milling processes (e.g., of wheat, flour and rice). Thus ordinary white flour contains little thiamine, while the wholemeal flour is a good source of the vitamin, Pulses and nuts are among the richest natural sources of the vitamin.
Deficiency of thiamine causes beriberi (characteristics of beriberi: retarded growth, loss in appetite and weight, nervous degeneration, muscular atrophy, oedema, etc). The physiological action of thiamine is due to its pyrophosphate known as thiamine pyrophosphate (TPP), or co-carboxylase.
Vitamin B2 (Riboflavin) : The most important source is milk. Ordinary cooking processes cause little destruction of riboflavin. Considerable loss of the vitamin may occur in milk exposed to bright sunlight.
In riboflavin deficiency (in man) the skin becomes rough and scaly; the pink parts of the lips become bright red, swollen and cracked (cheilosis).
Vitamin B6 (Pyridoxine) : Most abundant sources for the vitamin is rice polishings followed by seeds and cereals viz. wheat, maize, ctc, Fish liver, kidney, milk, fresh vegetables, egg and meat also contain small amounts of vitamin B6. Its deficiency causes anaemia.
Vitamin B12 (Cyanocobalamines) : Cobalamine has by far the most complicated structure of all the vitamins. It has cobalt atom in the centre. It also has -CN groups in its structure.
It is found in liver, egg, milk, meat, and fsh. Minute amounts are probably present in all animal cells. Peculiarly, unlike other B vitamins, B12 is not found in significant amounts in green plants.
It is involved in the formation of red blood cells. Thus its deficiency in man causes pernicious anemia (immature, enlarged and nucleated RBCs deficient in haemoglobin) which is accompanied by degradation of the spinal cord and thus vitamin B12 is known as antipernicious anemia factor.
6. Vitamin C (Ascorbic acid) : It is a derivative of carbohydrates. It is optically active.
Its important sources are leaves and flowers (e.g., rose hips, pine needles), fruits (e.g., lemons, oranges, black currants, grape fruit, guava, strawberies, apples, bananas, etc.) and green vegetables (e.g., cauliflowers, cabbage, green peas, beans, tomatoes, etc.). Citrus fruits are the best sources of ascorbic acid.
Deficiency of ascorbic acid causes an abnormal condition known as scurvy. The main features of scurvy are bleeding into the skin and deeper tissues, swollen bleeding gums and fragile bones.
