How does cholesterol turn into vitamin D?

December 28, 2025 •

4 min read

The human body is capable of producing its own vitamin D, but it can only do so by first converting a specific precursor found in cholesterol. This remarkable process is a photochemical reaction that hinges on exposure to ultraviolet B (UVB) radiation from sunlight.

Quick Summary

This article details the multi-step biochemical process by which a cholesterol precursor in the skin is converted to vitamin D3 upon exposure to UVB light. It explains the subsequent transformations in the liver and kidneys that produce the active hormonal form of vitamin D.

Key Points

UVB is the Catalyst: Sunlight's ultraviolet B (UVB) rays initiate the conversion of a cholesterol precursor into previtamin D3 in the skin.
Precursor is 7-DHC: The specific cholesterol intermediate used for vitamin D synthesis is 7-dehydrocholesterol (7-DHC).
Thermal Rearrangement: After the initial photochemical reaction, body heat spontaneously converts previtamin D3 into cholecalciferol (vitamin D3).
Liver and Kidneys Activate: The inactive D3 must undergo two subsequent hydroxylation steps—first in the liver, then in the kidneys—to become the active hormone, calcitriol.
Melanin Affects Conversion: Darker skin pigmentation can reduce the amount of UVB radiation reaching the 7-DHC in the skin, lowering vitamin D synthesis.
Cholesterol and D3 Production are Balanced: The body regulates its cholesterol levels, and high cholesterol can downregulate its own production, potentially leaving more precursor available for vitamin D synthesis under proper conditions.

The Photochemical Spark: 7-Dehydrocholesterol and UVB

At the heart of vitamin D synthesis lies a cholesterol precursor molecule known as 7-dehydrocholesterol (7-DHC). This compound is an intermediate in the complex metabolic pathway that produces cholesterol within the body's cells. Unlike cholesterol itself, 7-DHC resides in the plasma membranes of skin cells, specifically in the epidermal layers. When your skin is exposed to UVB radiation from the sun, the energy from these light waves strikes the 7-DHC molecule, breaking a chemical bond in one of its rings. This initial, non-enzymatic reaction transforms 7-DHC into a new molecule called previtamin D3. This photochemical event is the essential first step that distinguishes vitamin D synthesis from the standard cholesterol pathway, which otherwise uses the same precursor.

The Role of Sunlight's Wavelengths

It is critical to note that only a specific range of UV light is effective for this conversion. Wavelengths between 290 and 320 nanometers, which fall under the UVB category, are the most efficient catalysts for the reaction. This is why sun exposure during peak daylight hours is most productive for vitamin D synthesis, while exposure during other times, or to UVA radiation, is less effective. The amount of vitamin D produced in the skin is directly influenced by the intensity and quality of the UVB radiation that reaches the deeper layers of the epidermis where 7-DHC is abundant.

The Thermal Isomerization to Vitamin D3

Once previtamin D3 is formed, it does not remain in this state for long. The heat within the body causes the molecule to undergo a spontaneous thermal isomerization, rearranging its chemical structure. This process does not require any additional energy from light. The result is the formation of vitamin D3, also known as cholecalciferol. This thermally driven step ensures that previtamin D3 is efficiently converted into a form that can be used by the body, although continued, prolonged sun exposure can also cause previtamin D3 and vitamin D3 to be broken down into other inactive photoproducts, preventing toxic levels from accumulating.

The Journey from Inactive to Active Hormone

Cholecalciferol (vitamin D3) is the form that is produced in the skin, but it is not yet biologically active. It must undergo further processing in two different organs to become the potent hormone that regulates calcium metabolism.

The Activation Pathway:

After its creation in the skin, vitamin D3 enters the bloodstream. It is then transported to the liver, where it undergoes its first metabolic transformation.
In the liver, the enzyme 25-hydroxylase adds a hydroxyl group to the 25th carbon position of the cholecalciferol molecule.
This hydroxylation results in the formation of calcidiol, also known as 25-hydroxyvitamin D [25(OH)D]. Calcidiol is the primary circulating form of vitamin D in the body and is what is measured to determine a person's vitamin D status.
Calcidiol is then transported from the liver to the kidneys.
In the kidneys, the enzyme 1-alpha-hydroxylase performs the final hydroxylation, adding a hydroxyl group to the 1-alpha position.
This final step creates calcitriol, or 1,25-dihydroxyvitamin D [1,25(OH)2D], which is the fully active, hormonal form of vitamin D.

