Vitamin D

Vitamin D helps the body absorb calcium, and promotes the immune system.

There is increasing evidence that vitamin D deficiency may increase cancer risk. Several studies and meta-studies (studies of studies) support support the evidence that breast, prostate, and colon cancer rates are much higher in populations that receive little sunlight, such as northern lattitudes. This may support the need for increased supplementation, but the research is not yet conclusive. One study by the National Cancer Institute failed to find a connection.
Deficiency

Deficiency can be caused by insufficient sun exposure and insufficient intake through the diet. Elderly people tend to be low on vitamin D, as well as those in northern latitudes as well as anyone else who does not get much sunlight, the obese, and darker skinned individuals. Deficiency diseases can include rickets (childhood bone deformity), osteomalacia (thin bones in adults), and osteoporosis (fragile bones).

Other diseases that have a possible connection with vitamin D deficiency are high blood pressure, Multiple Sclerosis, and diabetes, as well as depression and seasonal affective disorder.

Recently, there has been some preliminary research suggesting a connection between vitamin D and Parkinson's, but the research is not yet conclusive. [link]
Overdose

Vitamin D stays in the body for about a month. Normally only occurs with supplementation, not sunlight exposure, and has almost always been because of accidental overdose. Overdose would require intake of more than 40,000 IU per day.

Symptoms of vitamin D toxicity are nausea and vomiting, and anorexia, followed by increased urination and nervousness and finally kidney failure. Toxicity can be reversed by stopping supplementation and calcium intake. Severe and long lasting effects are rare.
Supplement

The American Dietary Guidelines recommends 1,000 IU for adults that are at risk of deficiency: dark skin, elderly, or lack of sunlight.