What Causes the Body to Produce Vitamin D?

December 14, 2025 •

3 min read

An estimated 40% of US adults are deficient in this vital nutrient. The body's primary method for generating this essential hormone is through a process triggered by sunlight. Learn what causes the body to produce vitamin D and its metabolic pathway.

Quick Summary

The body produces vitamin D when UVB radiation from sunlight strikes the skin, converting a cholesterol precursor. It is then activated through hydroxylation in the liver and kidneys.

Key Points

UVB is Key: Skin exposure to ultraviolet B (UVB) radiation is the primary cause for the body to start producing vitamin D3.
Two-Step Activation: After initial production in the skin, vitamin D must undergo two hydroxylation steps in the liver and kidneys to become its active hormonal form, calcitriol.
Multiple Influencers: Factors like latitude, season, time of day, skin pigmentation, age, and sunscreen use all significantly affect the amount of vitamin D your body can produce from sunlight.
D3 More Potent: Studies suggest that vitamin D3, derived from sunlight and animal sources, is more effective at raising blood vitamin D levels than vitamin D2, which comes from plants.
Dietary Alternatives: For those with limited sun exposure, dietary sources like fatty fish, fortified foods, and supplements are essential to maintain adequate vitamin D levels.
No Toxicity from Sun: The body has a protective mechanism to prevent vitamin D toxicity from overexposure by converting previtamin D3 into inactive forms.

The Role of Sunlight: The Primary Trigger

Exposure to sunlight is the most significant factor for the body's vitamin D production. Specifically, ultraviolet B (UVB) radiation with wavelengths between 290 and 320 nm initiates a photochemical reaction in the skin. This reaction converts a cholesterol derivative into vitamin D. The amount of vitamin D produced from sunlight varies based on factors like time of day, season, and latitude. For instance, in temperate regions, vitamin D synthesis is substantial only during spring and summer months when the sun is higher in the sky. The body regulates this process to avoid toxicity.

How UV Light Converts Skin Precursors

The process starts in the epidermis with 7-dehydrocholesterol (7-DHC), a cholesterol precursor. UVB light converts 7-DHC into previtamin D3. Previtamin D3 then changes into vitamin D3 (cholecalciferol) in the skin's plasma membrane over several hours. This vitamin D3 is then transported in the blood.

The Two-Step Metabolic Activation Process

Vitamin D3 from the skin is inactive and needs two hydroxylation steps to become active calcitriol. These conversions occur mainly in the liver and kidneys, highlighting vitamin D's function as a steroid hormone.

Step 1: Hydroxylation in the Liver

The first activation happens in the liver, where vitamin D3 is converted to 25-hydroxyvitamin D (calcidiol). This step is catalyzed by enzymes like CYP2R1. Calcidiol is the main form of vitamin D in the blood and is measured in blood tests.

Step 2: Hydroxylation in the Kidneys

Calcidiol then goes to the kidneys for the final step. Here, the enzyme CYP27B1 converts it to 1,25-dihydroxyvitamin D (calcitriol), the active hormone. Calcitriol regulates calcium and phosphate absorption and supports bone health. Parathyroid hormone and blood calcium and phosphate levels control calcitriol production in the kidneys, which is the primary source of circulating calcitriol, although other tissues can also produce this enzyme.

Factors That Influence Your Body's Vitamin D Production

Several factors impact how much vitamin D your body can produce:

Latitude and Season: Sunlight intensity decreases at higher latitudes, especially in winter, limiting vitamin D production.
Time of Day: Midday sun provides the most effective UVB for vitamin D synthesis.
Skin Pigmentation: More melanin in darker skin reduces UVB absorption, requiring more sun exposure for adequate vitamin D production.
Age: Older adults have less 7-DHC in their skin, reducing their capacity to produce vitamin D.
Sunscreen and Clothing: Sunscreen with SPF 8 or higher and clothing block UVB rays, preventing vitamin D production.
Body Weight: Obesity can lower vitamin D levels as it can be stored in fat tissue.

The Difference Between Vitamin D2 and Vitamin D3

While the body makes D3, dietary sources provide both D2 (ergocalciferol) and D3 (cholecalciferol). Both are activated, but they differ in origin and metabolism.


Feature	Vitamin D2 (Ergocalciferol)	Vitamin D3 (Cholecalciferol)
Primary Source	Plants and yeasts, often used to fortify foods.	Sunlight exposure and animal-sourced foods.
Chemical Difference	Contains a double bond and an extra methyl group.	Lacks the extra methyl group and double bond.
Stability	Less stable than D3.	More stable.
Bioavailability	May be less effective at raising 25(OH)D levels.	More potent at increasing blood vitamin D concentrations.
Processing	Activated through similar liver and kidney steps.	Activated through similar liver and kidney steps.

Food and Fortification as Alternative Sources

Dietary sources and fortified foods are crucial for those with limited sun exposure. Fatty fish like salmon are good natural sources. Egg yolks, red meat, and liver also contain some vitamin D. Fortification of foods like milk and cereals is common. UV-exposed mushrooms can provide vitamin D2. Supplements are also an effective option. For more information, the NIH Office of Dietary Supplements provides a fact sheet: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/.

Conclusion: The Sun and Beyond

The body's ability to produce vitamin D hinges primarily on skin exposure to UVB from sunlight. This synthesis, starting with a cholesterol precursor, is complemented by dietary intake of D2 and D3. The liver and kidneys convert these forms into the active hormone, calcitriol. Production efficiency varies due to factors like latitude, skin tone, and age. Many individuals cannot rely solely on sun exposure and require a blend of smart sun practices, diet, and potentially supplements to maintain healthy vitamin D levels and support overall wellness.

Frequently Asked Questions

No, in higher latitudes (e.g., Boston, 42°N), the sun's angle during winter months (roughly November to February) means there is insufficient UVB radiation to produce vitamin D. During these months, supplementation or dietary sources are necessary.

Individuals with darker skin have higher levels of melanin, which acts as a natural sunscreen by absorbing UVB radiation. This requires them to have significantly more sun exposure than those with fair skin to produce the same amount of vitamin D.

Yes, sunscreens with a sun protection factor (SPF) of 8 or higher can significantly block the UVB rays required for vitamin D synthesis. However, since people often don't apply it perfectly, some synthesis may still occur.

Calcitriol is the biologically active hormone form of vitamin D, specifically 1,25-dihydroxyvitamin D. Vitamin D (D2 or D3) is the precursor molecule that must be converted in the liver and kidneys to become active calcitriol.

No, prolonged sun exposure does not cause vitamin D toxicity. The skin has a self-regulating mechanism where excess previtamin D3 is converted into inactive photoproducts like lumisterol and tachysterol.

Vitamin D3 produced in the skin is transported to the liver where it is converted to 25-hydroxyvitamin D (calcidiol). This is then sent to the kidneys for the final conversion into the active hormone, calcitriol.

Some commercial tanning beds can produce vitamin D if they emit ultraviolet B (UVB) radiation in the correct wavelength range (290–320 nm). However, limiting UV exposure is prudent due to the risk of skin cancer.