Vitamins

Calcifediol

[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](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 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.