The Basics of Vitamin D

Types, Conversion and Role of Vitamin D

The Basics of Vitamin D

Types, Conversion and Role of Vitamin D

Vitamin D2 versus Vitamin D3

The term vitamin D is usually used to refer to vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol), although there are vitamins D1 through D5 and synthetic analogues made by the drug industry. Vitamin D2 largely works the same as vitamin D3 in the body but is obtained from food whereas vitamin D3 is made in and on the skin. Both forms are found in dietary supplements, although there has been a trend to use vitamin D3 because of its (at least perceived) better absorption by the body. We continue this discussion for vitamin D3.

Conversion of Vitamin D to Activated Vitamin D

Vitamin D is cholecalciferol. It is made in and on the skin from a precursor called 7-dehydrocholesterol under the illumination of UVB light. It is also contained in vitamin D supplements. It is not a vitamin in the true sense of the word, as it can be made in the body whereas a true vitamin cannot. Vitamin D is in fact a pro-hormone, which means that it is a molecule that converts into a hormone in the body.

There are two stages in the conversion of vitamin D to that hormone. First, vitamin D is converted to 25(OH)D, also called calcidiol. In a blood test, the level of vitamin D is measured as the level of 25(OH)D in blood plasma. Second, 25(OH)D is converted to activated vitamin D, the hormone, also called calcitriol. The first conversion stage largely takes place in the liver and the second stage in the kidneys. However, both stages of conversion occur in many other tissues, presumably where high concentrations of activated vitamin D are needed.

We often refer to the activated vitamin D (i.e., the hormone) as simply vitamin D (which strictly speaking is the pro-homone).

The Role of (Activated) Vitamin D as a Hormone

A hormone is a molecule that signals the body to do something, such as carry out a physiological function. Vitamin D is crucial to health because the body does not know what to do if there is insufficient vitamin D to act as the messenger signal. This explains why low levels of vitamin D are associated with higher risks of dozens of diseases, including all the big diseases -- cancer, cardiovascular disease, diabetes, obesity, and infertility. Vitamin D acts through a specific receptor (aptly named the vitamin D receptor) that is present in virtually every cell in the body. In this way, vitamin D turns thousands of genes on and off as required. These genes synthesize hundreds of types of protein that control essential physiological functions, such as the production of insulin for the pancreas, the production of enzymes that make neurotransmitters, functions of the immune system that fight disease, and the appropriate killing off of cells so that the cells do not become cancerous.