What does Vitamin D do?

The fat-soluble vitamin D’s most important role is maintaining blood levels of calcium, which it accomplishes by increasing absorption of calcium from food and reducing urinary calcium loss. Both effects keep calcium in the body and therefore spare the calcium that is stored in bones. When necessary, vitamin D transfers calcium from the bone into the bloodstream, which does not benefit bones. Although the overall effect of vitamin D on the bones is complicated, some vitamin D is necessary for healthy bones and teeth.

Vitamin D plays a role in immunity and blood cell formation and also helps cells "differentiate"—a process that may reduce the risk of cancer. From animal and human studies, researchers have hypothesized that vitamin D may protect people from multiple sclerosis,¹ autoimmune arthritis, and juvenile diabetes.²

Vitamin D is also needed for adequate blood levels of insulin.³ Vitamin D receptors have been found in the pancreas where insulin is made, and preliminary evidence suggests that supplementation may increase insulin secretion for some people with adult-onset (type 2) diabetes.⁴