Vitamin D is completely different from other forms of vitamins. Vitamin D is actually a hormone (rather steroid hormone) that is produced from cholesterol when the skin is exposed to the Sun. This is the reason why vitamin D is sometimes referred to as the sunshine vitamin. It is absolutely essential for optimal health. These days, we do not expose ourselves to sunlight for the adequate duration. So it becomes necessary for us to get more vitamin D from our diet or from supplements.
Let us begin with a simple question;
What is vitamin D?
Vitamin D is a fat-soluble vitamin, just like vitamins A, D, E and K. This means that it dissolves in fat or oil. Vitamin D can be stored in the body for a long time. There are two main forms of vitamin D found in our diet:

• Vitamin D3 (cholecalciferol): This is found in foods such as fishes that are high in omega-3 content and egg yolks.
• Vitamin D2 (ergocalciferol): Found generally in some mushrooms.

Of the above two, D3 (cholecalciferol) is almost twice as effective as the D2 form in increasing the levels of vitamin D in the blood stream.

• What does vitamin D do to the body?
  Before breaking down, vitamins are pretty useless. They need to go through two conversion steps before they are activated. Vitamin D is first converted to calcidiol in the liver. This is the storage form of the vitamin in the body. Second, calcidiol from the liver is then converted to calcitriol, mostly in the kidneys. Calcitriol is the active steroid hormone form of vitamin D. Calcidiol converted to calcitriol travels through the entire body. It is then absorbed in the nuclei of the cells. There, it interacts with a receptor called the vitamin D receptor (VDR), found in almost every single cell of the body. When the active form of vitamin D binds to this receptor, it turns genes on or off leading to changes in the cells. This is similar to how most other steroid hormones work. It is well known fact that vitamin D affects various cells related to bone health, for example telling the cells in the gut to absorb calcium and phosphorus. Latest findings indicate that vitamin D is involved in other processes as well. This includes helping in the functioning of the immune system and even prevention of cancer.

So people who have vitamin D deficiency will have calcitriol (the steroid hormone form) deficiency as well. Thus, they have a deficiency of one of the critical hormones of the body.