Everything about Vitamin D — from production and regulation to supplementation and testing.

Vitamin D

Vitamin D Supplementation

Vitamin D, often called the “sunshine hormone,” plays a vital role in overall health. Many people in the Western world are deficient in it, but before deciding to supplement, it’s essential to test your blood levels (Calcifediol). This can help you understand whether supplementation is necessary and how much you need to reach optimal levels.

Key considerations include your current blood levels, lifestyle, UVB exposure, skin tone, BMI, and health conditions. Vitamin D3 (Cholecalciferol) is the preferred supplement form, as it is more effective than Vitamin D2. Individuals with higher BMIs or certain health conditions may require higher doses or specialized forms like Calcifediol or Calcitriol.

Vitamin D

Calciol Production

Cholecalciferol Production

When exposed to sunlight, your skin produces [vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/). How much you create depends on your skin type, time in the sun, and how much skin is exposed. We can estimate this using two simple steps. First, we calculate how much [vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/) your skin makes per square centimeter per minute based on UV radiation and daylight hours. Then, we adjust that number for how long you’re outside, how much skin is exposed (like arms, legs, and face), and your skin type. For example, exposing 5,000 $cm^{2}$ of skin to the sun for 30 minutes will produce more [vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/) if done around noon when UV rays are strongest. Always avoid sunburn!

For more technical details, read on.

When our skin is exposed to UVB rays, e.g., during sun exposure, Cholecalciferol aka Calciol is produced from 7-Dehydrocholesterol. In a 1977 publication, PC Beadle estimated that about 160 IU Cholecalciferol could be produced at 40° Latitude when exposing 1 $cm^{2}$ of skin to the sun for the whole day. Dark skin would produce around 70 IU under the same conditions. This information is suboptimal to drive the decision of how to expose oneself to the sun.

Vitamin D

Ergocalciferol

Ergocalciferol (vitamin D2) is made by plants and is similar to Cholecalciferol (vitamin D3), but it’s less effective for humans. D3 is better at raising vitamin D levels and has stronger effects in the body, while D2 binds less efficiently to the vitamin D receptors.
Studies have shown that D3 is more effective at improving vitamin D stores and lowering parathyroid hormone. Though some medications use high doses of D2, newer research suggests D3 supplements are a better choice. Plants like mushrooms and algae produce D2 when exposed to UVB light, but D2 levels are much lower than D3 found in fatty fish, for example.
In summary, D2 is similar to D3 but provides less bang for the buck.

Vitamin D

Vitamin D Unit Converter

Vitamin D (Cholecalciferol) used to be measured in micrograms (mcg). At one point, international units were introduced (IU). Today, either or both IU and mcg are found on supplement labels. Sometimes, it is beneficial to know the amount in IU and the amount in mcg. Especially when you want to use the correct ratio of several vitamins simultaneously. An important example in this context would be the combination of vitamin D and vitamin K. While Cholecalciferol ensures that there is enough calcium in the blood, vitamin K is involved in bringing calcium to the correct location (bones). Calcification of soft tissues such as cartilage and arteries is definitely something no one wants. Therefore, both vitamins should be used together. How much of each is a discussion for another time. However, once you decide and settle on a ratio between vitamins D and K, you can use the below converter to ensure that you use the correct amounts.

Vitamin D

Vitamin D Regulation

Calcitriol, the active form of vitamin D, is a powerful hormone that needs tight regulation in the body. It acts like a “key” that fits into specific “locks” (receptors) to manage important functions like calcium and phosphate balance. Too much or too little can cause harm, so the body carefully controls how much is made.
The body converts stored vitamin D (Calcifediol) into Calcitriol in the kidneys, but if Calcitriol levels get too high, enzymes slow down its production and break it down. Calcium and phosphate levels in the blood also help regulate this process.
Low calcium triggers the production of more Calcitriol, while high calcium or phosphate slows it down. Additionally, low phosphate increases Calcitriol, while high phosphate decreases it. These processes work together to maintain balance, ensuring proper calcium and phosphate levels in the blood.
If vitamin D stores are low, the body can’t produce enough Calcitriol, which can lead to health problems.

