Vitamin D serves as a hormone in the human body. Supplied by food in two different forms (ergocalciferol or vitamin D2, in plant products, and cholecalciferol or vitamin D3,
in products of animal origin), it is also synthesised in the human body through exposure to UVB rays (vitamin D3). The liver and then the kidneys convert vitamin D into its active form. Vitamin D contributes to normal immune system function.
To act, vitamin D binds to specific receptors – which are also found in immune cells – that can metabolise it. Numerous studies have therefore shown a connection between low levels of vitamin D and increased sensitivity to aggressive external factors and immune disorders. Vitamin D works in two ways: it improves immune responses while regulating them.
The other main role of vitamin D is maintaining calcium homoeostasis: it stimulates the absorption of calcium in the intestines and its reabsorption in the kidneys and regulates the level of calcium in the blood by controlling calcium exchanges between blood and bone. By binding calcium to bones, it keeps them in good health.
Vitamin D is involved in the mineralisation of bones, cartilage and teeth and helps maintain normal muscle function. Calcium is essential to maintain good bone health.
Several studies have shown vitamin D deficiency for many population groups.
It is estimated that one billion people worldwide are vitamin D deficient; in Western countries, such a deficiency affects more than 40% of the population over the age of 50*.
In Europe, a study even showed that 80% of elderly people had a below-normal level**.
* Holick MF. Vitamin D deficiency. N Engl J Med 2007;357:266-81.
**Van Der Wielen RP, Löwik MR, van Den Berg H, et al. Serum vitamin D concentrations among elderly people in Europe. Lancet 1995;346:207-10.