Vitamin D is a necessary vitamin for the absorption of our bodies of minerals such as calcium, iron, magnesium, phosphate and zinc. Thus, your body is able to utilize these minerals to keep your body healthy and working properly. This is why some milk includes added vitamin D, which is added to enhance calcium absorption in the body. I hope this explanation was useful!

We use it to control the amount of calcium dissolved in our blood. We need dissolved calcium in order for our nerves and our cells to work, but too much would be poisonous.

The most interesting part of vitamins is that they are needed and also cannot be produced by your body on its own. This means that without an external source of vitamin D (including the sun), you couldn't make it. Vitamin D specifically is needed for healthy bones, since it plays an important role in how your body uses calcium. There are quite a few diseases (rickets, stunted growth, etc.) if one doesn't get enough vitamin D. In general, the best way to get vitamin D is from the sun: getting a little bit of UV rays from the sun allows your skin to make some vitamin D on it's own, and helps keep you healthy.