Vitamins: Functions and their sources

Vitamin refer to essential dietary factor which is required by an organism in small amounts and whose absence results in deficiency diseases. Vitamins are important for their regularity and protective functions. Unlike most other nutrients, they are required in very small amounts. But it is necessary to provide these in diet because many of them cannot be synthesized in our body. Excess intake of any vitamin however is harmful. Vitamins are generally classified on the basis of their solubility.

  1. Water soluble vitamin: vitamin C, B complex
  2. Fat soluble vitamin: vitamin A, D, E and K

Fat soluble vitamin:

Vitamin A:

  • Retinol (an alcohol) or retinal (an aldehyde) are often referred to as preformed vitamin A.
  • Retinal can be converted by body to retinoic acid, the form of vitamin A.
  • Preformed vitamin A is present in animal food source only such as fish liver oil, egg yolk.
  • Vitamin A is also formed in the body in the form of pro-vitamin A carotenoids, which include β – carotene and other carotenoids.
  • Pro-vitamin A occurs in plants such as carrot, mango, and dark green leafy vegetables like spinach, amaranth and coriander.
  • The chief source of vitamin A in human nutrition is β – carotene.
  • A molecule of β – carotene yield two molecules of vitamin A.

Stability of vitamin A:

  • Relatively stable to heat in the absence of oxygen
  • Susceptible to oxidation in the presence of light and oxygen
  • Unstable under influence of mineral acid but stable in alkali

Source of vitamin A: Fish liver oil, green leafy vegetables. Yellow fruits like papaya, mango, carrot, spinach, fortified margarine and vegetable oil.

Functions of vitamin A:

  1. Epithelium tissue function as barrier and form the body’s first line of defense against infection. Inadequate vitamin A supply results in suppression of normal secretions and produces a keratinized (dry, horny) type of epithelium. Such tissue may become susceptible to bacterial invasion.
  2. Immunity: Vitamin A is commonly known as anti-infective vitamin because it is required for normal functioning of immune system.
  3. Growth and development: during fetal development, retinoic acid (RA) function in limb development and formation of heart, eyes and ear. RA has been found to regulate expression of gene for growth hormone.
  4. Red blood cell production: Red blood cells are derived from precursor cells called stem cells. These stem cells are dependent on retinoid for normal differentiation into RBC.
  5. Regulation of gene expression: Retinoic acid act as hormones to affect gene expression and thereby influence numerous physiological processes.

Vitamin D:

  • Two most important forms are
  1. Vitamin D2 = Ergocalciferol
  2. Vitamin D3 = Cholecalciferol
  • The precursors of vitamin D2 and D3 are ergo sterol (plant steroid) and 7 – dehydrocholesterol (present in animal tissue). These precursors or provitamins can be converted into respective vitamin D by irradiation with UV light.
  • In animal tissue, 7 – dehydrocholesterol, which occurs naturally in the epidermal layers can be converted by UV radiation to vitamin D3. Therefore, vitamin D is sometimes called sunshine vitamin.

Deficiency of vitamin D: Vitamin D deficiency leads to rickets, osteomalacia and muscle weakness and pain. Rickets results in infant and children duet to failure of bone to mineralization.

Food sources of vitamin D: fish liver oil, egg, milk and fortified foods.

Functions of vitamin D:

  1. Vitamin D is essential for efficient utilization of calcium by body.
  2. The active form of vitamin D inhibits cell proliferation and stimulates differentiation of cells.
  3. Active form of vitamin D enhance innate immunity and inhibit the development of autoimmunity.
  4. Vitamin D plays a role in insulin secretion under condition of increased insulin demand.
  5. Adequate vitamin D level is important for decreasing the risk of high blood pressure.

Vitamin E:

  • Vitamin E refers to the family of eight antioxidants. I.e. four tocopherols (α, β, γ, δ) and four tocotrienols (α, β, γ, δ).
  • α- tocopherol is only form of vitamin E that is actively maintained in human body and therefore found in largest quantities in blood and tissue.

Stability of vitamin E: Vitamin E is lost in significant amount during refining and deodorization of vegetable oil. Heating, frying, storage etc. all reduces vitamin E concentration in oil.

Deficiency of vitamin E: Severe vitamin E deficiency results in neurological symptom including impaired balance and coordination (ataxia), muscle weakness (myopathy) and damage to retina of eyes (pigmented retinopathy).

Food sources of vitamin E: vegetable oil, nuts, whole grain and green leafy vegeables.

Function of vitamin E:

  1. Vitamin E acts as antioxidant and prevents chain reaction of lipid destruction.
  2. Vitamin E enhance vasodilation.

Vitamin K:

  • The word vitamin K is derived from German word “koagulation” which refers to coagulation (blood clotting).
  • Vitamin K is essential for function of several protein involved in blood clotting.
  • There are two naturally occurring forms of vitamin K. They are;
  1. K1 = Phylloquinone
  2. K2 = Menaquinone
  • K3 = menadione (synthetic product with vitamin K activity)

Stability of vitamin K: Vitamin K is slowly decomposed by atmospheric oxygen but is readily destroyed by light. It is stable against heat but is unstable against alkali.

