What is Vitamin D?

Vitamin D is a fat-soluble vitamin that is rarely found naturally in foods. It is more commonly added to foods and widely available in supplement form. The body also produces vitamin D when the skin is exposed to UV rays from sunlight, which trigger its natural synthesis.

However, vitamin D obtained through sun exposure or diet is not biologically active right away. It first undergoes a conversion in the liver to become 25-hydroxyvitamin D (25-OH D). It then goes through a second conversion in the kidneys to become the active form, 1,25-dihydroxyvitamin D (1,25-OH2D).

We test 25-OH D because it is the best indicator of vitamin D status in the body. In contrast, 1,25-OH2D has a very short half-life and typically does not get low enough to detect a problem until deficiency is severe, making it less useful for early detection.