Engineering Chemistry

Lesson 26 Vitamins

26.2 Classification

Vitamins differ widely in their chemical nature. They are classified based on their solubility. The vitamins A, D, E and K are soluble in fats and oils and are known as fat soluble vitamins. The vitamins of group B and vitamin C are water soluble and are known as water soluble vitamins.

26.3 Functions

Different vitamins have different functions. Most of the water soluble vitamins function as coenzymes of enzymes. The functions of various vitamins will be discussed in following sections.

26.4 Fat Soluble Vitamins

Fat soluble vitamins are generally present in foods containing high amounts of fat, such as butter, cream, oil seeds, fats of meat and fish, etc. They are not excreted in the urine, and can accumulate in human body leading to toxic results in high concentration.

26.4.1 Vitamin A

Vitamin A is an alcohol (C₂₀H₂₉OH). Its chemical name is retinol and is so named because of its association with retina of the eye. The active forms are aldehyde (retinal) and acid (retinoic acid). It is stable to heat to some extent. If heated for a long time in contact with air, it gets destroyed.

26.4.1.1 Functions of  Vitamin A

Vitamin A is essential for vision in night. It is also helpful in maintaining health of epithelial cells, which secrete mucous, which protects the skin and membranes against microorganisms. It also affects protein synthesis and is necessary for growth and maintenance of skeletal muscles and soft tissues.

26.4.1.2 Deficiency disorders of  Vitamin A

Prolonged deficiency may lead to night blindness, xerophthalmia (keratinization of cornea) and decreased defense against microorganisms because the mucous membranes cannot function properly.

26.4.1.3 Sources of  Vitamin A

It is found in food items like meat, milk, fish, etc. In animals, its highest concentration is found in the liver. It is also present in plant sources like carrot, pumpkin and green leafy vegetables. The plant sources contain precursor of Vitamin A known as carotenoids, which are the orange and yellow pigments of fruits and vegetables.

26.4.2 Vitamin D

It is  a steroid vitamin. Five forms of vitamin D are known as vitamin D₁ to D₅ . Out of these, the forms D₂ and D₃ are most important in humans. Vitamin D₂ is ergocalciferol and vitamin D₃ is cholecalciferol. Vitamin D is very stable and operations like storage, processing and cooking do not affect its activity.

26.4.2.1 Functions of  Vitamin D

Vitamin D is a precursor of hormone 1,25-dihydroxycholecalciferol, which is formed by liver and kidney. This hormone is necessary for growth and mineralization of bones and teeth. Vitamin D is essential for absorption and maintaining proper levels of calcium and phosphate.

26.4.2.2 Deficiency disorders of  Vitamin D

In children, deficiency of vitamin D, leads to Rickets, in which bones are weak and assume a bent form, joints are enlarged and chest is narrow due to distortion of ribs. In adults, deficiency leads to loss of calcium from bones which results in increased of fractures.

26.4.2.3 Sources of  Vitamin D

It is found in small quantities in butter, cream, egg-yolk and liver. The best source is fish liver oil. In the body, ergosterol which is obtained from plant sources is converted into vitamin D when ultraviolet light falls on the skin.

26.4.3 Vitamin E

The activity of Vitamin E is due to mixture of four tocopherols (α, β, γ and δ), out of which α-Tocopherol is the biologically most active form.

26.4.3.1 Functions of  Vitamin E

Vitamin E possesses antioxidant property. It prevents formation of peroxides from polyunsaturated fatty acids. It protects cell membrane from deterioration caused by peroxides.

26.4.3.2 Deficiency disorders of  Vitamin E      

The deficiency of Vitamin E results in increased fragility of red blood cells and muscle damage.

26.4.3.3 Sources of  Vitamin E

It is present in cereals, green plants, egg-yolk, milk fat, butter, meat, nuts and vegetable oils.

26.4.4 Vitamin K

It occurs in two forms. Vitamin K₁ (phylloquinine) is present in green plants and Vitamin K₂ (menaquinone) is formed in intestine by action of bacteria.

