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This is a fat soluble vitamin, supplied either by food, or by exposure to the sun - hence it's known as the 'sunshine vitamin', since the sun's ultra violet rays convert subcutaneous (under the skin) cholesterol to vitamin D. it's mainly stored in the liver.
There are two major types of the vitamin: D-D2 (ergocalciferol) and D3 (cholecaliferol) - the latter is the form added to milk and used in nutritional supplements.
The vitamin assists the assimilation of vitamin A and maintains a healthy nervous system, normal heartbeat and efficient blood clotting.
Deficiency symptoms include porous bones and teeth, leading to rickets, fatigue and arthritis and one report links short-sightedness (myopia) to a deficiency in this vitamin.
Best natural sources occur in fish liver oils, oily fish - such as herring, salmon, sardines and tuna - and in fortified milk. The Recommended Daily Allowance (RDA) is 400 i.u both for adults and children.
Prolonged doses above 1,600 i.u can lead to over-accumulation and toxicity symptoms such as diarrhoea, nausea, excessive urination, calcification of the arteries and kidney damage. See 'Vitamin', 'Vitamin B Complex', 'Vitamin B2', 'VitaminB5', 'Vitamin B6', 'Vitamin B12','Vitamin B15'; 'Vitamin C', 'Vitamin E', 'Vitamin K', 'Vitamin P'. For more information on nutrition, see 'Nutrients A to Z' by Dr. Michael Sharon; Prion Books.