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Chapter: Biochemistry: Vitamins

Water soluble vitamins

The members of this group are B complex vitamins and vitamin C. They are readily soluble in water.


Vitamins are generally classified into two main groups. (i) fat soluble vitamins (ii) water soluble vitamins.

2. Water soluble vitamins

The members of this group are B complex vitamins and vitamin C. They are readily soluble in water.

B Complex vitamins

i. Thiamine (B1)

The structure of vitamin B1 is given in Fig. 8.5.


Thiamine act as a coenzyme in the form of thiamine pyrophosphate in many enzyme systems. These are involved principally in the breakdown of glucose to yield energy.

Thiamine also aids in the formation of ribose, a sugar that is an essential constituent of DNA and RNA, the carriers of the genetic code. The adequate level of thiamine provides healthy nerves, a good mental outlook, a normal appetite and food digestion.


Meats, especially pork and liver are rich in thiamine and account for about one fourth of the average intake. Dry beans, peanuts and egg are good sources.

Whole grain breads and cereals supply about one third of the daily thiamine intake.


The requirement of thiamine depends on energy expenditure.

Infants - 0.3 - 0.5 mg / day

Children - 0.7 -1.2 mg / day

Adults - 1.2 - 1.5 mg/day

Pregnant women and lactating women - 1.3- 1.5 mg/day

Absorption and storage

Free thiamine is readily absorbed from the small intestine. Excess thiamine administered is not stored in the tissues. A part of the excess thiamine is excreted in urine and same of it is destroyed by the enzyme thiaminase.


The symptoms of thiamine deficiency occur because the tissue cells are unable to receive sufficient energy from glucose. Therefore, they cannot carry out their normal functions.

Early symptoms of thiamine deficiency include fatigue, irritability, depression and numbness of the leg and poor tone of the gastro intestinal tract together with constipation.

Beriberi, sometimes called “rice-eaters disease” is another deficiency symptom which is often seen in people whose chief diet is refined rice and is the most severe form of thiamine deficiency.

ii. Riboflavin (B2)

Structurally vitamin B2 consists of a ribitol moiety and a substituted isoalloxazine ring (Fig. 8.6).


Riboflavin is a constituent of a group of enzymes called “flavoproteins”. As with thiamine, the enzymes are necessary in the break down of the glucose to form energy. Riboflavin is essential for a healthy skin and for good vision in bright light. If the individual ingest more riboflavin than their body needs, the urinary excretion will increase, if the intake is inadequate, the body maintains its supply very carefully and the urinary excretion will practically stop.


About half of the intake of riboflavin daily is furnished by milk alone and cheese is a good source, although some of the vitamin has been lost in the whey.


A more generous estimate of requirements is the level of intake at which there is normalisation of the activity of the red cell enzyme glutathione reductase, which is a flavoprotein whose activity is especially sensitive to riboflavin nutritional status.

Infants - 0.4 - 0.6 mg / day

Children - 0.8 - 1.2 mg / day

Adults male - 1.5 -1.8 mg/day

Adults female - 1.1 - 1.4 mg/day

Pregnant women - 1.4 - 1.7 mg/day

Lactating women - 1.6 - 1.9 mg / day

Absorption and storage

The vitamin is phosphorylated in the intestinal mucosa during absorption. It is absorbed from the small intestine through the portal vein and is passed to all tissues being stored in the body. The major part is excreted in urine and a small part is metabolized in the body.


Riboflavin deficiency leads to cheilosis, a cracking of the skin at the corners of the lips and scaliness of the skin around the ears and nose. There may be redness and burning as well as itching of the eyes, and extreme sensitivity to strong light.

iii. Niacin (B3)

Niacin is pyridine-3-carboxylic acid (Fig. 8.7). It occurs in tissues as nicotinamide.


Niacin is required for the stepwise breakdown of glucose to yield energy. Niacin is essential for the healthy skin, normal functions of the gastro intestial tract and maintenance of the nervous system.


The meat group especially organ meats and poultry, is the chief source of preformed niacin. Dark green leafy vegetables, whole grain, enriched breads and cereals are fair sources.

Niacin is more stable to cooking procedures than thiamine or ascorbic acid (Vitamin C).


The recommended niacin allowance is 6.6 mg/1000 k cal. This can be supplied by exogenous niacin in the diet and by the tryptophan, an essential aminoacid that is the precursor of niacin biosynthesis.

Infants - 5 -8 mg / day

Children - 9 - 16 mg / day

Adults male - 16 - 20 mg/day

Adults female - 12 - 16 mg/day

Pregnant women - 14 - 18 mg/day

Lactating women - 16 - 20 mg / day

Absorption and storage

Nicotinic acid and nicotinamide are absorbed from the intestine through the portal vein into the general circulation. Excess nicotinic acid is not stored in the body.