26.4.4.1 Functions of  Vitamin K

It is necessary for synthesis of proteins like prothrombin, which are involved in blood clotting.

26.4.4.2 Deficiency disorders of  Vitamin K

The deficiency of vitamin K increases blood clotting time, which results in haemorrhage and uncontrolled bleeding.

26.4.4.3 Sources of  Vitamin K

Green leafy vegetables like spinach, cabbage and lettuce are best sources of vitamin K. It is also present in fruits, cereals, dairy products and meat. It is also produced in intestine by bacteria.

26.5 Water Soluble Vitamins

Water soluble vitamins are not stored in human body and if excess quantities are ingested, they are excreted in the urine. Therefore they should be taken in diet every day. These vitamins are lost easily during cooking process.

26.5.1 Thiamine (Vitamin B-1)

It is stable when dry but solutions are unstable in presence of heat or alkali. In acidic pH, it is heat stable. It is also destroyed by oxidation and reduction process.

26.5.1.1 Functions of  Thiamine

It reacts with phosphoric acid to form thiamine pyrophosphate, which as a coenzyme It is required during biological oxidation of glucose.

26.5.1.2 Deficiency disorders of  Thiamine

Mild deficiency results in fatigue, depression, irritability, loss of appetite and lethargy. Severe deficiency causes beri-beri, whose symptoms are disorders of nervous system, cardiovascular system and digestive tract.

26.5.1.3 Sources of Thiamine

The sources of vitamin B-1 are unpolished cereals, pulses, nuts, liver, meat, egg yolk and fish.

26.5.2 Riboflavin (Vitamin B-2)

Chemically it belongs to a group of flavins, which are yellow coloured fluorescent pigments. It is orange yellow in colour. On exposure to light, it loses its activity.

26.5.2.1 Functions of Riboflavin

It forms part of two coenzymes flavin mononucleotide (FMN) and flavin dinucleotide (FAD), which is required in catalyzing oxidation-reduction reactions of the cell.

26.5.2.2 Deficiency disorders of Riboflavin         

Its deficiency results in cracks in corner of mouth, reddened lips, swollen and reddened tongue. The deficiency also results in disorders of eyes.

26.5.2.3 Sources of Riboflavin

The main sources of this vitamin are milk and dairy products, liver, eggs and leafy vegetables. Dried yeast, pulses and meat also contain vitamin B-2.

26.5.3 Niacin

Nicotinic acid and nicotinamide are two naturally occurring chemicals with niacin activity. It is resistant to heat, light acids and alkali.

26.5.3.1 Functions of Niacin

It is a part of two coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). These coenzymes help in releasing energy from carbohydrates, fats and proteins.

26.5.3.2 Deficiency disorders of Niacin

The symptoms of niacin deficiency are weakness, indigestion, ulcers of mouth and tongue. Pellagra is the disease associated with prolonged deficiency of niacin. The symptoms of pellagra are dermatitis, diarrhea and dementia. Skin lesions on exposure to sunlight, neurological symptoms and mental changes are also associated with niacin deficiency.

26.5.3.3 Sources of  Niacin

Meat, yeast, fish, groundnuts, pulses and whole grain cereals are good sources of niacin. It can be synthesized by bacteria in the intestine.

26.5.4 Pyridoxine (Vitamin B-6)

Vitamin B-6 is a group of three substances having biological activity. They are pyridoxine, pyridoxal and pyridoxamine of which pyridoxine is the most stable form.

26.5.4.1 Functions of Pyridoxine

Pyridoxine in the form of pyridoxal phosphate is a coenzyme of many enzymes involved in carbohydrate, protein and lipid metabolism. It is required in transamination, deanimation, desulphuration and decarboxylation reactions.

26.5.4.2 Deficiency disorders of Pyridoxine

Deficiency of vitamin B-6 results in depression, confusion and convulsions. It also causes dermatitis around the eyes, eyebrows and mouth, soreness of mouth and reddening of tongue.

26.5.4.3 Sources of Pyridoxine          

Meat, liver, vegetables and whole grain cereals are good sources of vitamin B-6.

