Vitamin D deficiency is SO common. There’s research showing that approximately 35% of adults in the United States have vitamin D deficiency. Why does this matter? Well, this vitamin (or pro-hormone) plays a role in so many functions in our body. As most of us know, it’s critical for building bone, but it can also reduce inflammation, reduce occurrence of illness and may even reduce cancer cell growth. And although it’s such a simple deficiency to test for, most doctors aren’t doing adequate testing to ensure their patients aren’t deficient.