Understanding What Helps Our Body Produce Vitamin D

November 19, 2025 •

4 min read

According to the NIH, ultraviolet B (UVB) rays from the sun are the most significant natural source of vitamin D for most people. This process, along with dietary intake and supplementation, is what helps our body produce vitamin D, an essential nutrient for bone health, immune function, and overall well-being.

Quick Summary

The body creates vitamin D through sun exposure, specific food sources like fatty fish and mushrooms, and dietary supplements. Its production is a multi-step process involving the skin, liver, and kidneys.

Key Points

Sunlight is Key: The skin produces vitamin D3 from cholesterol when exposed to the sun's UVB rays, making it a primary natural source.
Factors Affect Sun Exposure: Your skin tone, geographic location, the time of year, and the use of sunscreen all impact how much vitamin D your body can create from sunlight.
Dietary Sources are Important: Fatty fish, eggs, and UV-exposed mushrooms are natural food sources, while many products like milk and cereals are fortified to boost intake.
Supplements Ensure Adequacy: Taking vitamin D supplements, particularly the D3 form, is a reliable way to maintain adequate levels, especially during winter months or for those with limited sun exposure.
Metabolism is a Two-Step Process: The vitamin D produced by your skin or consumed through diet must be converted by the liver and then the kidneys into its active form, calcitriol, before the body can use it effectively.
Monitor Your Levels: If you have concerns about your vitamin D status, a healthcare provider can order a blood test to determine if supplementation or dietary changes are needed.

Sunlight Exposure and Vitamin D Synthesis

Sunlight is the most efficient and natural way for the human body to produce its own vitamin D. The process begins in the skin, where a form of cholesterol called 7-dehydrocholesterol is converted into pre-vitamin D3 when exposed to UVB radiation. A thermal reaction then converts this pre-vitamin D3 into vitamin D3, which is then transported to the liver for further processing.

Several factors influence how much vitamin D your body produces from sun exposure:

Skin Tone: Individuals with darker skin have more melanin, a pigment that acts as a natural sunscreen and reduces the skin's ability to produce vitamin D from sunlight. As a result, they may need more sun exposure than those with lighter skin tones.
Geographic Location and Season: The intensity of UVB rays depends heavily on latitude and the time of year. For those living far from the equator, vitamin D synthesis from the sun is limited during the winter months when the sun's rays are weaker and more filtered by the atmosphere.
Time of Day: Midday, particularly between 10 a.m. and 4 p.m., is often the best time for vitamin D synthesis, as UVB rays are at their most intense.
Amount of Skin Exposed: Exposing a larger surface area of skin, such as the face, arms, and legs, significantly increases vitamin D production.
Sunscreen and Clothing: Sunscreens with an SPF of 8 or more can block the UVB rays needed for vitamin D production. Similarly, wearing clothing that covers most of the body can limit synthesis.

Dietary Sources of Vitamin D

For many, especially during winter or for those with limited sun exposure, relying on dietary sources is essential. Few foods naturally contain high levels of vitamin D, but some are excellent sources, while others are fortified during processing.

Naturally Rich Foods

Fatty Fish and Seafood: Some of the best natural sources include salmon, trout, tuna, mackerel, and sardines. A 3-ounce serving of cooked sockeye salmon can provide a substantial amount of the daily recommended intake.
Cod Liver Oil: One of the most concentrated food sources, a single tablespoon of cod liver oil can provide a significant amount of vitamin D.
Mushrooms: Certain mushrooms, particularly those exposed to ultraviolet light, can be a good source of vitamin D. Some wild mushrooms also contain it.
Egg Yolks: The vitamin D content in egg yolks is variable, but they contain a small amount.
Beef Liver: Beef liver contains a smaller amount of vitamin D compared to fatty fish.

Fortified Foods

Food fortification has made it easier to obtain vitamin D. Common examples include:

Milk and Milk Alternatives: Most cow's milk in the U.S. is fortified, as are many plant-based options like soy, almond, and oat milk.
Cereals and Orange Juice: Many breakfast cereals and some brands of orange juice are fortified with vitamin D.
Tofu: Certain types of packaged tofu are fortified with vitamin D.

