Understanding the Process: What Produces Vitamin D3 in the Body?

December 19, 2025 •

3 min read

According to the National Institutes of Health, vitamin D3 is primarily synthesized in human skin when exposed to ultraviolet B (UVB) radiation from sunlight. The entire process involves multiple steps beyond just sun exposure, transforming a cholesterol-based compound into a biologically active hormone. This article explains the fascinating journey of how your body produces vitamin D3.

Quick Summary

The body synthesizes inactive vitamin D3 in the skin from 7-dehydrocholesterol after exposure to UVB radiation. It is then activated through two conversion steps, first in the liver and then in the kidneys, to produce its active form.

Key Points

Sunlight is Key: The process of producing vitamin D3 in the body is initiated by the absorption of ultraviolet B (UVB) radiation from sunlight by the skin.
Cholesterol Precursor: Vitamin D3 synthesis begins with 7-dehydrocholesterol, a compound derived from cholesterol found in the skin.
Two Activation Steps: The inactive form of vitamin D3 made in the skin is converted into its active form through two essential steps, first in the liver and then in the kidneys.
Influencing Factors: Skin pigmentation, latitude, age, and sunscreen use all significantly affect the body's ability to produce vitamin D3.
Safe Production: The body has a mechanism to prevent vitamin D3 toxicity from sun exposure by converting excess previtamin D3 into biologically inert products.
Dietary Role: While the body can produce its own vitamin D3, dietary intake from food and supplements remains a critical source, especially for those with limited sun exposure.

The Initial Step: Skin and Sunlight

The production of vitamin D3, often called the "sunshine vitamin," begins in the skin. This process involves a cholesterol-derived precursor molecule, 7-dehydrocholesterol (7-DHC), found in the skin's epidermal layers. When UVB radiation from sunlight hits the skin, it triggers a photochemical conversion of 7-DHC into previtamin D3.

UVB Conversion: UVB light breaks a bond in the 7-DHC molecule, forming previtamin D3.
Thermal Isomerization: Previtamin D3 then changes into the inactive form of vitamin D3, cholecalciferol, over several hours due to body heat.

The body has a mechanism to prevent vitamin D toxicity from too much sun exposure by converting excess previtamin D3 and vitamin D3 into inactive substances.

The Activation Pathway: Liver and Kidneys

Inactive vitamin D3 needs to be converted into its active form for the body to use it. This happens in two steps, involving the liver and kidneys, which is why issues with these organs can lead to deficiency.

The Liver's Role: 25-Hydroxylation

After skin synthesis, vitamin D3 travels to the liver. In the liver, an enzyme converts vitamin D3 into 25-hydroxyvitamin D (calcidiol), the main form circulating in the blood and used to check vitamin D status.

The Kidney's Role: 1-alpha-Hydroxylation

Calcidiol then goes to the kidneys, where another enzyme completes the activation by converting it into 1,25-dihydroxyvitamin D (calcitriol). Calcitriol is the active hormone responsible for regulating calcium and phosphorus.

Factors Affecting Internal Production

Several factors impact how much vitamin D3 your body produces from sunlight:

Skin Pigmentation: Darker skin with more melanin absorbs more UVB, reducing vitamin D production compared to lighter skin.
Latitude and Season: Further from the equator, the sun's angle is too low during certain months for effective UVB penetration, leading to little or no vitamin D synthesis.
Age: Older skin has less of the precursor 7-DHC, making it less efficient at producing vitamin D3.
Sunscreen and Clothing: Sunscreen and clothing block UVB rays necessary for synthesis.
Time of Day: Midday sun provides the most effective UVB for vitamin D production.

Internal Synthesis vs. Dietary Intake: A Comparison


Feature	Internal Synthesis (Sunlight)	Dietary Intake (Food/Supplements)
Initiating Factor	UVB radiation exposure on skin.	Consumption of food or supplements.
Starting Material	7-dehydrocholesterol in skin.	Inactive vitamin D3 (cholecalciferol) or D2.
Initial Product	Previtamin D3 (then isomerized to D3).	Inactive vitamin D3 or D2.
Regulation	Photo-degradation prevents toxicity from over-exposure.	Excessive intake from supplements can lead to toxicity.
Best for Consistency	Unreliable due to location, season, and lifestyle factors.	Can provide a reliable, consistent source of the vitamin.
Considerations	Requires moderate, unprotected sun exposure with skin cancer risk.	Must account for absorption issues and dietary preferences.
Activation Path	D3 → Liver (calcidiol) → Kidneys (calcitriol).	D3/D2 → Liver (calcidiol) → Kidneys (calcitriol).

Conclusion: The Holistic View of Vitamin D3

Producing vitamin D3 involves skin exposure to sunlight and subsequent processing by the liver and kidneys. Relying solely on one source is often insufficient. A combination of careful sun exposure, vitamin D-rich foods, and supplements is usually recommended to maintain adequate levels. Understanding this internal production pathway highlights the various factors influencing your vitamin D status. Always consult a healthcare provider to determine the best approach for you, especially considering potential risks like skin cancer from overexposure to UV radiation. For further details, the NIH provides detailed information on vitamin D metabolism.

Frequently Asked Questions

No, your body has a built-in protective mechanism to prevent vitamin D3 intoxication from sun exposure alone. Prolonged exposure causes excess previtamin D3 and vitamin D3 to be broken down into inactive forms.

The inactive vitamin D3 molecule (cholecalciferol) is first made in the skin's epidermal layers. The molecule is then sent to the liver for the first activation step, and finally to the kidneys for the final activation into its hormonal form, calcitriol.

Supplements provide vitamin D from an external source, either D2 or D3, which is absorbed through the intestines. The body then processes this vitamin D in the liver and kidneys in the same way as the vitamin D3 synthesized in the skin.

Individuals with darker skin have higher levels of melanin. Melanin is a natural pigment that acts as a sunscreen, absorbing UVB radiation and reducing the amount that can penetrate the skin to trigger vitamin D synthesis.

No, you cannot. Glass blocks UVB radiation, which is the specific type of ultraviolet light required to initiate the production of vitamin D3 in the skin.

Calcitriol is the biologically active hormonal form of vitamin D. It is produced in the kidneys from calcidiol and is responsible for regulating calcium and phosphorus levels in the body.

Since the liver and kidneys are responsible for the critical activation steps of vitamin D, dysfunction in these organs can lead to deficiencies. Liver disease impairs the first conversion to calcidiol, while kidney disease impairs the second conversion to active calcitriol.