Vitamin D

Calcitriol

[Vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/), also known as calcitriol in its active form, is produced in the kidneys and helps the body absorb calcium, among other important functions like supporting the immune system. It is made from another form of vitamin D (/vitamins/vitamin-d/calcifediol) and is carefully regulated to keep its levels in check. While calcitriol is the active form, it’s not a good indicator of overall vitamin D storage in the body. Instead, vitamin D levels in storage are better measured by checking calcifediol levels. Some diseases, like sarcoidosis, can cause too much active vitamin D, while others, like chronic kidney disease, can prevent enough from being made.

For more technical details, read on.

The name Calcitriol (aka vitamin D) refers to a hormone that is produced via hydroxylation of Calcifediol in the human liver. Calcitriol is typically bound by the D-binding protein (DBP) but has a lower affinity DBP than Calcifediol but higher than Cholecalciferol. Calcitriol is the active form of the hormone and facilitates calcium uptake among a myriad of other functions, such as immune system-related ones.

Vitamin D

Calciol

Cholecalciferol

[Vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/), also known as cholecalciferol (aka Calciol), is a hormone that our skin makes when exposed to sunlight, specifically UVB rays. For this to happen, we need enough of a molecule called 7-dehydrocholesterol in our skin. If we don't get enough sunlight, whether it's because we stay indoors, wear too much clothing, or use sunscreen, we can't produce enough vitamin D. When vitamin D enters our blood, it can either float freely or bind to a protein called DBP. Once in the bloodstream, vitamin D can be taken up by fat and muscle cells for storage. Our body can also convert vitamin D into different forms, like calcifediol and calcitriol, which are more active in the body. The names of these forms refer to the number of hydroxyl groups (OH) added to the molecule—one for vitamin D, two for calcifediol, and three for calcitriol. Studies measuring vitamin D levels in people have shown differences over time. For example, a 1984 study found lower levels in healthy subjects (around 3-5 ng/ml), while a 2013 study reported higher levels (around 37-45 ng/ml). These variations might be due to changes in vitamin D supplementation or other factors. However, these levels don’t necessarily have clear biological or medical significance.

For more technical details, read on.

Vitamin D

Calcifediol

[Vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/), also called calcifediol (the short-term storage form), is made in the liver from another form of vitamin D. It usually binds to a protein in the blood, which helps it stay in the body for weeks. If there's not enough of this protein, vitamin D can be broken down and removed from the body through the kidneys. When the body needs more of the active form of vitamin D (/vitamins/vitamin-d/calcitriol), especially for things like low calcium or immune system support, stored vitamin D is quickly converted. [Vitamin D](https://www.allmer.de/blog/vitamins/vitamin-d/vitamin-d/) levels in the blood can indicate if someone has a deficiency or not. If levels are too low (see table below), the body may struggle to make enough active vitamin D, leading to various diseases.

For more technical details, read on.

The name Calcifediol (aka vitamin D) refers to a hormone that is produced via hydroxylation of Cholecalciferol in the human liver. Calcifediol is typically bound to the D-binding protein (DBP) but can be unbound if there is not enough DBP. If unbound, it is typically degraded and excreted via the kidneys. When bound to DBP, Calcifediol is relatively stable and can circulate in the human blood for weeks. Unless it is converted to Calcitriol and used by downstream processes. If demand for Calcitriol is high, e.g., low calcium or challenges to the immune system, ‘stored’ Calcifediol is converted to Calcitriol quicker.

Vitamin D

Vitamin D Solubility

Vitamin D, in fact a hormone, is said to be soluble in fat. Therefore, it has been cautioned, that too large an intake or production of D3 or more precisely Cholecalciferol can be dangerous since it could be stored in fat. Solubility is not that easy, especially when considering biological systems. First of, anything is soluble in anything to a varying degree. When saying D3 is fat soluble, we also need to consider that there are three different molecules that we mean when we say D3 (see vitamin D for a disambiguation).

Vitamin D

Vitamin D, technically a hormone, is made in the skin when UVB rays convert a cholesterol compound into D3 (Cholecalciferol, or Calciol). Many people don’t make enough due to lack of sun exposure, so we rely on food (like fatty fish) or supplements. Plants provide D2 (Ergocalciferol), but D3 works better in our bodies.

D3 has three key forms:

Calciol: Stored in fat for long-term use; this is the form we produce or ingest.
Calcifediol: Formed in the liver for short-term storage, circulating in blood and commonly tested to check D3 levels.
Calcitriol: The active form, made in the kidneys, with a short lifespan but crucial for biological effects.

We’ll be using the terms Calciol → Calcifediol → Calcitriol throughout future discussions.