How Does Vitamin D3 Come From Sunlight, Food, and Supplements?

December 5, 2025 •

5 min read

According to the National Institutes of Health, vitamin D deficiency is a global health concern affecting roughly one billion people worldwide. This fat-soluble vitamin, known for its critical role in bone health, is uniquely synthesized by the human body in a process initiated by sun exposure. However, modern lifestyles, limited sunlight, and dietary patterns mean that many rely on food and supplements to maintain adequate levels.

Quick Summary

Vitamin D3 originates from three main sources: the skin's exposure to UVB radiation from sunlight, consumption of certain animal-based foods, and fortified products or supplements. In the skin, a cholesterol precursor is converted to previtamin D3, which then thermally rearranges into D3. This inactive form is subsequently activated by the liver and kidneys to support bodily functions.

Key Points

Sunlight is Key: The most efficient natural method for producing vitamin D3 involves exposing the skin to UVB radiation, which converts 7-dehydrocholesterol into the vitamin.
Limited Natural Food Sources: Few foods naturally contain vitamin D3 in significant amounts, with oily fish like salmon and mackerel being the most potent.
Fortified Foods Bridge the Gap: Many staples like milk, cereals, and orange juice are fortified with vitamin D to help address widespread deficiency.
Supplements for Consistency: Supplements provide a reliable, controlled dose of vitamin D3, especially for those in northern latitudes or with limited outdoor time.
Internal Activation is Required: Whether from sun, food, or supplements, vitamin D3 is biologically inactive until it is converted by the liver and kidneys into its active hormonal form.
D3 is More Potent: Scientific evidence suggests that vitamin D3 is generally more effective at increasing and maintaining blood vitamin D levels than vitamin D2.
No Risk of Sun Overdose: The body has a protective mechanism that prevents vitamin D toxicity from excessive sun exposure by converting surplus D3 into inactive compounds.

The Primary Pathway: Skin Synthesis from Sunlight

The most natural and effective method for the body to produce vitamin D3 is through sunlight exposure. This process is not as simple as absorbing sunlight directly; it involves a complex biochemical reaction within the skin's layers.

The Role of 7-Dehydrocholesterol

Deep within the epidermal layers of the skin, specifically the stratum basale and stratum spinosum, a cholesterol precursor called 7-dehydrocholesterol (7-DHC) is present. When ultraviolet B (UVB) radiation from sunlight, with wavelengths between 290 and 315 nm, penetrates the skin, it is absorbed by 7-DHC. This absorption causes a molecular change, breaking a chemical bond and converting 7-DHC into previtamin D3.

Thermal Isomerization to Vitamin D3

Once formed, previtamin D3 is a relatively unstable compound. It undergoes a heat-dependent process called thermal isomerization, where its molecular structure rearranges to form the more stable vitamin D3, or cholecalciferol. This conversion is continuous and regulated by the body's temperature.

Limiting Toxicity from Sun Exposure

Interestingly, the body has a built-in safety mechanism to prevent vitamin D3 toxicity from excessive sun exposure. If previtamin D3 or vitamin D3 is overexposed to UVB, it is converted into biologically inactive photoproducts, such as lumisterol and tachysterol. This self-regulatory system ensures that the body's synthesis from sunlight does not lead to harmful, toxic levels.

Dietary Sources of Vitamin D3

While sunlight is a major source, many people supplement their intake through diet. Natural food sources containing vitamin D3 are relatively limited, making fortified foods and supplements crucial for a balanced approach.

Foods Naturally Containing Vitamin D3

Oily Fish: Some of the richest natural sources include salmon, mackerel, herring, and sardines. For example, a 3-ounce serving of salmon can provide a substantial amount of the daily recommended intake.
Egg Yolks: The yolks of eggs contain vitamin D3, though the amount is typically small. One large scrambled egg provides around 44 IU of vitamin D.
Beef Liver: This is another natural source, although its vitamin D content is relatively low compared to oily fish.

Fortified Foods with Vitamin D3

Because of the limited natural sources, many staple foods are fortified to help people meet their requirements. These include:

Milk (cow and some plant-based alternatives)
Breakfast cereals
Yogurt and some cheese
Certain brands of orange juice

The Role of Supplements

For those with limited sun exposure, darker skin tones, or specific health conditions, supplements are a common and reliable way to ensure adequate vitamin D3 levels. Supplements are available in various forms, including pills, capsules, and liquid drops.

D3 vs. D2 in Supplements

Supplements contain either vitamin D2 (ergocalciferol), derived from plants and fungi, or vitamin D3 (cholecalciferol), typically from animal sources like lanolin or lichen. Most evidence suggests that vitamin D3 is more effective at raising and sustaining blood levels of 25-hydroxyvitamin D (25(OH)D), the circulating form of the vitamin.

Bioavailability of Supplement Forms

The delivery system of supplements can impact their bioavailability, or how efficiently the body can absorb and use them. Studies suggest that oil-based and microencapsulated forms of vitamin D3 may have higher bioavailability compared to micellized or tablet forms. Some fortified foods, however, can provide vitamin D3 with higher bioaccessibility than supplements due to the presence of other nutrients.

