What are the vitamers of vitamin D? Exploring D2, D3, and Beyond

January 13, 2026 •

3 min read

Approximately one-quarter of Americans are at risk of inadequate vitamin D levels. To understand how the body utilizes this crucial nutrient, it's essential to first know what are the vitamers of vitamin D and their distinct characteristics.

Quick Summary

Vitamin D is a group of related compounds known as vitamers, primarily ergocalciferol (D2) from plants and cholecalciferol (D3) from animals and sun exposure.

Key Points

Two Main Vitamers: Vitamin D exists primarily as two forms: ergocalciferol (D2) from plants and cholecalciferol (D3) from animals and sunlight.
Activation is Required: Both D2 and D3 are biologically inactive and must be processed by the liver and kidneys to become the active hormone, calcitriol.
D3 is More Potent: Research suggests vitamin D3 is more effective at raising and maintaining blood levels of vitamin D than D2.
Sources Dictate Choice: Vegans and vegetarians often opt for D2 from supplements or fortified foods, as D3 is primarily animal-sourced.
Monitoring is Key: A blood test measuring 25-hydroxyvitamin D levels is the best way to assess your vitamin D status, regardless of the vitamer source.
Calcitriol is the Active Form: The final, active form of vitamin D, calcitriol, is responsible for regulating calcium and phosphate absorption for bone health.

What Exactly Are Vitamers?

A vitamer is one of several related chemical compounds that perform the same function as a vitamin in the body. For example, the different forms of vitamin B6, like pyridoxine and pyridoxal, are all vitamers of vitamin B6. The collective term “vitamin D” actually refers to a group of fat-soluble compounds, with the two most important for human health being vitamin D2 and vitamin D3. While they serve the same biological purpose, their origins and potencies can differ significantly.

The Two Primary Vitamers: Vitamin D2 and D3

Vitamin D2 (Ergocalciferol)

Ergocalciferol, or vitamin D2, is a plant-based vitamer found in foods like mushrooms and is often used to fortify cereals, plant-based milks, and other food products. It is produced when the plant compound ergosterol is exposed to ultraviolet B (UVB) radiation from the sun. This makes D2 a common choice for vegan or vegetarian supplements and fortified foods, as it is not animal-derived.

Vitamin D3 (Cholecalciferol)

Cholecalciferol, or vitamin D3, is the form produced naturally in the skin of humans and animals when exposed to sunlight. Specifically, UVB radiation triggers the conversion of 7-dehydrocholesterol, a compound in the skin, into vitamin D3. This is why vitamin D is often called the “sunshine vitamin.” D3 is also found naturally in animal-sourced foods such as fatty fish, egg yolks, and beef liver.

The Activation Pathway: From Precursor to Hormone

Before they can be used by the body, both D2 and D3 must undergo two enzymatic hydroxylation steps.

The Liver: Both vitamers travel to the liver, where they are converted into 25-hydroxyvitamin D (25(OH)D), also known as calcifediol. This is the primary circulating form of vitamin D, and a blood test measuring its levels is used to determine a person's vitamin D status.
The Kidneys: From the liver, calcifediol travels to the kidneys, where it is converted into the biologically active form of vitamin D, 1,25-dihydroxyvitamin D, or calcitriol. Calcitriol functions as a hormone, regulating calcium absorption in the gut and influencing bone health.

Comparing Vitamin D2 and D3


Feature	Vitamin D2 (Ergocalciferol)	Vitamin D3 (Cholecalciferol)
Primary Source	Plants and fungi (e.g., mushrooms), fortified foods, supplements	Sunlight exposure, fatty fish, egg yolks, animal products
Production Method	UV irradiation of plant sterol ergosterol	UV irradiation of 7-dehydrocholesterol in skin
Chemical Structure	Contains a double bond and an extra methyl group on the side chain	Different side chain structure compared to D2
Potency	Considered less potent and has a shorter duration of action	More potent and effective at raising blood vitamin D levels
Best For	Vegans, vegetarians, and those seeking plant-based supplements	Most individuals seeking supplementation due to higher efficacy

Key Differences Between D2 and D3

Sources: D2 is derived from plants and fungi, while D3 is sourced from animals or synthesized in the skin from sunlight exposure.
Effectiveness: Multiple studies indicate that D3 is more effective than D2 at increasing and maintaining blood vitamin D levels over time.
Fortification: D2 is the more common form found in fortified foods due to being cheaper to produce, whereas D3 is naturally occurring in animal products.
Stability: Some research has raised questions about the shelf stability of D2 compared to D3, though the human health relevance is debated.

Conclusion

The vitamers of vitamin D, most notably D2 (ergocalciferol) and D3 (cholecalciferol), are crucial for proper bodily function, especially for bone health and the immune system. While both forms effectively increase vitamin D levels, research generally suggests that D3 is more potent and sustained in raising blood concentrations. For most people, adequate vitamin D comes from a combination of sun exposure, diet, and potentially supplements. Understanding the distinction between D2 and D3 can help individuals make informed choices about their dietary intake and supplementation strategy. As always, consulting with a healthcare provider can help determine the best approach for your specific needs.

Learn more about vitamin D research from the National Institutes of Health.

Frequently Asked Questions

The main difference is their origin: vitamin D2 comes from plants and fungi, while vitamin D3 is produced in animal skin when exposed to sunlight and is also found in animal products like fatty fish and egg yolks.

While both forms can raise vitamin D levels, studies suggest that vitamin D3 is more effective at increasing blood concentrations and sustaining them longer than D2.

Yes, but it depends on factors like your geographic location, skin tone, and time spent outdoors. Many people rely on fortified foods or supplements to meet their needs, especially in less sunny climates.

Many fortified foods, such as milk, cereals, and plant-based alternatives, often use vitamin D2 because it is cheaper to produce. However, some foods and supplements may also be fortified with D3.

Calcitriol is the biologically active, hormonal form of vitamin D. It is synthesized primarily in the kidneys from calcifediol (25(OH)D), and it plays a critical role in regulating calcium and phosphate levels in the body.

A blood test that measures the concentration of 25-hydroxyvitamin D (calcifediol) is the standard method for assessing a person's vitamin D status.

Yes, other vitamers such as D4 and D5 exist, but they are far less common and are not typically considered major sources in human nutrition compared to D2 and D3.