Vitamin D is a group of fat-soluble substances (calciferols) of which the most important are D2 and D3.The liver and kidneys convert vitamin D to the active form called calcitriol that is used to regulate many processes in the body.

The best natural source of vitamin D (D3) is fat fish. The skin also makes vitamin D3 with the help of energy from sun light.

Vitamin D promotes the uptake of calcium from the intestines and the re-uptake of calcium from the kidneys. It stimulates the white blood cells to catch foreign or dangerous elements. It also regulates (mostly stimulates) the production and maturing of new cells in many tissues.

Lack of vitamin D is caused by poor diet, by lack of sun exposure and by certain conditions that hinders the uptake of vitamin D in the digestive tract.

Vitamin D deficiency causes diseases that gives deformed bones, low calcium content in the bones and brittle bones, like rickets in children and osteomalacia or osteoporosis.in adult persons.

Growing evidence show that too low intake of vitamin D contributes to circulatory problems like hypertension, narrowing of blood vessels, heart failure and stoke. Persons with diabetes seem to get the most increased risk by lack of vitamin D. Vitamin D deficiency also seems to increase the risk of getting cancer, especially colo-rectal, breast and prostate cancer.

Low intake of vitamin D gives poor immune function and can make a person susceptible to infections of various kind or to autoimmune diseases like diabetes type1, and also to diabetes type 2.

Low levels of vitamin D in the body probably also contributes to increased aging signs on the cellular levels and age-realted diseases like diabetes, Alzheimer's disease Parkinson's disease and peripheral neuropathy.

Low levels of calcium in the body also seems to contribute to the risk of getting many of the listed ailments, especially the bone disorders and cancer.

A commonly recommended daily intake of vitamin D is 200-600 IU (international units)., with the highest doses for older people. According to growing evidence, this seems to be too little and up to 2000 IU may be good for the health. Obese persons seem to require more vitamin D than normally weighted individuals.

However, a too high daily intake of vitamin D is toxic and a much higher intake that 2000 IU may give risk for poisoning.

Fat fish and fish liver oil are the best natural sources of vitamin D3. Another good natural way to get vitamin D3 is by exposing great skin areas for sunlight some time every week. Some food products are often fortified in vitamin D, especially milk and juice. In the United States fortified products are probably the most important supply of this vitamin for many people.

Food from plant sources contain little vitamin D, but fatty plant products contain some vitamin D2. People that do not eat fish often, do not expose their skin to sunlight or do not consume fortified products have a risk of getting too little vitamin D.

If it is difficult to get enough vitamin D by the food alone or by the amount of sun exposure, a supplement can be useful, for example supplements based on fish liver as a source. ØIf the diet is low in calcium or if the purpose is to improve bone integrity, it will usually be wise to take a supplement of calcium together with vitamin D. A too high intake of both calcium and vitamin D can however result in unwanted calcium deposits in the body.