Can the Body Manufacture Vitamin D? The Role of Sunlight and Skin

January 13, 2026 •

3 min read

According to the NIH, ultraviolet B (UVB) radiation from the sun is the primary driver for vitamin D synthesis in most humans. This remarkable process means your body can manufacture vitamin D, though its efficiency depends on several key factors, including your location and skin type.

Quick Summary

The body synthesizes vitamin D3 in the skin when exposed to UVB radiation, but many factors affect this process. The precursor molecule 7-dehydrocholesterol is converted to previtamin D3, which then thermally isomerizes to vitamin D3. Dietary intake and supplements are also crucial sources, particularly when sun exposure is limited.

Key Points

Skin Synthesis: The body produces vitamin D3 in the skin when exposed to the sun's UVB rays, converting 7-dehydrocholesterol.
Activation Process: The newly synthesized vitamin D3 is converted in the liver and kidneys to its active hormonal form, calcitriol.
Influencing Factors: Production is influenced by latitude, season, skin pigmentation, age, sunscreen use, and time of day.
High-Risk Groups: People with darker skin, older adults, breastfed infants, and those with limited sun exposure are at higher risk for deficiency.
Balanced Approach: Relying solely on sun exposure is often insufficient and carries health risks, making dietary intake and supplements important.
Health Impact: Deficiency can cause bone disorders like rickets and osteomalacia, as well as fatigue, bone pain, and impaired immune function.

How the Body Makes Its Own "Sunshine Vitamin"

For most people, the most significant source of vitamin D comes directly from the sun. This process begins when the sun's ultraviolet B (UVB) rays strike the skin. Inside the skin's epidermal layers, there is a cholesterol precursor known as 7-dehydrocholesterol (7-DHC). The energy from the UVB radiation converts this 7-DHC into previtamin D3. This new molecule then undergoes a temperature-dependent process called thermal isomerization to become vitamin D3 (cholecalciferol). Once in the bloodstream, the liver and kidneys further metabolize vitamin D3 into its active hormonal form, calcitriol.

The Multi-Step Conversion of Vitamin D

While the skin produces vitamin D3, it is not yet in its biologically active form. A series of steps transforms it into the hormone the body uses for vital functions like calcium absorption.

Skin Synthesis: UVB radiation converts 7-DHC in the skin into previtamin D3, which then isomerizes to vitamin D3.
Liver Processing: An enzyme converts D3 into 25-hydroxyvitamin D. This is the major circulating form and is measured in blood tests.
Kidney Activation: A different enzyme converts 25-hydroxyvitamin D into the active form: 1,25-dihydroxyvitamin D (calcitriol).

Factors That Influence Vitamin D Production in the Skin

The amount of vitamin D the body can produce from sunlight is not constant and is affected by various factors. These include geographic location, season, time of day, skin pigmentation (melanin content), age, and lifestyle choices such as sunscreen use and clothing. Melanin acts as a natural sunscreen, meaning people with darker skin require more sun exposure to produce the same amount of vitamin D. The sun's angle and intensity vary by season and time of day, impacting UVB exposure.

Can You Rely on Sun Exposure Alone?

Relying solely on sun exposure for vitamin D can be problematic due to skin cancer risk from excessive UV exposure. Dermatologists advise against unprotected sun exposure to increase levels. Factors like location and season can make sun synthesis insufficient in many areas. Dietary sources and supplements are important for maintaining adequate levels.

Comparison of Vitamin D Sources


Source	Pros	Cons	Reliability	Who is it for?
Sunlight	Free and natural.	Risk of skin cancer, highly dependent on location, season, and time of day.	Highly Variable	Those with fair skin in sunny climates, with short, controlled exposure.
Diet	Safe and consistent intake.	Few foods naturally contain significant vitamin D; depends on regular consumption of fortified foods.	Moderate to High	Everyone, especially when sun exposure is low.
Supplements	Ensures adequate intake regardless of sun exposure or diet; can be precisely dosed.	Risk of toxicity if dosage is not monitored, requires consistent use.	High	At-risk individuals (older adults, dark-skinned people, those with malabsorption issues) or those with confirmed deficiency.

Understanding Vitamin D Deficiency

A lack of sufficient vitamin D can lead to health problems. In children, it can cause rickets. Adults can develop osteomalacia and it contributes to osteoporosis. Symptoms may include fatigue, depressed mood, hair loss, and increased susceptibility to infections.

Who Is at High Risk?

Certain populations face a higher risk of vitamin D deficiency and may need to pay extra attention to their intake. This includes breastfed infants, older adults, individuals with dark skin, people with certain medical conditions, and those with limited sun exposure.

Conclusion

While the human body can manufacture vitamin D from sunlight, this process is significantly influenced by factors such as latitude, season, skin pigmentation, and age, making sun exposure alone often unreliable. Due to this variability and the risks of excessive sun exposure, health experts recommend a combined approach that includes controlled sunlight exposure, a diet rich in vitamin D, and targeted supplementation, particularly for those at higher risk of deficiency. To accurately assess vitamin D status, a blood test is recommended. A balanced strategy is essential for maintaining adequate vitamin D levels and supporting overall health. Consulting a healthcare professional for personalized guidance is advisable.

Frequently Asked Questions

When UVB radiation from the sun hits your skin, it converts a cholesterol precursor called 7-dehydrocholesterol into previtamin D3. This previtamin then thermally isomerizes into vitamin D3, which is processed by the liver and kidneys into the active form the body can use.

The amount varies based on several factors, including your location, skin color, and the season. For example, during midday in sunny climates, just 5–30 minutes of sun exposure to your face, arms, and legs, a couple of times per week, might be sufficient for lighter-skinned individuals. Those with darker skin need more time.

No, you cannot. Glass blocks the UVB rays from the sun that are needed to start the vitamin D synthesis process in your skin. Therefore, sun exposure through a window is not effective for making vitamin D.

Yes, sunscreen is designed to block UVB rays, which are necessary for vitamin D production. However, most people do not apply it perfectly, so some synthesis still occurs. Experts prioritize sun protection to prevent skin cancer and recommend other sources like diet and supplements for vitamin D.

Supplements are considered a safer and more reliable method than relying solely on sun exposure. Unprotected sun exposure carries the risk of skin cancer, and the effectiveness of sun synthesis is highly variable. A balanced diet and supplementation can provide a consistent intake.

Symptoms of vitamin D deficiency can include fatigue, bone and muscle pain, muscle weakness, depressed mood, and frequent infections. Severe deficiency can lead to bone disorders like rickets in children and osteomalacia in adults.

No, the body has a self-regulating mechanism that prevents vitamin D toxicity from sun exposure alone. When exposed to too much sun, the body converts previtamin D3 and vitamin D3 into inactive compounds. Toxicity is almost always a result of excessive supplementation.