Vitamins are used for the prevention and treatment of specific deficiency states or when the diet is known to be inadequate. Large doses of vitamins (megavitamin therapy) have been proposed for a variety of disorders, but adequate evidence of their value is lacking. Excessive intakes of most water - soluble vitamins have little effects due to their rapid excretion in urine, but excessive intakes of fat - soluble vitamins accumulate in the body and are potentially dangerous.
Vitamin A (Retinol) is a fat - soluble substance stored in body organs, principally the liver.
Deficiency of Vitamin A (Retinol) is associated with occular defects (particularly xerophthalmia) and an increased susceptibility to infections particularly measles and diarrhoea. Despite initial epidemiological evidence suggesting that vitamin A or carotene may have a protective effect against some epithelial cancers, the claims have not been substantiated.
Massive overdose can cause rough skin, dry hair, an enlarged liver, and a raised erythrocyte sedimentation rate and raised serum calcium and serum alkaline phosphatase concentrations.
In view of evidence suggesting that high levels of vitamin A may cause birth defects women who are (or may become) pregnant are advised not to take vitamin A supplements (including tablets and fish liver oil drops), except on the advice of a doctor or an antenatal clinic; nor should they eat liver or products such as liver pate or liver sausage.
Vitamin B is composed of widely differing substances which are, for convenience, classed as 'vitamin B complex'. Thiamine (Vitamin B1) is used orally for deficiency due to inadequate dietary intake, severe deficiency may result in 'beri-beri'. Thiamine is given by intravenous injection in doses of up to 300mg daily (parenteral preparations may contain several B group vitamins) as initial treatment in severe deficiency states. Potentially severe allergic reactions may occur after parenteral administration; facilities for resuscitation should be immediately available. Pyridoxine (Vitamin B6) deficiency is rare as the vitamin is widely distributed in foods, but deficiency may occur during isoniazid therapy and is characterized by peripheral neuritis. High dose are given in some metabolic disorders, such as hyperoxaluria.
Nicotinic acid inhibits the synthesis of cholesterol and triglyceride and is used in some hyperlipidaemias.
Nicotinic acid and nicotinamide are used to prevent and treat nicotinic acid deficiency (pellagra). Nicotinamide is generally preferred as it does not cause vasodilation.
Folic acid is essential for the synthesis of DNA and certain proteins. Deficiency of folic acid or vitamin B12 is associated with megaloblastic anaemia. Folic acid should not be used in undiagnosed megaloblastic anaemia unless Vitamin B12 is administered concurrently, otherwise neuropathy may be precipitated.
Supplementation with folic acid 400 micrograms daily is recommended for women of child - bearing potential in order to reduce the risk of serious neural tube defects in their offspring.
Ascorbic acid (Vitamin c) is used for the prevention and treatment of scurvy. Claims that ascorbic acid is of value in the treatment of common colds are unsubstantiated.
The term Vitamin D covers a range of compounds including ergocalciferol (Vitamin D2) and colecalciferol (Vitamin D3). These two compounds are equipotent and either can be used to prevent and treat rickets.
Simple deficiency of Vitamin D occurs in those who have an inadequate dietary intake or who fail to produce enough colecalciferol (Vitamin D3) in their skin from the precursor 7 - dehydrocholesterol in response to ultraviolet light.
Children with dark skin must continue vitamin D prophylaxis for up to 24 months because of their inability to produce enough vitamin D3 in their skin. Dark skin with a high melanin content must be exposed to daylight longer than light skin in order to obtain the same synthesis of vitamin D3. Vitamin D is also used in deficiency states caused by intestinal malabsorption or chronic liver disease and for the hypocalcaemia of hypoparathyroidism.
Vitamin K is necessary for the production of blood clotting factors (see sec. 10.1)