Vitamin D - Calciferol

Calciferol might as well be characterized as a hormonal precursor with vitamin-like effects.

Vitamin D can be found in several forms, each a little different in its chemical structure. Of greatest relevance to us is D2 (ergocalciferol) and D3 (cholecalciferol) which is the most effective form.

Vitamin D increases the intestinal absorption of calcium and is therefore important to the prevention of osteoporosis and rickets, which is a condition with soft, deformed bones, blood sugar problems, and development of short-sightedness. Vitamin D is necessary for the development of healthy lung function. It also has significance to the blood's ability to clot. Studies indicate that vitamin D participates in the regulation of the immune defence and that the vitamin may play a role in the prevention (but treatment) of hypertension and several forms of cancer.

The vitamin D that we get from our food or from dietary supplements demands must be metabolised in the liver followed by the kidneys in order to function. People with kidney or liver problems have an increased risk of acquiring osteoporosis because they cannot optimally process vitamin D.

When exposed to ultraviolet radiation from e.g. sunlight, cholesterol is transformed into vitamin D in the skin - popularly speaking. For this reason a certain amount of sunlight is healthy. Pale skin absorbs more vitamin D than dark skin and vitamin D is much more easily absorbed from the skin than from food. The amount of vitamin D coming from the skin is deposited in e.g. the liver and kidneys and the depots can cover the body's requirement for a couple of months. It is estimated that an adult person whose entire body is exposed to the sun for a whole day will be able to produce about 10,000 I.U. of vitamin D in the skin. However, vitamin D from sunlight is only produced when the sun is sufficiently high in the sky.
When the sun is low, its ultraviolet radiation is reduced from having to penetrate more atmospheric air. For this reason, in latitudes corresponding to those of Britain, little vitamin D is acquired from sunlight except for during the summer months.

According to one theory, our need for vitamin D is far higher than assumed because our development as humans has taken place under more sunny latitudes and without modern clothing. In ancient times we have therefore been accustomed to far greater blood levels of vitamin D than we have today.

Vitamin D can also be found in fatty saltwater fish, salmon, sardines, mackerel, tuna etc. and in eggs, bean sprouts, butter, milk, cod-liver oil, oats, and vegetable oils. Diet alone, however, rarely provides enough vitamin D to cover our needs.

Today it is common to give infants a supplement of vitamin D but vitamin D deficiency can still be observed in women of childbearing age and in many elderly people who do not get out into sunlight also can benefit from a daily supplement. Unspecific muscle pains can be a symptom of vitamin D deficiency. Deficiency symptoms can be seen in people who are not getting enough sunlight, people who have had a part of their intestines surgically removed or who have diseases connected to the alimentary canal causing the absorption of vitamin D to be decreased.

RDI
300 I.U. (7,5 mcg.) 

Optimal dosage
One tsp. of cod-liver oil can be taken daily in the winter as a general supplement.
Pregnant- and breast-feeding women need at least 500 I.U. daily.
Elderly people who do not get enough sunlight and children may benefit from 500 - 1.000 I.U. daily.
In case of diseases like rickets and osteoporosis a daily supplement of 2,000 - 10,000 I.U. can be taken. Sustained supplements of vitamin D of more than 4,000 I.U. a day should be avoided if not supervised by a medical expert.

Vitamin D used to be considered to pose the greatest threat of a possible vitamin poisoning. However, no valid scientific documentation can establish a risk from taking up to 40,000 I.U. daily of vitamin D3.
One of the consequences of a sustained vitamin D overdosing is that the body's inner soft parts calcify. However, in practice the risk is not very big and if the occasion should arise, there would be a long line of previous symptoms as a warning.

Poisoning symptoms
General uneasiness, itching, frequent urination, great thirst, headache, insomnia, fatigue, and muscle weakening.

Warning
Do not take large doses of vitamin D if you have too high levels of calcium in your blood (hypercalcaemia). A large therapeutic vitamin D supplement can require monitoring of your body's calcium balance. Certain circumstances disturb the absorption and metabolism of vitamin D: some cholesterol-lowering remedies, gastric acid inhibitors, and cortisone drugs. Some diuretics also interfere with the relation between calcium and vitamin D.

Also see
http://www.vitaviva.com/en/Health/Health_Subjects_Details.6.35.aspx