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A 2025 review of randomized trials found that people taking vitamin D2 had lower levels of one blood marker, 25-hydroxyvitamin D3, than control groups. That is a biochemical finding—not proof that vitamin D2 causes harm, lowers total vitamin D status by the same amount, or leads to worse health. Whether it should change supplement advice remains unresolved.
What the review found
Emily I. G. Brown and colleagues published a systematic review and meta-analysis in Nutrition Reviews on September 18, 2025. The review covered 20 studies, and 11 had data suitable for meta-analysis. The authors searched for relevant published studies from January 1, 1975, through February 1, 2023.
They compared serum 25(OH)D3 levels after vitamin D2 supplementation with control groups. The two pooled estimates describe different comparisons:
| Measure | Pooled result | What it describes |
|---|---|---|
| End-of-trial between-group weighted mean difference | −17.99 nmol/L (95% CI, −25.86 to −10.12; P < .00001) | The difference between the D2 and control groups at the end of the trials. |
| Absolute-change weighted mean difference | −9.25 nmol/L (95% CI, −14.40 to −4.10; P = .0004) | The difference in change over the trials between the D2 and control groups. |
Both estimates concern 25(OH)D3 specifically. They do not show that total vitamin D status fell by either amount, nor do they measure illness, symptoms, or other clinical outcomes.
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What 25(OH)D3 means—and what it does not
Vitamin D2, also called ergocalciferol, and vitamin D3, or cholecalciferol, are different forms of vitamin D. D3 is produced in human skin after exposure to UVB light. Both forms can enter the body’s vitamin D metabolism.
A 25(OH)D blood test is commonly used to estimate vitamin D status. The review’s signal was a reduction in the D3-derived form of that marker in the D2 groups; it was not evidence that D2 fails to contribute to vitamin D status. The authors discuss increased metabolic clearance as a possible explanation, but say that mechanism needs further study.
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Does this mean vitamin D2 is harmful or that everyone should switch?
No. The review identifies a difference in a blood biomarker, but does not establish that the difference causes harm or that D3 supplementation produces better long-term health outcomes. The authors say further research is needed to determine whether D2 and D3 affect overall vitamin D metabolism differently in a way that should change clinical advice.
Brown, the review’s lead researcher and a research fellow at the University of Surrey, interpreted the findings this way: “This study suggests that subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2.” That is her qualified interpretation of the findings, not a universal instruction to change supplements.
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What the immune-system claims do—and do not—show
In coverage published by ScienceDaily using materials from the University of Surrey on October 8, 2026, University of Surrey professor Colin Smith said that D3, but not D2, appeared to stimulate type I interferon signaling, a part of the immune system’s first-line response. Changes in gene activity or immune signaling do not demonstrate that a supplement prevents infections; the report itself cautions against treating them as evidence of fewer illnesses.
The same coverage quoted John Innes Centre group leader Cathie Martin calling for plant-based vitamin D3 to be accessible in the UK. The report notes that lichen-derived D3 is available as an animal-free option. Availability does not establish that a particular product is right for an individual, and the review does not establish an infection-prevention benefit for either supplement form.
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How to read a supplement label
Check the form named on the label: “ergocalciferol” is D2, while “cholecalciferol” is D3. If choosing a plant-based product matters to you, the University of Surrey report describes lichen-derived D3 as an animal-free option. The review does not establish a long-term health-outcome advantage for either form, so its biomarker finding alone is not a reason for everyone to switch.
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The primary study is Brown EI, Darling AL, Robertson TM, et al., “Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials,” Nutrition Reviews, published September 18, 2025, DOI 10.1093/nutrit/nuaf166.
Quick Recap
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