Which Part of the Body Makes Most Vitamin D?

November 27, 2025 •

3 min read

Over 90% of the body's vitamin D is produced through sun exposure, which means the skin is the organ primarily responsible for kickstarting this vital process. Although this is where the journey begins, it's a multi-step process involving several organs to create the usable form of vitamin D.

Quick Summary

The skin produces the initial form of vitamin D with the help of sunlight, and the liver and kidneys perform subsequent conversions to make it biologically active. This article details the entire process, including the key organs and biological steps involved in the body's vitamin D synthesis.

Key Points

The Skin is the Primary Manufacturer: When exposed to UVB radiation from the sun, the skin initiates the process by converting a cholesterol precursor into a preliminary form of vitamin D.
The Liver is a Critical Intermediary: The vitamin D produced in the skin travels to the liver, which converts it into the main circulating form, calcidiol, which is measured to determine vitamin D status.
The Kidneys Create the Active Hormone: The kidneys perform the final, tightly regulated conversion of calcidiol into calcitriol, the active hormonal form essential for bone health and immune function.
Sunlight is Not Always Sufficient: Geographic location, season, skin pigmentation, age, sunscreen use, and clothing all influence the amount of vitamin D the skin can produce.
No Risk of Sun Overdose: The skin has a natural feedback mechanism that prevents the production of toxic levels of vitamin D from excessive sun exposure, unlike supplements which require careful dosing.
The Entire Pathway is Essential: The skin, liver, and kidneys must all function properly for the body to produce and utilize active vitamin D effectively, illustrating a complex, multi-organ collaboration.

The Skin's Role in Initiating Vitamin D Production

Your skin is the primary site for initiating vitamin D production, a process triggered by sunlight. When ultraviolet B (UVB) radiation from the sun hits the skin, it interacts with 7-dehydrocholesterol, a cholesterol precursor. This reaction creates previtamin D3, which then naturally converts to vitamin D3 (cholecalciferol). This synthesis mainly occurs in keratinocyte cells in the epidermis.

Many factors affect the amount of vitamin D the skin produces, including sun exposure duration, time of day, season, latitude, and skin pigmentation. Darker skin, for example, requires more sun exposure due to melanin acting as a natural sunblock.

The Role of the Liver in Activation

After the skin produces vitamin D3, it travels through the bloodstream to the liver. The liver adds a hydroxyl group to convert D3 into 25-hydroxyvitamin D (calcidiol), the main circulating form used for blood tests. This step is crucial for creating a transportable and storable form of the vitamin.

The Kidneys' Final Conversion Step

The liver sends calcidiol to the kidneys for the final activation. Here, another hydroxyl group is added, resulting in 1,25-dihydroxyvitamin D (calcitriol). Calcitriol is the active hormone essential for calcium absorption and bone health. The kidneys regulate this final conversion based on factors like parathyroid hormone and mineral levels.

Comparison of Organs in Vitamin D Production


Organ	Primary Role in Vitamin D Pathway	Chemical Conversion	Primary Output	Regulatory Importance
Skin	Initial synthesis via sunlight exposure	7-dehydrocholesterol to Vitamin D3	Previtamin D3 & Vitamin D3	Initiates the entire process; depends on external factors.
Liver	First activation step	Vitamin D3 to 25-hydroxyvitamin D	Calcidiol (storage form)	Creates the main circulating form, often unregulated.
Kidneys	Final activation step	25-hydroxyvitamin D to 1,25-dihydroxyvitamin D	Calcitriol (active hormone)	Converts to the potent, active form and is tightly regulated.

Factors Affecting Skin-Based Vitamin D Production

Factors like season, latitude, melanin content, sunscreen and clothing use, aging, and cloud cover can limit the skin's ability to produce vitamin D.

The Complete Vitamin D Production Pathway

The creation of active vitamin D is a collaborative process involving the skin, liver, and kidneys. This teamwork ensures a regulated supply for health.

Conclusion

While the skin starts vitamin D production with sunlight, the liver and kidneys are essential for its activation into the form the body uses. Healthy function of all three is key to optimal levels.

Can you get all the vitamin D you need from sunlight alone?

For some, sensible sun exposure may suffice, but for many, it doesn't. Season, geography, skin tone, age, sunscreen, and clothing limit production, often requiring supplements.

What happens if the liver or kidneys aren't functioning properly?

Impaired liver or kidney function can hinder vitamin D activation, potentially causing deficiency even with sun exposure.

Can you get too much vitamin D from the sun?

The skin prevents toxic levels from sun exposure through a protective mechanism. Overdosing is only possible through excessive supplementation.

Is vitamin D from supplements the same as from sunlight?

Both follow the same activation path in the liver and kidneys. Skin-produced D3 might have a longer half-life than oral D3.

What is 7-dehydrocholesterol's role?

This precursor in skin plasma membranes absorbs UVB and converts to previtamin D3, starting synthesis.

Do the arms and legs produce the most vitamin D?

Exposing the face, arms, hands, and legs to midday sun is recommended. Larger areas like the back and legs can produce more, but consistent exposure of smaller areas also works.

Why is the final conversion to calcitriol important?

Calcitriol is the active hormone binding to receptors, promoting calcium absorption, cell growth, and immune support. The body can't use vitamin D effectively without this conversion.

Frequently Asked Questions

For some people in certain latitudes, sensible sun exposure can be enough, but for many others, it is not. Factors like season, geography, skin pigmentation, age, and lifestyle choices such as sunscreen use and clothing can severely limit production, necessitating dietary intake or supplementation.

Poor liver or kidney function can disrupt the activation process of vitamin D, potentially leading to a deficiency even with adequate sun exposure. A person with kidney disease, for instance, might have an impaired ability to convert vitamin D to its final, active form.

No, it is impossible to get a toxic dose of vitamin D from sun exposure alone. The skin has a protective mechanism that prevents overproduction by converting previtamin D3 and vitamin D3 into inactive forms when exposed to prolonged sunlight. Excessive intake from supplements, however, can be toxic.

Vitamin D from both supplements and sunlight goes through the same activation pathway in the liver and kidneys to become the active form. However, some studies suggest that D3 from the skin may have a longer half-life in circulation compared to orally ingested D3.

7-dehydrocholesterol is a precursor molecule found in the skin's plasma membranes. It is this molecule that absorbs UVB radiation and is converted into previtamin D3, the first step in the synthesis pathway.

Exposing the face, arms, hands, and legs to midday sun is typically recommended for sufficient vitamin D synthesis. Generally, parts of the body with larger surface areas, like the back and legs, have the potential to produce more, but consistent exposure of a smaller area can still be effective.

Calcitriol is the active hormone that binds to vitamin D receptors throughout the body, performing crucial functions such as promoting calcium absorption, regulating cell growth, and supporting the immune system. Without this final conversion, the body cannot effectively use vitamin D.