Vitamin D (Calciferol) refers to several related fat-soluble vitamin variants, all of which are sterol (cholesterol-like) substances. D2, or activated ergo-calciferol, is the major synthetic form of provitamin D; D3, or cholecalciferol, is found in animals, mainly in fish liver oils. These are converted in the liver and kidneys to 25-hydroxycholecalciferol, and 1, 25-dihydroxylcholecalciferol, the major circulating active forms of vitamin D.

Vitamin D is also known as the "sunshine" vitamin because it is actually manufactured in the human skin when in contact with the ultraviolet light in the sun¹s rays. The sunlight interacts with 7-dehydrocholesterol to form cholecalciferol, which is then transferred to the liver or kidneys and converted to active vitamin D. Wintertime, clouds, smog, and darkly pigmented skin reduce the body¹s production of the "sunshine" vitamin.

Vitamin D absorption takes place in the upper part of the small intestine with the aid of bile salts. Vitamin D is stored in the liver where it is metabolized into calcidiol. Further metabolic reactions take place in the kidneys where it is converted to calcitriol and then absorbed into the blood where it is stored in fat and muscle. Calcitriol is a hormone that helps regulate the body's calcium needs.

Vitamin D helps with increasing the absorption of calcium, assists in bone growth and the integrity of bone and promotes strong teeth. It also helps regulate the amount of phosphorus in the body as well as assisting in a healthy heart and nervous system. In some recent studies it has also shown great promise in assisting psoriasis, the immune system, thyroid function, as well as normal blood clotting.

A shortage can lead to softening of the bones and muscle twitching and convulsions, and in children it causes rickets, resulting in bowed legs. In adults, the shortage causes loss of minerals from the bones, (osteomalacia) where the bones are sore, tender, and weak muscles with the possibility of deafness developing. In older people, osteoporosis may appear when protein is also lost from the bone. Vitamin D in short supply is also linked to having a burning sensation in the mouth and throat, diarrhea, insomnia and visual problems.

Some clinical guidelines for toxicity are sometimes set at 5,000 to 10,000 iu per day to cause toxicity, but other researchers place the value much higher to reach toxicity. You are however advised to keep your supplement intake to no more than 600 iu per day. Having too much vitamin D in your system could leave a too elevated calcium level, a lower appetite, increased thirst, nausea, vomiting, drowsiness, abdominal pain. A long-term effect of too much vitamin D is the deposit of calcium in soft tissues of the body including the blood vessel walls and kidneys where it can cause serious damage.

When you are very seldom exposed to sunlight, or if you always wear sunscreens with a SPF factor higher than 8, you might need extra vitamin D. This is also the case if you are on a strict vegan diet. Older people are also advised to check their level of vitamin D. People with compromised kidneys or liver are at risk of too little of this vitamin, since the kidneys and liver are required to activate this vitamin in processes taking place in those organs.

Vitamin D is present in fatty fish like kipper, sardines, salmon, tuna and mackerel, liver, egg yolk and butter. Smaller amounts are also present in dark leafy vegetables.