Pellagra is the major deficiency disease resulting from the lack of niacin. Dermatitis, especially of the skin exposed to the sun, soreness of the mouth, swelling of the tongue, diarrohea, and mental changes including depression, confusion, disorientation, and delirium are typical of the advancing stages of the disease, which ends to death if not treated. (The disease is sometimes referred to as the “4D’S” - dermatitis, diarrohea, dementia and death).

iv. Pyridoxine (B6)

Pyridoxine is 3-hydroxy 4,5 dihydroxy methyl - 2- methyl pyridine (Fig 8.8) The metabollically active form of vitamin B6 is pyridoxal phosphate.


Three forms of vitamin B6 exist in nature which are pyridoxine, pyridoxal and pyridoxamine. The functions of vitamin B6 are closely related to protein metabolism, the synthesis and breakdown of amino acids, conversion of tryptophan to niacin, the production of antibodies, the formation of heme in hemoglobin, the formation of hormones important in brain function and others.


Meat, especially organ meats, whole grain cereals, peanuts and wheat germ are rich sources. Milk and green vegetables supply smaller amounts.


Although most of the body’s vitamin B6 is associated with glycogen phosphorylase in muscle, this is relatively stable and well conserved.

The requirement depends not on energy expenditure and glycogen metabolism, but on the intake of protein. The average requirement is 13 μg/g dietary protein.

Infants - 0.3 mg / day

Children - 0.6 - 1.2 mg / day

Adults - 1.6 - 2 mg/day

Pregnant and lactating women - 2.5 mg/day

Absorption and storage

Pyridoxine is readily absorbed from the small intestine. The excess amount if ingested is not stored in the body but is excreted in urine.


Deficiency of vitamin B6 is extremely rare. Nervous disturbances such as irritability, insomnia, muscular weakness, fatigue and convulsion have been recorded in infants. The cause of the convulsions severe impairment of the activity of the enzyme glutamate decarboxylase, which is dependent on pyridoxal phosphate. The product of glutamate decarboxylase is GABA (γ-amino butyric acid) which is a regulatory neurotransmitter in the central nervous system.

v. Folic acid

Folic acid contains a pteridine group linked to para amino benzoic acid and l-glutamic acid (Fig. 8.9). It is slightly soluble in water and stable to heat.


·           Folic acid serve as coenzymes in reactions involving the transfer of one carbon units like formyl and methyl groups.

·           It participates in the reactions concerned with the synthesis of purine, pyrimidine and nucleic acids.

·           It is essential for maturation of red blood cells.

·           Folic acid is required for the metabolism of amino acids like histidine.

·           Along with vitamin B12, folic acid helps in the trans methylation reactions. eg: uracil to thymine.


Folic acid is particularly present in green leafy vegetables, cauliflower and dried yeast. Egg, liver and kidneys are rich animal sources.


There is no definite requirement for normal human being. However, an increased amount is required during pregnancy and lactation.

Infants - 50 μg / day

Children - 100 - 300 μg / day

Adults - 400 μg/day

Pregnant women - 800 μg / day

Lactating women - 600 μg/day

Absorption and storage

Absorption of folic acid takes place along the whole length of the mucosa of the small intestine. Folic acid about (5-15 mg/g) is in the liver and folate is also incorporated into the erythrocytes during erythropoiesis (Red blood cells production).


Deficiency of vitamin B12 also leads to functional folic acid deficiency.

·           Folic acid deficiency leads to megaloblastic anemia characterised by the release of large sized immature red blood cells into the circulation.

·           Sprue and symptoms like glossitis and gastro intestinal disturbances have also been reported.

·           Macrocytic anemia of pregnancy responds to treatment with folic acid.


vi. Vitamin B12

Structure of vitamin B12

Structurally, vitamin B12 consists of a corrin nucleus attached with 5,6 dimethyl benzimidazole moitey, an aminopropanol unit, a ribose unit and a phosphate group. A cobalt atom (Co) is present at the centre of the corrin ring structure. One of the valencies of cobalt is filled by either CN- (cyano cobalamin) or H2O (aqua cobalamin) or OH- (hydroxo cobalamin) or CH3 (methyl cyano cobalamin).


Of all vitamins, vitamin B12 is the most complex. The trace mineral cobalt is an essential part of the molecule. Vitamin B12 is required for the maturation of red blood cells in the bone marrow and for the synthesis of proteins.


Milk, eggs, cheese, meet, fish and poultry supply ample amounts of vitamin B12. Plant foods supply no vitamin B12 and use of an exclusively vegetarian diet for a long period of time will lead to symptoms of deficiency.


Early estimates of vitamin B12 requirements were based on the amounts required to maintain normal RBC maturation in patients with pernicious anemia due to lack of intrinsic factor secretion. There is a considerable enterohepatic circulation of vitamin B12. It is secreted in the bile and re-absorbed in the small intestine. However, in patients with defective secretion of intrinsic factor, the vitamin cannot be re-absorbed, but is excreted in the feces. This means that patients with impaired secretion of intrinsic factor have much higher requirement for vitamin B12 than normal.

The average requirement of vitamin B12 is 3 μg/day.

Infants - 0.3 μg / day

Children - 1 - 2 μg / day

Adults - 3 μg/day

Pregnant and lactating women - 4 μg / day

Absorption and Storage

For the absorption of vitamin B12 from the intestines, a factor called “Intrinsic Factor” (IF) secreted by the stomach is essential. Vitamin B12 is stored in fair amounts in the liver.


