The name 'vitamin' is actually a misnomer when it comes to vitamin D. Rather than a vitamin this nutrient is actual the precursor to the body's most potent steroid hormone, often referred to as 'activated vitamin D'.
Vitamin D can either be obtained from the diet in the form of cholecalciferol (vitamin D3), from animal sources, or ergocalciferol (vitamin D2), from plant foods, or it can be made in the skin when it is exposed to the ultraviolet (UV) radiation from the sun's rays.
Vitamin D from all sources is metabolised by the liver, kidneys and other tissues to its activated form.
The most well known function of vitamin D is its role in maintaining blood levels of calcium through mechanisms including controlling the rate of absorption of this mineral from foods. As such vitamin D, along with calcium, is known to be important in the maintenance of bone mass and the prevention of diseases such as osteoporosis.
More recent research however has revealed that vitamin D has a whole host of other important functions within the body and there are no doubt many more yet to be discovered. Some of these functions include:
Immune System Regulation - Vitamin D is known to stimulate the innate immune system; the part of the immune system which is non-specific and responds first to infection. This part of the immune system includes neutrophils, by far the largest sub-type of white blood cell, and the first on the scene when bacteria and other pathogens invade the body. Vitamin D has also been found to play a role in suppressing other portions of the immune system that may lead to the development of autoimmune disorders such as rheumatoid arthritis and systemic lupus erythamatosus (SLE).
Cancer Prevention - Connected to its role in immune system regulation is the fact that vitamin D appears to help prevent certain forms of cancer. Since activated vitamin D is a potent steroid hormone it is involved in turning genes on and off. Some of these genes are involved in processes that control cell growth and replication. If the wrong genes are overly expressed cancer may develop. It appears that vitamin D regulates gene expression so that cellular growth and replication occurs in a healthy way and cancerous cells do not develop. Forms of cancer that vitamin D has been shown to be protective against include breast, ovarian, prostate, and colon cancers.
Mood Regulation - Vitamin D activates the enzyme that leads to the production of the catecholamine neurotransmitters (dopamine, noradrenaline etc) that improve mood, motivation, focus and attention. Vitamin D also increases levels of serotonin, the "happy" neurotransmitter associated with feelings of contentment. Studies have shown that vitamin D levels are lower in depressed patients than in healthy individuals, that it improves mood even in healthy individuals, and in high doses is a more effective treatment for seasonal affective disorder (SAD) than bright light therapy.
Autism - Although studies are needed to confirm any link between vitamin D levels of either mothers during pregnancy or children in their early years and the development of autism, the Vitamin D Council postulates that such a link can account for many of the epidemiological trends seen in the disorder.