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A review of clinical trials published in 2025 found that people taking vitamin D2 had lower blood concentrations of a vitamin D3-derived marker than people who did not take D2. The finding does not show that D2 is harmful or that it lowers total vitamin D status; researchers say the mechanism and health consequences remain uncertain.
A review published in 2025 found that vitamin D2 supplements were associated with lower blood levels of 25-hydroxyvitamin D3, a form used to assess vitamin D status, than no D2 supplementation. The analysis does not show that D2 is harmful or that it reduces total vitamin D; researchers say the health significance of the change is not yet known.
The study, published in Nutrition Reviews, brought together evidence from randomized controlled trials. Researchers from the University of Surrey, the John Innes Centre and Quadram Institute Bioscience reviewed 20 studies, with 11 providing data for the main meta-analysis. Compared with people who did not receive D2, people taking it had significantly lower concentrations of 25-hydroxyvitamin D3. Depending on the analysis method, the average reduction was about 9 to 18 nanomoles per liter.
That measurement is not the whole picture. Vitamin D2 raises 25-hydroxyvitamin D2, and the supplement can contribute to a person’s overall vitamin D status. The result instead points to a possible interaction between the two forms: as D2-derived levels rise, the D3-derived marker may fall. The review does not establish that D2 removes vitamin D from the body, causes deficiency, or leads to illness.
Emily Brown, the study’s lead researcher and a research fellow at the University of Surrey, said the findings suggest that, subject to individual considerations, D3 supplements may be more beneficial for most people than D2. That is the researchers’ interpretation of the evidence, not proof that everyone should switch supplements. People taking vitamin D for a medical reason should discuss any change with a health professional.
What the D2 Finding Could Mean
The finding matters because D2 and D3 are often treated as interchangeable, even though research summarized by the U.S. National Institutes of Health indicates that D3 tends to raise the main measured vitamin D level more and keep it elevated longer. A possible reduction in the D3-derived marker adds another reason researchers are examining whether the two forms have different effects.
Vitamin D helps the body absorb calcium and supports bone, muscle, nerve and immune function. But the study measured blood markers, not fractures, infections, or other health outcomes. It cannot show that the observed difference causes a meaningful change in health, or establish that D2 supplementation is unsuitable for particular groups.
The result may also be relevant to people seeking plant-based products. D2 is commonly made from fungi or yeast exposed to ultraviolet light, while D3 has often come from sheep’s-wool lanolin. Animal-free D3 made from lichen is available, though the review itself does not compare specific retail products or make individual treatment recommendations.
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How the Two Vitamin Forms Differ
Vitamin D3 is the form the body produces in skin exposed to ultraviolet B sunlight. D2, also known as ergocalciferol, is generally obtained from fungi or yeast sources. After either form enters the body, the liver converts it into a related compound called 25-hydroxyvitamin D, commonly measured in blood tests. D2 and D3 produce distinct versions of that compound.
In the UK, public guidance advises people to consider taking 10 micrograms of vitamin D daily during darker months, when sunlight may be too weak for the skin to make enough. That general advice is separate from the study’s result and does not determine which form is right for each person. The NIH says D3 generally raises blood vitamin D levels more and for longer than D2, while both forms can increase vitamin D status.
A separate study in Frontiers in Immunology examined gene activity in blood after D2 and D3 supplementation and reported differing responses, including differences involving type I interferon signaling. Those results concern biological signals, not evidence that a supplement prevents infections. They do not establish a clinical immune benefit from choosing D3.
“Vitamin D2 supplements can actually decrease levels of vitamin D3 in the body.”
— Emily Brown, lead researcher and research fellow at the University of Surrey
Plant-based vitamin D3 from lichen
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Health Effects Still Need Study
The review does not explain why D2 supplementation was linked to lower 25-hydroxyvitamin D3. Researchers have proposed that the body may regulate rising vitamin D metabolites by breaking them down more quickly, potentially affecting the D3-derived form. That is a possible mechanism, not a settled explanation.
It is also unclear whether a reduction of roughly 9 to 18 nanomoles per liter, as reported under different analysis methods, has a meaningful effect on health. The review examined trial data on blood concentrations; it did not establish impacts on symptoms, disease risk, or long-term outcomes. Results can also depend on how the analyses are conducted.
The available material does not specify which doses, durations, or participant characteristics account for the different estimates, nor does it establish how the association applies to every person taking D2. The separate immune-signaling research likewise does not prove that D3 supplementation prevents viral or bacterial illness. These questions remain open.
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What Future Research Must Establish
Further studies would need to test why the two vitamin forms appear to affect each other’s blood markers and whether that change translates into measurable health outcomes. Research comparing D2 and D3 across different doses and populations could help clarify how consistent the finding is and who, if anyone, is most affected. The source material does not identify a scheduled follow-up study or a new clinical recommendation.
For now, the result is a reason to distinguish between a change in a laboratory marker and proof of harm. People already taking supplements should not treat the review as a diagnosis or an instruction to stop or switch. Anyone with a health condition, a prescribed regimen, or questions about dosage can ask a clinician or pharmacist for advice.
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Key Questions
Does vitamin D2 lower total vitamin D?
The review found lower levels of 25-hydroxyvitamin D3 in people taking D2 compared with people who did not receive it. D2 also raises its own related blood marker, so the finding does not establish that total vitamin D falls.
Does the study show vitamin D2 is harmful?
No. The review identified a difference in a blood marker, but it did not show that D2 causes illness, deficiency, or other harm. The health significance remains uncertain.
Is vitamin D3 better than vitamin D2?
The NIH says D3 tends to raise vitamin D levels more and keep them elevated longer than D2. The researchers also interpreted their findings as supporting D3 for many people, subject to personal considerations. This is not an individual medical recommendation; supplement choices can depend on a person’s circumstances.
Can vegans get vitamin D3?
Yes. The source material notes that animal-free D3 derived from lichen is available, alongside D2 products made from fungi or yeast. Check product information and seek professional guidance if you have questions about supplementation.
Should I stop taking my vitamin D supplement?
The review does not direct people to stop or change supplements. If you take vitamin D on medical advice, have a pre-existing condition, or are unsure about dose or form, speak with a clinician or pharmacist before making changes.
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