Vitamin D is not really a vitamin at all – it’s really a hormone. It’s made in the skin and is actually a hormone precursor – meaning our body changes it into a hormone that then has some profound effects on our body.
The most well known effect of vitamin D is the ability to increase the absorption of calcium and improve the health of our bones. Vitamin D has many other effects on lots of cells in our body. Immune cells, for instance, are sensitive to vitamin D and a deficiency in vitamin D is associated with increased autoimmune disease.
Most adults in Australia only get 5-10% of the vitamin D they need from food (fish and eggs being the main source). In Australia we are also sun safe (for good reason – we are the skin cancer capital of the world). Our skin needs UVB to make vitamin D and the wavelength required is when the sun is overhead meaning the easiest time to get burnt.
Vitamin D was discovered in the early 1920s, a few years after the discovery of vitamins A, vitamin B and vitamin C.
Over-the-counter vitamin D is 1000IU, which sounds like a lot but is actually only 25ug (where 1 ug is 1000 times smaller than a mg). Prescription vitamin D is far stronger.