Vitamin D is a unique vitamin. It actually functions as a steroid hormone in the body.
The skin makes Vitamin D when it is exposed to ultraviolet rays from the sun.
This is how we got most of our daily requirement throughout evolution.
However, today a large part of the world is deficient in this critical nutrient.
In many places, the sun simply isn’t available throughout most of the year.
Even where there is sun, people tend to stay inside a lot and use sunscreen when they go out, but sunscreen effectively blocks Vitamin D generation in the skin.
If you’re Vitamin D deficient, then you’re actually deficient in a major hormone in the body, and a deficiency is associated with many serious diseases, including diabetes, cancer, osteoporosis and others
The best way to know is to see a doctor and have your blood levels measured.