26.5.5 Folic Acid

It belongs to a group of compounds called pterins. Many chemical forms of Pteroylmonoglutamic acid (PGA, Folic acid) have vitamin activity. Folic acid is unstable to heat in acid medium, and is inactivated also by oxidation.

26.5.5.1 Functions of Folic Acid

In the body folic acid is converted into many coenzyme forms which are required for transfer of one-carbon groups.

26.5.5.2 Deficiency disorders of Folic Acid

Deficiency of folic acid results in poor growth, blood disorders and disturbances of gastrointestinal tract and metabolism.

26.5.5.3 Sources of Folic Acid

Green leafy vegetables (spinach, asparagus and broccoli), dried beans, liver and kidney are rich sources of folic acid.

26.5.6 Biotin

It is a sulphur containing vitamin, stable to heat and light but unstable in strong acid and alkali. It is susceptible to oxidation.

26.5.6.1 Functions of Biotin

Biotin is a coenzyme taking part in reactions in which carbon dioxide is added or removed.

26.5.6.2 Deficiency disorders of Biotin

Anorexia, nausea, vomiting, mental depression and dermatitis are some of the symptoms associated with biotin deficiency.

26.5.6.3 Sources of Biotin

Liver, kidney, egg yolk and groundnuts are good sources of biotin.  It can be synthesized by bacteria in the intestine.

26.5.7 Pantothenic acid

It is stable to heat in solution and is destroyed by dry heat and alkalies.

26.5.7.1 Functions of Pantothenic acid

It is a part of coenzyme A which is involved in acylation reactions.

26.5.7.2 Deficiency disorders of Pantothenic acid

Deficiency of this vitamin is rare. The symptoms of deficiency include pain in arms and legs, loss of appetite, nausea, indigestion and increased susceptibility to infections.

26.5.7.3 Sources of Pantothenic acid

It is abundant in animal tissues, whole grain cereals and pulses. It also occurs in milk, vegetables and fruits. It is also synthesized by bacteria in the intestine.

26.5.8 Cobalamine (Vitamin B-12)                  

This vitamin contains cobalt metal ion and hence the name is given as Cobalamine. It is the most complex vitamin. It is water soluble and stable in pH range 4-6.

26.5.8.1 Functions of Cobalamine

It is a part of coenzymes which are essential in transfer of methyl groups. They are necessary in normal metabolic functioning in all cells and also in nucleic acid metabolism.

26.5.8.2 Deficiency disorders of Cobalamine

Deficiency of this vitamin is uncommon. One of the diseases associated with deficiency of this vitamin is pernicious anaemia, which in severe cases leads to damage of spinal cord, leading to sensory disturbances, weakness and paralysis.

26.5.8.3 Sources of Cobalamine

Milk, cheese, eggs, sea food, meat, liver and kidney are good sources of this vitamin. Vitamin B-12 is not present in plant sources like grains, vegetables, fruits etc. It is synthesized by bacteria in the intestine.

26.5.9 Ascorbic acid (Vitamin-C)

 It is a hexose derivative and can be converted to dehydro ascorbic acid by oxidation. Both the forms have vitamin activity.

26.5.9.1 Functions of Ascorbic acid

It helps in healing of wounds, fractures, bruises and reduces liability to infection. It is also involved in metabolism of some amino acids.

26.5.9.2 Deficiency disorders of Ascorbic acid

Deficiency of vitamin C leads to scurvy, in which the collageneus structure weakens which results in haemorrhage. Other symptoms include weakness, loss of appetite, anemia, tenderness to touch, swollen gums, loosening of teeth and diseases of bone joints.

26.5.9.3 Sources of Ascorbic acid

Citrus fruits, berries, guava, capsicum and green leafy vegetables are the excellent sources of vitamin C.

References & Further Reading:

1. Satyanarayana U. & Chakrapani U. (2011), “Biochemistry”. Books and Allied (P) Ltd. pp.116-159.

3. N. Shakuntala Manay & M. Shadaksharaswamy (2001), “Foods: Facts and Principles”. New Age International Publishers, pp. 59-79.