The physiology of vitamin D


When vitamin D comes into your body, it goes through all sorts of processes and is turned into different forms to be effective. In this article, we’ll teach you a little about this process, and how vitamin D works in the body.

Where does vitamin D come from?

Vitamin D₃ is made in your skin. When you expose your skin to ultraviolet B, something called “photolysis” happens of a chemical called 7-dehydrocholesterol, which turns it into previtamin D₃. This happens on the outermost layer of your skin, called the epidermis.

This previtamin D₃ is “rearranged” into vitamin D₃ and then “ejected” into what is called “extracellular fluid space.” From here, vitamin D₃ binds to a protein (binding protein) that is able to ship vitamin D and vitamin D related things all over your body. After you make vitamin D₃ in the skin, this binding protein ships it to your liver.

Vitamin D₃ can be taken as a supplement and is also sometimes found in small amounts in food, either naturally (fatty fish) or fortified foods. Vitamin D₂ can also be found as a supplement and in food, either in irradiated mushrooms or fortified foods. Vitamin D, when taken as food or supplement, is absorbed in your intestine. From here, vitamin D is collected by that binding protein and taken to the liver, too.

Now what does vitamin D do?

In the liver, vitamin D is turned into a chemical called calcidiol. The process it goes through is called “hydroxylation.” This chemical the liver makes is called a number of things, including:

  • calcidiol
  • 25(OH)D
  • 25-hydroxyvitamin D
  • vitamin D level
  • blood vitamin D
  • vitamin D serum concentration

We like to call it either 25(OH)D or “vitamin D level” because we think it’s easiest to remember and understand. Basically, it’s believed that this chemical that the liver turns vitamin D into is how your body stores its vitamin D and provides vitamin D for your body.

When your doctor measures your vitamin D, this is what he is measuring, 25(OH)D, or sometimes he/she will just call it your vitamin D level.

At this point, this new form of vitamin D [25(OH)D] is still not really doing anything in your body. It’s simply making itself available for tissues all over the body. What do your tissues do with this 25(OH)D?

They take 25(OH)D and turn it into a chemical called 1,25(OH)₂D. This process is also called “hydroxylation.” This chemical is called a number of things, including:

  • Calcitriol
  • 1,25-dihydroxyvitamin D
  • 1,25(OH)₂D
  • Activated vitamin D

We like to call it either 1,25(OH)₂D or “activated vitamin D” because we think these terms are easiest to remember and understand. When vitamin D is in this final form, 1,25(OH)₂D, it’s ready to perform and help your body out, hence its other name “activated vitamin D.”

When your kidney turns 25(OH)D into 1,25(OH)₂D, it circulates this 1,25(OH)₂D all around your blood, helping your body maintain the right calcium balance in your blood. This is really important for your daily survival, and your body is just about never out of 1,25(OH)₂D. Sometimes doctors and scientists call this vitamin D’s “endocrine function.”

When other tissues in your body turn 25(OH)D into 1,25(OH)₂D, they make this activated vitamin D inside and around cells. Tissues like you brain, skin, breast, colon, cardiac muscle, immune cells, and more all make 1,25(OH)₂D with 25(OH)D. Sometimes doctors and scientists call this vitamin D’s “autocrine function.”

Sometimes doctors mistakenly measure 1,25(OH)₂D instead of 25(OH)D, because they think since it’s “activated” it must be the right kind of vitamin D to measure in your body. The problem with this, though, is that when you measure 1,25(OH)₂D, you’re likely only measuring how much 1,25(OH)₂D the kidney produced, and not how much the rest of your body is making. And how much your kidney produces is dependent on a lot of things, like how much calcium and PTH you have in your blood at that moment, and not how much vitamin D you have stored.

How does “activate vitamin D” work?

The last key piece to the vitamin D system is what is called the “vitamin D receptor” (VDR). When 1,25(OH)₂D is made, it binds to VDRs all over your body. These bounded VDRs can now tell local genes how to express themselves.

When genes express themselves, they are expressed in proteins and enzymes. These proteins and enzymes tell the body what to do, how to express itself and how to function. When you get enough vitamin D, this is a smooth process and allows cells all over your body to communicate properly. When you don’t get enough vitamin D, this process is broken down a little bit, and cells may not communicate quite as well. How this affects diseases and conditions, researchers are still trying to figure out.

How does vitamin D work to make sure you have the right calcium balance?

Vitamin D plays an important role in making sure your body and blood have the right amount of calcium. Depending on how much calcium you have in your blood, glands called the parathyroid will tell your kidney to either produce or not to produce more 1,25(OH)₂D.

When you do produce more 1,25(OH)₂D, this helps you absorb more calcium in your gut from your diet. 1,25(OH)₂D and your parathyroid glands also work together to pull calcium from your bones if needed.

Vitamin D is important for bone health because it ensures your body has all the vitamin D it needs to produce 1,25(OH)₂D if need be. Without enough vitamin D, your body might not produce as much 1,25(OH)₂D and have a more difficult time absorbing calcium from your diet. Without getting adequate calcium in your diet and being able to adequately absorb it, you can’t have good bone health.

See our Parathyroid glands and vitamin D page for more information on how vitamin D works with calcium and parathyroid.

How does the vitamin D system work outside of bone health?

Vitamin D’s function outside of calcium balance is a relatively new discovery, and it is what has doctors and scientists enthusiastic about vitamin D. As mentioned above, tissues all over your body produce 1,25(OH)₂D, tissues including the skin, immune system, breast, colon, cardiovascular system, lung, pancreas, brain, teeth, stomach, spinal cord and more. How they use this 1,25(OH)₂D, researchers are still trying to figure out. Getting enough vitamin D, however, ensures that tissues have all the vitamin D they need to produce as much 1,25(OH)₂D as they want and need.

See our Health conditions section for more information on conditions vitamin D plays a role in, including both bone health and non-bone health conditions. In each condition, we explain how vitamin D may act and play a role in that condition.

  1. Jones G and Prosser DE. The Activating Enzymes of Vitamin D Metabolism (25- and 1α-Hydroxylases). In Vitamin D (Third Edition) by Feldman D, Pike JW and Adams JS, 2011.
  2. Plum LA and DeLuca HF. The Functional Metabolism and Molecular Biology of Vitamin D Action. In Vitamin D: Physiology, Molecular Biology and Clinical Applications (Second Edition), by Holick MF, 2010.
  3. Pike JS, Meyer MB and Lee SM. The Vitamin D Receptor : Biochemical, Molecular, Biological, and Genomic Era Investigations. In Vitamin D (Third Edition) by Feldman D, Pike JW and Adams JS, 2011.