Pernicious anemia is the disease resulting from vitamin B12 deficiency. It is a genetic defect with the absence of intrinsic factor, hence the vitamin B12 in the diet cannot be absorbed. Since vitamin B12 is important for the maturation of red blood cells the deficiency of this vitamin leads to the formation of macrocytic red blood cells.


vii. Pantothenic acid (B5)

The structure of pantothenic acid consists of an alanine chain in peptide linkage with dihydroxy, dimethyl butyric acid (Fig. 8.10).

Pantothenic acid is highly soluble in water.


Pantothenic acid exists in the free form and in combination with b- mercapto ethylamine, adenine ribose and phosphoric acid. The later form is called as co-enzyme A (CoA).

The metabolic functions of pantothenic acid are due to its coenzyme derivative CoA, which participates in several metabolic reactions. CoA gains further importance after its conversion to form acetyl CoA.

·           Acetyl - Co A plays a key role in carbohydrate, protein and lipid metabolism.

·           Acetyl - Co A is the precursor of cholesterol. It is the main source for the synthesis of cholesterol as well as steroid hormones.

·           Acetyl - Co A combines with choline to form acetyl choline.

·           Some of the amino acids require Co A for their activation.


Dried yeast, liver, royal gelly are the rich sources of pantothenic acid. Egg yolk, meat, fish, milk are good sources.


The recommended daily allowance of pantothenic acid as follows:

Infants - 1.5 - 2.5 mg/day

Children - 5 - 8 mg/day

Adults - 5-12 mg/day

Pregnant and lactating women - 10-15 mg/day.

Absorption and sotrage

Pantothenic acid and its salts are readily absorbed from the small intestine through the portal vein into the general circulation. If ingested in excess of the requirements, it is not stored in the body; but is excreted in urine or metabolised by the tissues.


Deficiency of this vitamin results in nausea, vomitting, certain gastro intestinal tract disorders, inadequate growth, anemia, fatty liver and failure in gaining weights.

viii. Biotin

Biotin is a heterocyclic, sulphur containing monocarboxylic acid (Fig. 8.11). Biotin is sparingly soluble in cold water and is freely soluble in hot water.


Biotin is required as the co-factor for a small number of carboxylation reactions, it acts as the carrier for carbon dioxide.

·           It helps to maintain the skin and the nervous systems in good condition.

·           It assists in the deamination of amino acids like aspartic acid, serine and threonine.

·           It helps in the synthesis of purine.

·           For the conversion of ornithine to citrulline in the synthesis of urea, biotin is required.


Biotin occurs widely both in foods of vegetable and animal origin. Wheat germ, liver, peanut, and rice polishings are rich sources. Whole cereals, legumes, mutton and egg are good sources.


Since, intestinal bacteria and diets supply biotin in adequate amounts the deficiency of this vitamin in human being is rare.

Infants - 10- 15 μg/day

Children - 20-40 μg/day

Adults - 50-60 μg/day

Absorption and Storage

Biotin is readily absorbed from the small intestine through the portal vein into the general circulation. Excess of the requirements is not stored in the body but is mostly excreted in the urine.

Egg white injury factor (Avidin)

There is a protein in egg white called avidin which is responsible for producing egg white injury. Avidin binds with biotin tightly in the intestinal tract and prevents absorption of biotin from intestines. Avidin is denatured by cooking and then loses its ability to bind with biotin. The amount of avidin in uncooked egg white is relatively small, and problems of biotin deficiency have only occurred in people eating abnormally large amounts of raw eggs for many years.


Deficiency of biotin is rare in human beings.


Vitamin C (Ascorbic Acid)

Vitamin C is also called as ascorbic acid (Fig. 8.12).


·           Vitamin C is essential for building collagen the connective tissue protein which cements the cells and tissues together. The effect of this material is to provide firm tissues of all kinds. This vitamin helps forming strong blood vessels, teeth firmly held in their sockets, and bones firmly held together.

·           It has a general antioxidant role, especially in the regeneration of oxidized vitamin E in membranes.

·           Ascorbic acid reduces the ferric ion (Fe3+) to ferrous (Fe2+) ion and thus helps in the absorption of iron. It is also essential for rapid healing of wounds.


Raw fresh vegetables contain vitamin C, but some foods are more outstanding than others. Orange, grape, lime and lemon are especially rich in vitamin C.


Recommended amount of vitamin C for different age group is as follows:

Infants - 35 mg / day

Children - 40 mg / day

Adults - 45 mg / day

Pregnant women - 60 mg / day

Lactating women - 80 mg / day

Absorption and storage

Ascorbic acid is rapidly absorbed from the intestines and passed on through the portal vein to the general circulation. Liver and other organs and tissues have an optimal level of ascorbic acid. Excess intake do not increase further the optimal levels.


Severe deficiency of vitamin C leads to scurvy. This is characterised by easy bruising and hemorrhaging of the skin, lossening of the teeth, bleeding of the gums and distruption of the cartilages that support the skeleton.


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