Vitamin D is the substance that facilitates the absorption of calcium in the body and transports it into the bloodstream and bones. Thus, fragile or resistant bones are dependent on the amount of this vitamin in the body.
It is produced by the body (skin) naturally on exposure to sunlight, but can also be found in some foods such as tuna, salmon, cheese and egg yolk. Some other foods like milk and fruit juices could be fortified with vitamin D. Despite its presence in foods, the amount available is insufficient to meet the body's needs. While this may seem to be an issue, it is not, because the body is capable of producing in a very little time, sufficient amounts to make up what is missing in food.
When the skin is exposed to the ultraviolet rays (UV rays) from the early morning sunlight, a process called "photolysis" takes place, and this results in the production of vitamin D. It is no wonder this vitamin is called "sunshine vitamin".