Food sources of vitamin K: Spinach, cabbage, oranges and fish, liver, meat and egg

Water soluble vitamin:

  1. Thiamin (vitamin B1)
  • Thiamin helps in glucose utilization by nerve tissues.
  • Deficiency disease : Beriberi (pyruvate in blood rises to abnormal level)
  • Sources: yeast, liver, outer layer of cereal grain, lean meat, soybean
  • Stability: reduced by heat, oxygen, Sulfur dioxide, leaching neutral or alkaline pH.
  1. Riboflavin (vitamin B2)
  • Helps in oxidation –reduction reaction.
  • Exhibit antioxidant function
  • Critical for metabolism of carbohydrate, fat and protein
  • Food sources: yeast, liver, milk, wheat germ, egg, green leafy vegetables, germinated legumes.
  • Stability: acid stable, thermo stable, alkali labile and photo labile, oxidized by strong oxidizing agent.
  • Deficiency: condition called “ariboflavinosis” characterized by sore throat, inflammation and redness of tongue and decreased RBC count.
  1. Niacin (vitamin B3)
  • Also known as nicotinic acid and nicotinamide
  • Plays role as coenzyme in as many as 200 enzymes
  • Important for synthesis for fatty acid and cholesterol
  • Food sources: Yeast, liver, legumes, peanuts, outer coat of cereals, meat, fish
  • Stability: Niacin is most stable B vitamin. Unaffected by light, heat, oxygen, acid and alkali. Blanching of vegetables may cause loss of about 15%
  • Deficiency: disease called “Pellagra” characterized by 4D’s i.e. dermatitis, diarrhea, dementia and death.
  • Pellagra is common in region where corn is staple food. Occurrence of pellagra has been related to low level of tryptophan present in corn.
  1. Pantothenic acid (vitamin B5)
  • Found throughout living cell in the form of coenzyme A.
  • Generate energy from food (fat, carbohydrate and protein)
  • Important for synthesis of fat, cholesterol and steroid hormone.
  • Food sources: yeast, liver, kidney, egg yolk, legumes and nuts.
  • Stability: stable in air, labile to dry heat. Stable in solution in pH range 5-7.
  1. Pyridoxine (vitamin B6)
  • There are different forms of vitamin B6. Animal tissue contain pyridoxal and pyridoxamine while plant contain pyridoxine.
  • Plays role in generation of glucose form amino acid (gluconeogenesis)
  • Important for synthesis of neurotransmitter and heme (component of hemoglobin).
  • Food sources: meat, fish, egg, soybean, spinach.
  • Stability: stable to heat and strong alkali or acid but is sensitive to light esp. ultraviolet light.
  • Deficiency: condition characterized by irritability, depression, confusion and inflammation of tongue.
  • Alcoholics are thought to be at risk of vitamin B6 deficiency.
  1. Biotin (vitamin B7)
  • Biotin is required by all organism but can only ne synthesized by bacteria, yeast, mold, algae and some plant species.
  • Biotin is required for synthesis of fatty acid
  • Required for metabolism of leucine (essential amino acid)
  • It is critical enzyme in gluconeogenesis and metabolism of cholesterol and fatty acid.
  • Food sources: egg yolk, liver, yeast. Considerable amount is synthesized by intestinal bacteria.
  • Stability: highly soluble in water. It is photo stable, thermo stable but acid labile.
  1. Folic acid (vitamin B9)
  • Play important role in DNA metabolism
  • Required for metabolism for several important amino acid
  • Food sources: yeast, liver, spinach, amaranth, mint. Intestinal bacteria also synthesize large amount of folate.
  • Stability: easily destroyed by cooking but fermentation of milk increases folate level.
  • Deficiency: megaloblastic or macrocytic anemia referring to large immature red blood cells
  1. Cyanocobalamin (Vitamin B12)
  • Largest and complex of all vitamins. It contains metal ion, cobalt. .
  • Plays role in production of energy from fat and protein. Plays role in synthesis of hemoglobin.
  • Functions as cofactor for many enzymes
  • Only bacteria can synthesize vitamin B12.
  • Sources: meat, poultry, fish and to lesser extent in milk. It is not generally present in plant product or yeast.
  • Stability: fairly thermo stable in alkaline pH.
  • Deficiency: pernicious anemia, relating to destruction of stomach cell by one’s own antibodies.
  1. L- Ascorbic acid (Vitamin C)
  • Vitamin C occurs in all living tissue.
  • Man, apes, monkeys and some other animal cannot synthesize vitamin C because they lack enzyme “Gulunolactone oxidase”.
  • Vitamin C is required for synthesis of collagen and neurotransmitter.
  • It is strong antioxidant.
  • Sources: amala, guava, cabbage, citrus fruits. Animal sources such as milk, and liver also contain vitamin C.
  • Stability: least stable of all vitamin and is easily destroyed during processing and storage. Exposure to oxygen, prolonged heating in presence of oxygen and exposure to light destroys vitamin C.
  • Deficiency: “scurvy” relating to bleeding of gums, joint pain and hair and tooth loss.



About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.