Vitamins

Summary:
Information about vitamins, their classification as fat soluble and water soluble and functions of Vitamin A, D, E, K, the B complex and C.

Vitamins

Vitamins are chemical substances that are required by the body in very minute quantities for normal metabolism and healthy growth. Vitamins are found widely distributed in foods. On the basis of their solubilitythey are classified into into two main groups:

Fat-soluble vitamins: include vitamin A, D, E and K
Water soluble vitamins: include vitamin B complex and C.

Fat soluble vitamins

Vitamins A (Retinol)

Vitamin A, also known as retinol, is present in foods like cream, egg yolk, liver, fish oil, cheese and butter. Our body can synthesize vitamin A from certain carotenes. Carotenes are present in green vegetables, fruits and carrots. Fat absorption is essential for the absorption of vitamin A. Vitamin A and carotene are absorbed from the small intestine only if fat absorption is normal. The body needs about 600 to 700 µg of vitamin A per day. Vitamin A plays the following roles in the body:

Generation of rhodopsin, a light sensitive pigment in the retina of the eye
Growth and differentiation of cells
Provides immunity and defense against infection
Promotion of growth, e.g. in bones.

The first symptom of vitamin A deficiency is night blindness. Night blindness is caused due to defective retinal pigment. Other consequences of vitamin A deficiency include xeropthalmia, drying and thickening of the conjunctiva and, ultimately, there is ulceration and destruction of the conjunctiva. Xeropthalmia is a common cause of blindness in developing countries. Vitamin A deficiency also causes atrophy and keratinisation of other epithelial tissues. This leads to increased incidence of infections of the ear, and the respiratory, genitourinary and alimentary tracts. Immunity is compromised and bone development may be hampered.

Vitamin D

Vitamin D is found mainly in animal fats such as eggs, butter, cheese, fish liver oils. Humans and other animals can synthesize vitamin D by the action of the ultraviolet rays of the sun on a form of cholesterol in the skin.

Vitamin D regulates calcium and phosphate metabolism by increasing their absorption in the gut and stimulating their retention by the kidneys. It therefore promotes the calcification of bones and teeth.

Deficiency causes rickets in children and osteomalacia in adults, due to deficient absorption and utilization of calcium and phosphate. The daily requirement is 10 micrograms and stores in fat and muscle are such that deficiency may not be apparent for several years.

Vitamin E

This is a group of eight substances called tocopherols. They are found in nuts, egg yolk, wheat germ, whole cereal, milk and butter.

Vitamin E is an antioxidant, which means that it protects body constituents such as membrane lipids from being destroyed in oxidative reactions. Deficiency is rare, because of the widespread occurrence of this vitamin in foods, and is usually seen only in premature babies and in conditions associated with impaired fat absorption. Hemolytic anemia occurs, as abnormal red blood cell membranes rupture. White blood cells can likewise be affected, and vitamin E supplements boost immune function. Neurological abnormalities such as ataxia and visual disturbances may occur if the deficiency is severe. Recently, vitamin E has been shown to protect against coronary artery disease. Recommended daily intake is 10 mg for men and 8 mg for women, but this should be increased in high fat diets.

Vitamin K

The sources of vitamin K are fish, liver leafy green vegetables and fruit. It is synthesized in the large intestine by microbes and significant amounts are absorbed. Absorption is dependent upon the presence of bile salts in the small intestine. The normal daily requirement is 1 microgram.kg body weight and only a small amount is stored in the liver and spleen.

Vitamin K is required by the liver for the production of prothrombin and factors essential for clotting of blood. Deficiency therefore prevents normal coagulation. It may occur in adults when there is obstruction to the flow of bile, severe liver damage and in malabsorption conditions, such as coeliac disease. Newborn infants may be given vitamin K because their intestines are sterile and require several weeks to become colonized with vitamin K-producing bacteria.

Water-soluble vitamins

Vitamin B complex

This is a group of water soluble vitamins that promote activity of enzymes at various stages in the chemical breakdown of nutrients to release energy.

Vitamin B1 (thiamine). This vitamin is present in nuts, yeast, egg yolk, liver, legumes, meat and germ of cereals. It is rapidly destroyed by heat. The daily requirement is 0.8 to 1 mg and the body stores only about 30 mg. Thiamine is essential for the complete aerobic release of energy from carbohydrate. When it is absent there is accumulation of lactic and pyruvic acids, which may lead to accumulation of tissue fluid and heart failure. Thiamine is also important for nervous system function because of the dependency of these tissues on glucose for fuel.

