☀️ Vitamin D Metabolism: From Sunlight to the Cell and the VDR Receptor
How vitamin D turns from a "sunshine" vitamin into an active hormone and works in cells
Vitamin D is a unique vitamin that the body can synthesize under the influence of sunlight. But for it to "work", it must go through several activation steps in the liver and kidneys, and then interact with a receptor in cells (VDR).
Understanding vitamin D metabolism helps explain why its status matters and how to assess the level correctly. Let's look at the path of vitamin D from sunlight to the cell.
What is vitamin D
Vitamin D is a group of compounds, the main ones being vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). D3 is synthesized in the skin under sunlight; D2 comes from some plants.
The activation path
Vitamin D goes through several steps to become active.
Synthesis or intake
D3 forms in the skin from cholesterol under UVB or comes from food/supplements.
Liver
Converted to 25(OH)D — the main storage status marker.
Kidneys
Converted to 1,25(OH)₂D (calcitriol) — the active form.
The role of the liver and kidneys
Two organs are critical for vitamin D activation.
25(OH)D
Converts vitamin D into the main storage form 25(OH)D.
Calcitriol
Convert 25(OH)D into active 1,25(OH)₂D (calcitriol).
The VDR receptor and work in the cell
The active form of vitamin D (calcitriol) acts through the vitamin D receptor (VDR), present in many cells of the body.
The 25(OH)D marker
To assess vitamin D status, 25(OH)D — the storage form in blood — is measured.
| 25(OH)D level | Interpretation |
|---|---|
| < 30 nmol/L | Deficiency |
| 30–50 nmol/L | Insufficiency |
| 50–75 nmol/L | Adequate (for many) |
| > 125 nmol/L | Possible excess |
Sources and norms
Vitamin D comes from the sun, food and supplements.
| Source | Description |
|---|---|
| Sun (UVB) | D3 synthesis in the skin |
| Fatty fish | Salmon, mackerel, sardines |
| Eggs | The yolk contains a small amount |
| Supplements | D3 (cholecalciferol) or D2 |
| Indicator | Value |
|---|---|
| Norm/day (adults) | ~600–800 IU |
| With deficiency | Higher doses as prescribed |
What matters
How to sensibly approach vitamin D.
Check your status
The 25(OH)D test shows whether there is a deficiency or sufficiency.
Correct by the test
The dose depends on the level. Large doses — only as prescribed by a doctor.
Consider the kidneys and liver
With their diseases, vitamin D activation can be impaired.
Watch your nutrition
Fatty fish and supplements are sources, especially in the low-sun season.
How is vitamin D activated?
First in the liver to 25(OH)D, then in the kidneys to the active form calcitriol.
What is the VDR receptor?
The vitamin D receptor through which the active form acts on genes in many cells.
Why measure 25(OH)D?
25(OH)D is stable and reflects the total vitamin D reserve in the body.
What is the liver's role?
The liver converts vitamin D into the storage form 25(OH)D.
What is the kidneys' role?
The kidneys convert 25(OH)D into the active form calcitriol.
What is calcitriol?
The active form of vitamin D (1,25(OH)₂D), acting through the VDR receptor.
What 25(OH)D level is normal?
For many, 50–75 nmol/L is considered adequate. Lower — deficiency or insufficiency.
Is vitamin D overdose dangerous?
Yes, overdose is toxic. Take it as prescribed and by test.
How much vitamin D per day?
About 600–800 IU for adults. With deficiency, higher doses — as prescribed.
Do kidney diseases affect vitamin D?
Yes, with kidney disease vitamin D activation can be impaired.