What are the two main sources of vitamin D?

December 25, 2025 •

4 min read

An estimated 35% of American adults have low vitamin D levels, highlighting the importance of understanding how our bodies obtain this essential nutrient. To get the recommended amount, it is crucial to know what are the two main sources of vitamin D and how to incorporate them into your daily life.

Quick Summary

The two primary ways the body gets vitamin D are through direct exposure to sunlight, which triggers skin synthesis, and by consuming certain foods and supplements. Many people cannot rely on just one source due to geographical location, skin type, or dietary restrictions, making a combination of both a necessary strategy for maintaining optimal levels.

Key Points

The Sun is the Top Source: Exposure to UVB rays causes skin to produce vitamin D3.
Diet and Fortified Foods Matter: Food sources like fatty fish and fortified milk are crucial, especially when sun exposure is limited.
Reliability is Key: Factors like latitude, skin tone, and age make sun-based production inconsistent.
Supplements Ensure Sufficiency: Vitamin D supplements (D2 or D3) are a reliable way to meet needs, particularly during winter.
Combine Sources for Optimal Health: A balance of sunlight and dietary intake is the best approach for maintaining healthy vitamin D levels year-round.
Darker Skin Needs More Sun: Higher melanin content requires longer sun exposure for the same vitamin D synthesis.
Age Decreases Production: The elderly have a reduced capacity for synthesizing vitamin D from the sun.

Sunlight Exposure: The Primary Natural Source

The most natural and often most significant source of vitamin D is exposure to sunlight. When the skin is exposed to ultraviolet B (UVB) radiation from the sun, it produces vitamin D3 (cholecalciferol) from a cholesterol precursor. This is why vitamin D is often referred to as the "sunshine vitamin." The body's ability to produce vitamin D from sunlight is influenced by several factors, which is why reliance on this method can be unreliable for many people.

Factors Influencing Vitamin D Synthesis from Sunlight

Geographic location: People living farther from the equator experience less intense UVB radiation, especially during winter, and may not produce any vitamin D from sunlight for several months of the year.
Time of day: The intensity of UVB rays is strongest at midday, between 10 a.m. and 3 p.m.. Sensible, short-term exposure during this period is most effective for vitamin D production, though longer exposure increases skin cancer risk.
Skin pigmentation: Individuals with darker skin have higher levels of melanin, which acts as a natural sunscreen and reduces the skin's ability to produce vitamin D. Darker-skinned people therefore require longer sun exposure than those with lighter skin to produce the same amount.
Age: The skin's capacity to synthesize vitamin D decreases with age. Older adults are often more at risk for deficiency, partly due to this reduced synthesis and spending more time indoors.
Clothing and sunscreen: Covering skin with clothing or using sunscreen with a sun protection factor (SPF) of 8 or higher significantly reduces or blocks UVB absorption and, therefore, vitamin D synthesis.

Dietary Intake: The Essential Supplement

Since relying solely on sunlight is impractical or ineffective for many people, dietary intake is the second critical source of vitamin D. Few foods naturally contain significant amounts of this nutrient, but many common foods are fortified. Dietary intake is especially important during winter months in higher latitudes or for individuals with limited sun exposure.

Types of Dietary Sources

Foods Naturally Rich in Vitamin D

Fatty Fish: Salmon, mackerel, tuna, and sardines are among the best natural sources of vitamin D. Wild-caught salmon generally contains more vitamin D than farmed salmon.
Fish Liver Oils: Cod liver oil is a potent source of vitamin D.
Mushrooms: Some mushrooms contain vitamin D2, but only if they have been exposed to UV light. Commercially grown mushrooms are often exposed to UV light to increase their vitamin D content.
Egg Yolks: Small amounts of vitamin D are found in the yolks of eggs, particularly from chickens raised outdoors.
Beef Liver: Contains small quantities of vitamin D.

Fortified Foods and Beverages

Milk: Most cow's milk in the U.S. and Canada is fortified with vitamin D. Plant-based milks like soy and almond milk are also frequently fortified.
Cereals: Many breakfast cereals have added vitamin D.
Orange Juice: Certain brands of orange juice are fortified with vitamin D.
Yogurt and Margarine: Some dairy products and margarine spreads are also fortified to help increase intake.

Supplements as a Reliable Alternative

For many, especially those at high risk for deficiency, supplements are the most reliable and consistent way to ensure adequate vitamin D intake. Supplements are available in two forms: D2 (ergocalciferol) and D3 (cholecalciferol). While both can raise blood levels, D3 is often cited as being more effective at raising and sustaining blood concentrations for a longer period.

A Comparison of the Main Sources of Vitamin D


Feature	Sunlight Exposure	Dietary Sources (Natural/Fortified)	Supplements (D2/D3)
Availability	Dependent on season, latitude, time of day.	Can be limited for natural sources; widespread for fortified foods.	Readily available and consistent dosage.
Risks	Skin damage and increased risk of skin cancer from excessive exposure.	Few risks from food intake; potential for high mercury content in some fatty fish.	Possible toxicity from excessive intake, but not from recommended doses.
Forms of Vitamin D	Produces D3 (cholecalciferol) directly in the skin.	Primarily D3 in animal products; D2 in UV-exposed mushrooms; D2 or D3 in fortified foods.	Available as both D2 (ergocalciferol) and D3 (cholecalciferol).
Reliability	Highly variable; inconsistent due to environmental and personal factors.	Requires conscious effort to choose and consume rich or fortified foods.	Most reliable method for achieving and maintaining target levels.

Conclusion

To ensure sufficient vitamin D levels, a combination of sensible sun exposure and dietary intake is often necessary. While the skin's ability to produce vitamin D from sunlight is a powerful and natural mechanism, its effectiveness is subject to numerous variables, including geography, season, and skin tone. For this reason, relying on food, especially fortified products and supplements, is a crucial part of a comprehensive strategy. Understanding what are the two main sources of vitamin D empowers individuals to make informed choices that support bone, immune, and overall health throughout the year. For more detailed information on recommendations, consult the NIH Office of Dietary Supplements.

Frequently Asked Questions

Vitamin D2 (ergocalciferol) is primarily found in plants and irradiated fungi like mushrooms, while vitamin D3 (cholecalciferol) is produced in the skin from sunlight and found in fatty animal products. D3 is generally considered more effective at raising and sustaining blood levels.

Sensible sun exposure of 5 to 30 minutes, particularly between 10 a.m. and 3 p.m., a few times per week to the face, arms, and legs, is often sufficient for many people. However, this varies based on location, season, and skin tone.

Fatty fish such as salmon, mackerel, tuna, and sardines are excellent natural sources. Small amounts are also found in egg yolks, beef liver, and UV-exposed mushrooms.

Fortified foods are products with added vitamin D, such as cow's milk, some plant-based milks, orange juice, and cereals. They are a reliable source and provide a significant portion of dietary vitamin D in many countries.

No, you cannot. The glass in windows blocks the sun's UVB rays, which are necessary for the skin's vitamin D production.

Those at highest risk include older adults, people with darker skin, individuals with limited sun exposure (e.g., housebound or institutionalized), and people with certain medical conditions that affect nutrient absorption.

No, the body has a self-regulating mechanism that prevents excess vitamin D production from sun exposure. Toxicity almost always results from over-supplementation.