Comparing the Cholesterol and Vitamin D Pathways

While they share a precursor, the ultimate fate of 7-DHC differs significantly based on the presence of UV light.


Feature	Cholesterol Synthesis	Vitamin D Synthesis
Initial Precursor	7-dehydrocholesterol	7-dehydrocholesterol
Catalyst	Enzyme DHCR7	UVB Radiation
Initial Product	Cholesterol	Previtamin D3
Primary Location	Liver (enzymatic)	Skin (photochemical)
Key Intermediates	Desmosterol, Lathosterol	Previtamin D3, Calcidiol
Final Active Form	Used as a structural component for membranes and hormones	Calcitriol, a hormone regulating calcium

Factors Influencing Vitamin D Synthesis from Cholesterol

Several factors can affect how efficiently your body turns cholesterol into vitamin D. The amount of 7-DHC available in the skin, as well as the intensity of UVB radiation, play crucial roles.

Skin Pigmentation: Individuals with darker skin have more melanin, which acts as a natural sunscreen and reduces the penetration of UVB rays into the skin. This can lead to lower vitamin D production and an increased risk of deficiency.
Latitude and Season: Geographic location and time of year significantly impact UVB radiation levels. People living at higher latitudes experience weaker sun exposure, especially during winter months, which limits vitamin D synthesis from cholesterol.
Age: The body's ability to produce vitamin D from 7-DHC declines with age. This is partly due to a decrease in 7-DHC levels in the skin and changes in skin morphology.
Sunscreen and Clothing: Sunscreen and protective clothing block the UVB rays necessary for the initial conversion step. While essential for skin cancer prevention, prolonged use can inhibit vitamin D synthesis.
Other Factors: Cloud cover, pollution, and the use of certain medications can also affect the amount of UVB light reaching the skin.

Conclusion: The Intricate Balance

Cholesterol serves as the essential raw material for vitamin D production, highlighting the interconnectedness of these vital biological processes. The conversion begins with a cholesterol precursor in the skin, 7-DHC, and requires the activation energy of UVB sunlight to initiate the transformation. Following the initial photochemical reaction and thermal isomerization, the resulting inactive vitamin D3 is sent on a journey through the liver and kidneys to become the active hormone, calcitriol. This sophisticated process is a testament to the body's ability to repurpose a fundamental molecule like cholesterol for multiple critical functions. Understanding this pathway is key to appreciating both the importance of moderate sun exposure and the factors that can influence a person's vitamin D status.

For more in-depth information on vitamin D metabolism, refer to the detailed pathway explanations provided by the National Institutes of Health.

Frequently Asked Questions

7-dehydrocholesterol (7-DHC) is the immediate precursor molecule for vitamin D3 production. When exposed to UVB light, 7-DHC undergoes a photolytic reaction in the skin that breaks a chemical bond, converting it first into previtamin D3 and then, via heat, to vitamin D3.

While statins block an early step in the shared cholesterol and vitamin D synthesis pathway, research suggests they have a negligible impact on overall vitamin D status. The body has complex feedback mechanisms that ensure vitamin D production continues.

Sunlight, specifically the UVB portion of the spectrum, provides the specific energy needed to break the chemical bond in the 7-dehydrocholesterol molecule. Without this photochemical reaction, the conversion cannot begin.

After vitamin D3 is synthesized in the skin, it is carried to the liver, where it is converted into calcidiol. This calcidiol is then sent to the kidneys for a second conversion into the active hormone, calcitriol.

Not directly. While high cholesterol may lead to a buildup of the 7-DHC precursor, vitamin D production is still dependent on adequate UVB exposure. One study showed that the body has sophisticated feedback loops where high cholesterol can actually suppress its own synthesis, potentially increasing the 7-DHC pool available for vitamin D production if UV light is present.

Melanin, the pigment that gives skin its color, acts as a natural protective filter against UV radiation. Higher levels of melanin in darker skin block more of the UVB rays needed to convert 7-DHC into previtamin D3, leading to less synthesis.

No, it is not possible to get toxic levels of vitamin D from prolonged sun exposure. The body has a built-in safety mechanism where excess previtamin D3 and vitamin D3 are broken down into inactive forms by the sun's radiation itself.