Deficiency of thiamine causes beriberi which occurs mainly in countries where polished rice if the chief constituent of the diet. In beriberi there is:

Severe wasting of muscle
Stunted growth in children
Polyneuritis, causing degeneration of motor, sensory and some autonomic nerves
Susceptibility to infections.
If untreated, death occurs due to cardiac failure or severe microbial infection.

The main cause of thiamine deficiency in developed countries is chronic alcohol abuse, where the diet is usually poor. Neurological symptoms include memory loss, ataxia and visual disturbances; sometimes these are reversed with oral thiamine supplements.

Vitamin B2 (riboflavine). Riboflavine is found in yeast, green vegetables, milk, liver, eggs, cheese and fish roe. The daily requirement is 1.1 to 1.3 mg and only small amounts are stored in the body. It is concerned with carbohydrate and protein metabolism, especially in the eyes and skin. Deficiency leads to:

Blurred vision, cataract formation and corneal ulceration
Cracking of the skin, commonly around the mouth (angular stomatitis)
Lesions of intestinal mucosa.

Folic acid. This is found in liver, kidney, fresh leafy green vegetables and yeast. It is synthesized by bacteria in the large intestine, and significant amounts derived from this source are believed to be absorbed. The daily requirement is 200 micrograms, and, as only a small amount is stored in the body, deficiency is evident within a short time. It is essential for DNA synthesis, and when lacking mitosis is impaired. This manifests in rapidly dividing cells such as blood and folate deficiency therefore leads to a form of megaloblastic anemia, which is reversible with folate supplements. Deficiency at conception and during early pregnancy is linked to an increased incidence of spina bifida.

Niacin (nicotinic acid). This is found in liver, cheese, yeast, whole cereals, eggs, fish and nuts; in addition, the body can synthesize it from the amino acids tryptophan. It is associated with energy releasing reactions in cells. In fat metabolism it inhibits the production of cholesterol and assists in fat breakdown. Deficiency occurs mainly in areas where maize is the chief constituent of the diet because niacin in maize is in an unusable form. The daily requirement is 12 to 17 mg.

Pellagra develops within 6 to 8 weeks of severe deficiency. It is characterized by:

Redness of the skin in parts exposed to light, especially in the neck
Anorexia, nausea, dysphagia and inflammation of the lining of the mouth
Delirium, mental disturbance and dementia.

Vitamin B6 (Pyridoxine).This is found in egg yolk, peas, beans, soya beans, yeast, meat and liver. The daily requirement is about 1.2 to 1.4 mg and dietary deficiency is rare, although certain drugs, e.g.: alcohol and antituberculous drugs, antagonize the vitamin and can induce deficiency status. It is associated with amino acid metabolism, including the synthesis of non-essential amino acids and molecules such as haem and nucleic acids.

Vitamin B12 (cynacobalamin). Vitamin B12 consists of a number of cobalamin compounds. It is found in liver, meat, eggs, milk and fermented liquors. The daily requirement is 1.5 micrograms.

Like folic acid, vitamin B12 is essential for DNA synthesis, and deficiency also leads to a megaloblastic anemia, which is correctable with supplements. However, vitamin B12 is also required for formation and maintenance of myelin, the fatty substance that surrounds and protects some nerves. Deficiency accordingly causes peripheral neuropathy and / or spinal cord degeneration. Such neurological changes are irreversible. The presence of intrinsic factor in the stomach is essential for vitamin B12 absorption and deficiency is usually associated with insufficient intrinsic factor.

Pantothenic acid. This is found in many foods and is associated with amino acid metabolism. The daily safe intake is 3 to 7 mg and no deficiency diseases have been identified.

Biotin. This is found in yeast, egg yolk, liver, kidney and tomatoes and is synthesized by microbes in the intestine. It is associated with the metabolism of carbohydrates. The daily safe intake is 10 to 200 micrograms. Deficiency is rare.

Vitamin C (ascorbic acid)

This is found in fresh fruit, especially black currants, oranges, grapefruit and lemons, and also in rosehips and green vegetables. The vitamin is very soluble in water and is easily destroyed by heat, so cooking may be a facto in the development of scurvy.

The daily requirement is 40 mg and after 2 to 3 months, deficient intake becomes apparent.

Vitamin C is associated with protein metabolism, especially the laying down of collagen fibers in connective tissue.

Vitamin C, like vitamin E, acts as an antioxidant, protecting body molecules from damaging oxidative reactions. When deficiency occurs, collagen production is affected, leading to fragility of blood vessels, delayed wound healing and poor bone repair. Gums become swollen and spongy and the teeth loosen in their sockets.

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