How Skin Cells Produce Vitamin D from Sunlight

December 14, 2025 •

2 min read

Sunlight exposure remains the primary natural source of vitamin D for most people, activating a remarkable process within the body's largest organ. This guide will detail exactly how skin cells produce vitamin D, revealing the intricate biochemical pathway initiated by UV radiation.

Quick Summary

This article explains how ultraviolet B (UVB) radiation converts a cholesterol precursor in skin cells into previtamin D3, which then thermally isomerizes into vitamin D3.

Key Points

UVB Initiation: Vitamin D synthesis begins in the skin upon exposure to UVB radiation from sunlight.
Cholesterol Precursor: The precursor molecule is 7-dehydrocholesterol, found in skin keratinocytes.
Photochemical Conversion: UVB converts 7-DHC into previtamin D3.
Thermal Rearrangement: Body heat changes previtamin D3 into stable vitamin D3.
Extensive Metabolism: Vitamin D3 is activated in the liver and kidneys to calcitriol.
Melanin's Influence: Skin color affects production; darker skin requires more sun exposure.
Production Factors: Age, sun exposure amount, and sunscreen also impact synthesis efficiency.

The Initial Spark: UVB Radiation

Vitamin D production in the skin begins in the epidermis upon exposure to ultraviolet B (UVB) radiation from the sun (wavelengths 290-320 nm). The energy from UVB photons is the necessary trigger for the biochemical reaction. Factors like geographical location, time of day, and season influence the intensity of this radiation and thus the efficiency of vitamin D synthesis.

The Precursor Molecule: 7-Dehydrocholesterol

Within the epidermal layers (stratum basale and stratum spinosum), keratinocytes contain 7-dehydrocholesterol (7-DHC), a cholesterol derivative, in their plasma membranes. This molecule serves as the raw material for vitamin D production. The concentration of 7-DHC decreases with age, affecting the ability of older adults to synthesize vitamin D from sun exposure.

The Two-Step Conversion Process

Step 1: Photolysis to Previtamin D3

Upon absorbing UVB photons, the 7-DHC molecule undergoes a photochemical reaction. The energy breaks a specific bond in its steroid structure, converting 7-DHC into the unstable intermediate, previtamin D3.

UVB Absorption: 7-DHC absorbs UVB energy.
Ring-Breaking: Energy breaks a bond in the 7-DHC structure.
Precursor Formation: Previtamin D3 is created.

Step 2: Thermal Isomerization to Vitamin D3

Following previtamin D3 formation, body temperature facilitates a spontaneous thermal isomerization, converting it into the stable form, vitamin D3 (cholecalciferol). The resulting vitamin D3 is then released into the bloodstream.

Heat-dependent conversion: Body heat drives the rearrangement.
Stabilization: Previtamin D3 becomes vitamin D3 (cholecalciferol).
Release: Vitamin D3 enters the bloodstream.

The Role of Melanin and Other Factors

Melanin, the skin pigment, affects vitamin D production by absorbing UVB radiation, competing with 7-DHC. Darker skin, with more melanin, requires more sun exposure for equivalent vitamin D synthesis compared to lighter skin. Other factors include sunscreen use (blocking UVB) and age (reduced 7-DHC concentration).

Table: Factors Affecting Vitamin D Production in Skin


Factor	Effect on Vitamin D Production	Mechanism
Skin Pigmentation	Decreases production in darker skin	Melanin absorbs UVB radiation, competing with 7-DHC.
Sunscreen Use	Reduces production significantly	Blocks and scatters UVB radiation from reaching the skin.
Age	Declines with increasing age	Lower skin concentration of 7-dehydrocholesterol.
Latitude & Season	Lower production at higher latitudes and in winter	Reduced UVB intensity due to solar zenith angle.
Time of Day	Most efficient at midday	The sun's UVB rays are most intense when the sun is highest.

Post-Production: Activation and Transport

Skin-produced vitamin D3 is not yet active. It's transported via the bloodstream to the liver, converted to 25-hydroxyvitamin D3 (calcidiol), the main circulating form. Calcidiol then goes to the kidneys for conversion into 1,25-dihydroxyvitamin D3 (calcitriol), the active hormone.

Conclusion

Vitamin D production in skin cells is a fascinating process initiated by UVB radiation converting 7-dehydrocholesterol to vitamin D3, which then requires further metabolism in the liver and kidneys for activation. Factors like skin tone and age influence this process. Given sun exposure risks, dietary sources and supplements are also important for maintaining adequate vitamin D levels. For more information, visit the Linus Pauling Institute.

Frequently Asked Questions

Keratinocytes in the epidermis contain 7-dehydrocholesterol, which is converted into vitamin D3 when exposed to UVB radiation.

Sunlight provides the UVB radiation whose energy triggers the initial photochemical step, converting 7-dehydrocholesterol to previtamin D3.

No, excessive sun exposure does not lead to vitamin D toxicity. The skin has regulatory mechanisms that convert excess precursors and vitamin D3 into inactive forms.

Melanin acts as a natural sunscreen, reducing the amount of UVB reaching 7-dehydrocholesterol. Darker skin tones require longer sun exposure to produce adequate vitamin D.

The liver converts skin-produced vitamin D3 into calcidiol, and the kidneys further convert calcidiol into the active hormone, calcitriol.

Yes, sunscreen blocks UVB rays needed for synthesis. However, studies suggest typical sunscreen use doesn't cause overall deficiency as application is often imperfect.

Yes, the skin's ability to produce vitamin D declines with age, mainly due to decreased concentrations of 7-dehydrocholesterol.

After forming in the skin, vitamin D3 enters the bloodstream bound to vitamin D-binding protein (DBP). It then travels to the liver for the next activation step.