Supplementation Options

When sun exposure and dietary intake are insufficient, supplements are a safe and effective way to ensure adequate vitamin D levels. Supplements come in two forms, D2 (ergocalciferol) and D3 (cholecalciferol).

Vitamin D2 vs. D3

Vitamin D2 comes from plant sources, such as UV-irradiated yeast.
Vitamin D3 is derived from animal sources, such as lanolin from sheep's wool, but animal-free versions from lichen are also available.
Studies suggest that vitamin D3 supplements may be more effective than D2 at raising and maintaining blood vitamin D levels over time.

The Metabolic Journey of Vitamin D

Regardless of whether vitamin D is produced in the skin or consumed through diet, it is biologically inactive until it undergoes a two-step conversion process.

Liver Conversion: The initial conversion happens primarily in the liver, where vitamin D is turned into 25-hydroxyvitamin D (25(OH)D). This is the major circulating form of vitamin D in the body and the one measured in blood tests to determine a person's vitamin D status.
Kidney Conversion: When needed, the kidneys convert 25(OH)D into the biologically active form, calcitriol (1,25-dihydroxyvitamin D). Calcitriol is the hormone responsible for most of vitamin D's functions, particularly regulating calcium and phosphorus absorption.

Sources of Vitamin D: Comparison Table


Feature	Sunlight Exposure	Dietary Sources (Natural)	Fortified Foods & Supplements
Availability	Dependent on season, latitude, time of day, and weather.	Dependent on regular consumption of specific, limited foods.	Readily available year-round in various forms.
Control	Variable; difficult to control exact dosage and is affected by skin type and age.	Some control, but natural food sources are inconsistent in their vitamin D content.	Highly controlled dosage; easy to track and adjust intake.
Effectiveness	Highly effective for most people during sunnier months and latitudes.	Can contribute, but often not enough to meet daily needs alone.	Very reliable for maintaining or increasing vitamin D levels.
Side Effects	Risk of skin cancer and sunburn with excessive exposure.	Generally safe, unless consuming very large amounts of certain items (e.g., cod liver oil).	Safe within recommended dosages; excessive intake can be toxic.

Conclusion

Maintaining adequate vitamin D levels is crucial for overall health. The body's natural production, triggered by sunlight, is a powerful and efficient mechanism, but its effectiveness varies based on factors like geography, skin type, and season. For this reason, relying solely on sunlight is often insufficient, and most people can benefit from a combination of diet and, if necessary, supplementation. By incorporating sources like fatty fish, fortified milk, and eggs into your diet, and considering a high-quality D3 supplement when sun exposure is limited, you can ensure your body has what it needs to produce this vital nutrient and support robust health all year long.

Authoritative Outbound Link

For more detailed information on vitamin D, including recommended intakes and food sources, consult the National Institutes of Health Office of Dietary Supplements.

Frequently Asked Questions

For many, exposing the face, arms, and legs to sunlight for about 5 to 30 minutes, particularly between 10 a.m. and 4 p.m., at least twice a week, can lead to sufficient vitamin D synthesis during sunnier months. However, this varies based on skin tone, age, and location.

It is difficult for most people to get sufficient vitamin D from natural (non-fortified) foods alone, as very few foods contain high amounts. A combination of sun exposure, fortified foods, and supplements is often necessary.

Both vitamin D2 and D3 are effective, but some evidence suggests that D3 may be better at increasing and maintaining overall vitamin D blood levels for a longer period.

Sunscreens with an SPF of 8 or more can block UVB rays and therefore limit vitamin D synthesis. However, most people do not apply sunscreen perfectly, so some vitamin D is likely still produced.

Groups at higher risk of deficiency include older adults, people with darker skin, breastfed infants, those with limited sun exposure, and people with conditions that limit fat absorption or obesity.

No, it is not possible to get toxic levels of vitamin D from sun exposure alone. The body has a built-in mechanism that regulates production, creating inert byproducts when exposure is prolonged.

After initial synthesis in the skin, the liver converts vitamin D into 25-hydroxyvitamin D. The kidneys then convert this into the active hormonal form, calcitriol, which the body can then use.