Vitamin D (Calciferol)

Vitamin D actually refers to a group of steroid molecules. Vitamin D is called the sunlight vitamin because the body produces it when the sun's ultraviolet B (UVB) rays strike the skin. It is the only vitamin the body manufactures naturally and is technically considered a hormone. Vitamin D is important for the proper absorption of calcium from food. It is vital for the control of the levels of calcium in the blood and also controls the rate at which the body excretes calcium in the urine. 
Health Benefits
Vitamin D and osteoporosis
Adequate amounts of vitamin D is necessary for preventing bone loss. Low levels of vitamin D and insufficient sunlight exposure are associated with osteoporosis. The body cannot absorb calcium from food or supplements without an adequate intake of vitamin D. After menopause, women are particularly at risk for developing this condition. Vitamin D  taken along with calcium plays a critical role in maintaining bone density. Vitamin D functions by increasing the uptake of calcium from the intestine through interaction with the parathyroid glands in controlling bone resabsorption and serum calcium levels. Vitamin D also increases reabsorption of phosphate by the kidney tubule, and may directly affect the osteoblast, the cell which forms bone. 
Vitamin D and cancer
Vitamin D's immunomodulatory abilities may also play a role in its anti-cancer activity. Vitamin D demonstrats a dose-dependent inhibition of cell proliferation in a number of cancer cell lines. It also has a pro-differentiation effect on these cells, resulting in potent anti-cancer activity in some preliminary work. vitamin D increases the potency of cytokines and enhances the phagocyte activity and antibody-dependent cytotoxicity of macrophages and that it boosts natural killer cell activity and helps regulate T cells, among other things. Vitamin D's analogues show significant experimental activity against colorectal, renal cell, breast and prostate cancers, among others. 
Vitamin D and autoimmune diseases
Maintaining sufficient vitamin D levels may help decrease the risk of several autoimmune diseases such as insulin-dependent diabetes mellitus, multiple sclerosis, and rheumatoid arthritis. A daily multi-vitamin supplement containing vitamin D may reduce the risk of developing multiple sclerosis. Low intakes of vitamin D may be linked to an increased risk of arthritis of the hip in older women. One recent study showed that taking 400 IU or more of vitamin D daily was effective in delaying or stopping the progression of osteoarthritis of the knees. Autoimmune responses are mediated by immune cells called T cells. The biologically active form of vitamin D can modulate T cell responses, such that the autoimmune responses are diminished.
Vitamin D and psoriasis
Vitamin D is sometimesused in the treatment for psoriasis. Because vitamin D and its analogues are potent antiproliferative agents for keratinocytes and stimulators of epidermal cell differentiation. Calcipotriol has been demonstrated to significantly improve psoriatic lesions in a number of double-blind, placebo-controlled trials. Because it plays a role in skin cell metabolism and growth, vitamin D may be helpful in treating the itching and flaking associated with this skin ailment.
Vitamin D and other bone disorders
Vitamin D protects against the preventable bone diseases rickets and osteomalacia (softening of the bones in adults caused by inability to properly deposit calcium). An adequate level of Vitamin D in the body is necessary to maintain strong bones and to help prevent fractures in older people. Vitamin D supplements are also used for people with genetic diseases that interfere with the metabolism of Vitamin D.
Functions
1) Absorption of Calcium & Phosphorus
2) Essential for normal growth  development.
Food Sources
1) Synthesis with sunlight (10 mins/day)
2) cod liver fish, halibut [type of flatfish], salmon, sardine, egg yolk 3) Fortified Vit. A products.
Deficiency
• Tetany ( abnormal muscle twitching and cramps
• Rickets (defective bones, retarded growth)
• Osteomalasia  ( softening of the bones)
• Toxicity
• Stone formation on kidney
• Demineralisation of the bone
• Polyuria 
• Weight Loss
• Hypercalcemia

Reference:
Basic nutrition by Ana Marie M. Somoray RND