This vitamin has recieved a lot of attention lately. The recommendations we have for it today are probably too low and especially here in Sweden and other northern countries.
There are different types of this vitamin. We can only get D2 from the food. The D-vitamin we get from the sun (and food) is called D3. The vitamin is a prohormone; which mean that it will turn into a hormone in the body. The hormone is called calcitriol and it participates in the regulation of the calcium and phosphate levels in our body - which is obviously very important for our bones.
Due to the phosphate regulation, Vitamin D also has an indirect effect on the blood sugar. And ofcourse many other things.
You can find vitamin D in oily fish and some lean fish, egg yolk and in sweden they add vitamin D to the milk products with lower fat. That is because this is a fat-soluble vitamin so if they take away the fat, the already existing D-vitamin will also go away, so they need to add what they lost.
You never get enough through the food though and it's important to be in the sun so your body can store what it needs. If you shower and wash yourself with soap before sunbathing, then you have taken away some of the fat on your skin which means that you won't get as much vitamin D as if you didn't.
They are coming with new recommendations next summer so it will be very interesting to see if they will change the recommendations or not. It seems like a lot of doctors think they should higher it and especielly here in Sweden where we don't get enough sun during the fall/winter/spring. I personally take supplements of D3 during the winter and I feel a big difference when I do that.