Where is vitamin D produced? The journey from sunlight to active hormone

December 10, 2025 •

4 min read

Did you know that vitamin D is technically a prohormone, not a vitamin? This fat-soluble nutrient is uniquely produced by the body itself, starting with skin exposure to sunlight. The subsequent journey of where vitamin D is produced involves a multi-step process spanning multiple organs for full activation.

Quick Summary

Vitamin D is primarily synthesized in the skin using sunlight, then biochemically activated by the liver and kidneys into its potent hormonal form. Supplements and certain foods can also provide it.

Key Points

Skin Synthesis: The body primarily produces vitamin D3 in the epidermis when exposed to ultraviolet B (UVB) radiation from sunlight, which converts a cholesterol precursor called 7-dehydrocholesterol.
Liver Activation: The newly synthesized vitamin D3 is transported to the liver, where it is converted into 25-hydroxyvitamin D (calcidiol), the main circulating form measured in blood tests.
Kidney Activation: The final, active hormonal form of vitamin D, called calcitriol, is produced in the kidneys through a second hydroxylation process involving calcidiol.
Controlling Factors: Vitamin D synthesis from sunlight is influenced by latitude, season, skin pigmentation, age, sunscreen use, and clothing, making dietary and supplemental intake important for many.
Alternative Sources: Beyond sun exposure, vitamin D can be obtained from specific dietary sources like fatty fish and fortified foods, as well as from supplements, especially during periods of limited sun.

The First Step: Production in the Skin

The initial phase of vitamin D production occurs in the epidermis, the outer layer of your skin. This process is dependent on a specific type of ultraviolet light. Solar ultraviolet B (UVB) radiation, with wavelengths between 290 and 315 nm, strikes the skin.

The Role of 7-Dehydrocholesterol

Within the skin's cells, a cholesterol precursor called 7-dehydrocholesterol (7-DHC) is present. When UVB rays are absorbed by 7-DHC, a chemical reaction breaks one of its rings, converting it into an unstable molecule called pre-vitamin D3. The body's warmth then spontaneously converts this pre-vitamin D3 into vitamin D3 (cholecalciferol) over several hours.

The Activation Process: Liver and Kidneys

Once formed in the skin, vitamin D3 is biologically inactive and must be processed further. Regardless of whether it comes from sun exposure or diet, it is transported through the bloodstream to two key organs for activation.

The Role of the Liver

In the liver, an enzyme called 25-hydroxylase adds a hydroxyl group ($OH$) to the vitamin D3 molecule. This converts it into 25-hydroxyvitamin D ($25(OH)D$), also known as calcidiol. This is the major circulating form of vitamin D in the body and is what is typically measured in blood tests to determine a person's vitamin D status.

The Role of the Kidneys

The second and final step of activation occurs primarily in the kidneys. Here, another enzyme called 1-alpha-hydroxylase performs a second hydroxylation, converting calcidiol ($25(OH)D$) into 1,25-dihydroxyvitamin D ($1,25(OH)2D$). This final compound, also known as calcitriol, is the biologically active hormonal form of vitamin D that regulates calcium absorption and many other bodily functions.

Factors Influencing Vitamin D Synthesis

Many variables can affect how much vitamin D your body produces from sunlight alone.

Latitude and season: The sun's angle is a major factor. In regions farther from the equator, the winter months see little to no vitamin D synthesis because the sun's UVB rays are too weak to penetrate the atmosphere effectively.
Skin pigmentation: Melanin, the pigment that determines skin color, acts as a natural sunscreen. People with darker skin have more melanin and therefore require significantly more sun exposure to produce the same amount of vitamin D as a person with lighter skin.
Age: As people age, their skin's ability to produce vitamin D from sunlight decreases.
Sunscreen and clothing: Both sunscreen and clothing effectively block the UVB radiation necessary for vitamin D production.
Time of day: The intensity of UVB rays is highest around midday, making it the most efficient time for vitamin D synthesis.

Comparison of Vitamin D Sources: Sun, Food, and Supplements

While sun exposure is a primary source, dietary intake and supplementation are crucial, especially for those with limited sun exposure.


Source	Pros	Cons	Key Vitamin Form	Notes
Sunlight	Free, natural, self-regulating (doesn't cause toxicity).	Production is affected by latitude, season, skin tone, and age. Risk of skin cancer from overexposure.	Vitamin D3	Main natural source for most people.
Dietary Sources	Accessible year-round. Can be the primary source for people with limited sun exposure.	Few foods naturally contain significant amounts. Must rely on fortified options or fatty fish.	D2 (from plants/fungi), D3 (from animals).	Primarily found in fatty fish, fortified dairy, and certain UV-treated mushrooms.
Supplements	Consistent and controlled dosage. Available in D2 or D3 form.	Requires diligent intake. Can lead to toxicity if taken in excessive amounts.	D2 or D3	D3 is often considered more effective at raising blood levels long-term.

Dietary Sources of Vitamin D

For those needing to supplement sun exposure, some foods provide vitamin D naturally or through fortification. Good sources include:

Fatty fish, such as salmon, mackerel, and trout.
Cod liver oil.
Fortified milk (dairy and plant-based alternatives).
Fortified breakfast cereals.
Mushrooms exposed to ultraviolet light.
Egg yolks.
Beef liver.

For more detailed information on dietary sources, the National Institutes of Health provides a comprehensive fact sheet.

Conclusion

The production of vitamin D is a fascinating biological process that begins in the skin, is regulated by sun exposure, and is completed by two essential organ systems. The initial synthesis in the skin from UVB light is followed by two critical hydroxylation steps in the liver and kidneys to produce the active, functional hormone, calcitriol. While sunlight is a powerful and natural source, factors like geography, season, and skin type can limit its effectiveness. This highlights the importance of dietary sources and supplements to ensure adequate levels for good health. Understanding the complete journey of where vitamin D is produced empowers individuals to make informed choices about maintaining sufficient levels for strong bones, immune function, and overall well-being.

Frequently Asked Questions

Sunlight triggers vitamin D production when ultraviolet B (UVB) rays strike the skin. These rays convert a compound called 7-dehydrocholesterol, which is naturally present in your skin cells, into pre-vitamin D3. The warmth of your body then converts pre-vitamin D3 into the final form, vitamin D3.

After being produced in the skin or consumed in food, vitamin D travels to the liver. There, it is converted into 25-hydroxyvitamin D (calcidiol) through a process called hydroxylation. This is the main form of vitamin D that circulates in the body.

The kidneys perform the final step in activating vitamin D. They convert the 25-hydroxyvitamin D from the liver into 1,25-dihydroxyvitamin D (calcitriol). Calcitriol is the active hormonal form that allows your body to absorb calcium.

No, it is not possible to get too much vitamin D from sun exposure. The skin self-regulates the amount it produces. Once sufficient vitamin D is made, further sun exposure degrades excess pre-vitamin D3 and vitamin D3 into inactive byproducts.

Yes, skin color significantly affects vitamin D synthesis. Individuals with darker skin have more melanin, which acts as a natural sunblock. This means they require longer periods of sun exposure to produce the same amount of vitamin D as someone with lighter skin.

During winter, especially at latitudes far from the equator, the sun is lower in the sky, and its UVB rays are less intense. These rays are largely filtered by the atmosphere, resulting in insufficient UVB exposure to trigger vitamin D synthesis in the skin.

While both vitamin D2 and D3 effectively raise blood levels, most evidence suggests that vitamin D3 is more potent and maintains optimal vitamin D levels for a longer period. D3 is typically derived from animal sources or lichen, while D2 comes from plants and fungi.