D is not really a true vitamin as it can be synthesized by the body.
The main way you get your vitamin D is through sunlight on the skin but
small amounts can be obtained via cold water oily fish and fish oil.
That is why fish should be a really important part of the diet in parts of
the world with short summers.
D has an important role in the absorption of calcium and phosphate and
therefore is important for maintaining healthy bones and teeth.
Lack of Vitamin D can also affect longevity as it appears to have a role
in heart health and the immune system. This nutrient highlights the fact
that it is important to get enough fresh air and sunshine. It can be valuable to take this supplement in the winter months but in the summer if the sun is out, take a walk in the sunshine and your body will make plenty.