What is Vitamin D?

Vitamin D is a fat-soluble vitamin that is found rarely naturally occurring in foods, found more often added to foods, and found in supplements. Our body produces vitamin D when our skin is exposed to UV rays from the sun which trigger vitamin D synthesis. Vitamin D that is ingested or made in our skin is not biologically active until it undergoes two reactions in the body to convert it first to 25-hydroxyvitamin D (25-OH D) in the liver and then a second reaction in the kidney to the active form called 1,25-dihydroxyvitamin D (1,25-OH2D).

We test 25-OH D because this form of vitamin D is the best indicator of vitamin D status in the body. 1,25-OH2D has a very short life in the body and levels typically do not get low enough to detect a problem until deficiency is severe. 

Related: Vitamin D Test .