Comparison of Vitamin D3 Sources

To better understand the differences, here is a comparison of the three main sources of vitamin D3:


Feature	Sunlight	Food	Supplements
Mechanism	UVB radiation converts 7-DHC in the skin into D3.	Ingestion of D3 from animal sources or fortified products.	Oral intake of concentrated D3, typically in oil or capsule form.
Toxicity Risk	Extremely low risk; the body self-regulates by creating inactive photoproducts from excess UV exposure.	Very low risk, as food sources contain relatively small amounts.	Possible with excessive, high-dose intake; should be carefully monitored.
Availability	Dependent on latitude, season, time of day, and skin pigmentation.	Limited natural sources; fortified options are widely available.	Widely available and provides a consistent, controlled dose.
Side Effects	Increased risk of skin damage and cancer from prolonged, unprotected exposure.	No specific side effects, assuming normal intake.	High doses can cause nausea, vomiting, and dangerously high blood calcium levels.
Recommended For	General population in optimal conditions (adequate sunlight, moderate exposure).	People seeking to complement sun exposure and supplementation.	Those with limited sun access, darker skin, or diagnosed deficiency.

The Final Activation Process

Regardless of its origin—whether from the skin, diet, or supplements—vitamin D3 is biologically inactive and must undergo two hydroxylation steps to become active.

First Hydroxylation in the Liver: Upon reaching the liver, vitamin D3 is converted into 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. This is the major circulating form of vitamin D in the body and what is typically measured in blood tests.
Second Hydroxylation in the Kidneys: Calcidiol then travels to the kidneys, where it undergoes a second hydroxylation to become 1,25-dihydroxyvitamin D, or calcitriol. Calcitriol is the physiologically active hormone form that performs vitamin D's primary functions.

Conclusion

Vitamin D3 is sourced primarily through three distinct pathways: the body's natural synthesis in the skin via sun exposure, consumption of specific foods and fortified products, and dietary supplements. The skin's synthesis process is an elegant, self-regulating system that uses UVB radiation to convert a cholesterol precursor into previtamin D3 and then into D3. However, factors like geography, season, and lifestyle can make dietary and supplemental sources essential for maintaining adequate levels. While natural foods and fortified options contribute to intake, supplements offer a reliable way to ensure consistency, particularly for those at risk of deficiency. The journey of vitamin D3, from its origins to its final activation, highlights its unique status as both a vitamin and a hormone critical for bone health and immune function.

Keypoints

Sun-activated Synthesis: The body produces vitamin D3 naturally when UVB radiation from sunlight strikes the skin, converting a cholesterol precursor into vitamin D3.
Dietary Sources: Vitamin D3 can be obtained by consuming oily fish, egg yolks, beef liver, and fortified foods like milk and cereal.
Supplementation Necessity: For those with limited sun exposure, darker skin, or dietary restrictions, supplements are a necessary and consistent source of vitamin D3.
Final Activation: Regardless of the source, vitamin D3 must be metabolized by the liver and kidneys to its active form, calcitriol.
Bioavailability Factors: The absorption of vitamin D3 from supplements and food can be influenced by the delivery system and matrix composition, affecting overall bioavailability.
No Sun Toxicity: Prolonged sun exposure will not cause vitamin D toxicity, as the body converts excess previtamin D3 into inactive compounds.
D3 Superiority: Vitamin D3 supplements are often more effective at raising and maintaining blood levels compared to vitamin D2.

Frequently Asked Questions

The body doesn't absorb vitamin D3 directly from the sun. Instead, UVB radiation from sunlight strikes the skin, converting a cholesterol precursor called 7-dehydrocholesterol into previtamin D3, which then thermally converts into active vitamin D3.

It is difficult to get enough vitamin D3 from food alone, as few foods are naturally rich in it. While oily fish, egg yolks, and fortified foods help, many people require additional sources like sun exposure or supplements to meet their needs.

Vitamin D3 supplements are generally safe when taken in recommended doses. However, excessive intake can be toxic and lead to harmful side effects like high blood calcium levels. It is important to follow dosing instructions or consult a doctor for proper guidance.

The body can effectively use vitamin D3 from both sources. Sunlight provides a natural, self-regulating process, but its effectiveness varies depending on factors like location, season, and skin type. Supplements offer a reliable and consistent intake, which is often necessary to prevent deficiency.

Vitamin D2 (ergocalciferol) is typically derived from plant sources like fungi, while vitamin D3 (cholecalciferol) comes from animal sources or lichen. Evidence suggests that D3 is more effective at raising and maintaining blood vitamin D levels than D2.

Several factors impact the skin's ability to produce vitamin D3, including latitude, season, time of day, skin pigmentation (melanin), age, and the use of sunscreen. Darker skin tones and older adults, for example, produce less vitamin D3 from the same amount of sun exposure.

After entering the body, inactive vitamin D3 is converted by the liver into 25-hydroxyvitamin D. This is then sent to the kidneys, where it is converted into the active hormone form, calcitriol, which the body uses for various functions.

No, you cannot get too much vitamin D3 from the sun. The body has a built-in feedback mechanism where excessive UV exposure converts excess previtamin D3 into biologically inactive photoproducts, preventing toxicity.

Vitamin D3 is vital for regulating calcium and phosphorus absorption, which is critical for maintaining healthy bones and teeth. It also plays a key role in supporting immune function and can modulate cell growth.

Symptoms of a vitamin D3 deficiency can include fatigue, bone pain, muscle weakness, and an increased susceptibility to infections. In severe cases, it can lead to bone disorders like rickets in children and osteomalacia in adults.