"This essential nutrient is called a vitamin, but dietary Vitamin D is actually a precursor hormone—the building block of a powerful steroid hormone in your body called calcitriol. It has been known for years that vitamin D is critical to the health of our bones and teeth, but a deeper insight into D's wider role in our health is quite new.

"Vitamin D works in concert with other nutrients and hormones. . . . It supports key mineral absorption and metabolism (especially calcium and phosphorus in the blood and bones). It regulates normal cell differentiation and proliferation (e.g., prevention of cancer). It promotes insulin sensitivity and blood sugar regulation (insulin secretion). . . . Studies show clear links between Vitamin D deficiency and obesity, insulin resistance, heart disease, certain cancers and depression.