People who took vitamin D2 supplements in randomized trials ended up with lower blood levels of vitamin D3 than people who took none, by roughly 9 to 18 nanomoles per liter depending on how the data were pooled. The drop comes from a meta-analysis of 20 reviewed trials, 11 of which went into the main calculation, and it applies to one specific form of the vitamin: ergocalciferol, the plant- and yeast-derived D2.
Emily Brown, a PhD research fellow at the University of Surrey, led the work and put the finding in plain terms: vitamin D2 supplements can actually decrease levels of vitamin D3 in the body. The analysis appeared in the journal Nutrition Reviews in 2025 and was recapped again this week, which is why the warning is circulating anew.
Ergocalciferol and the 25-hydroxyvitamin D3 drop
The Surrey-led team pooled randomized controlled trials that compared D2 supplementation with no D2 and tracked 25-hydroxyvitamin D3, the circulating form that clinicians use as the marker of D3 status. According to the summary of the paper, people given D2 had significantly lower D3 than controls, and the average reduction ran from about 9 to about 18 nmol/L depending on the analysis used. The University of Surrey’s own news release adds that in many of the included studies D3 fell below the level seen in the control group, not merely failing to rise.
The scale of that change matters against the clinical thresholds. The National Institutes of Health’s vitamin D fact sheet describes total 25-hydroxyvitamin D under 30 nmol/L as a risk of deficiency, 30 to under 50 nmol/L as potentially inadequate, and 50 nmol/L or more as sufficient for most people. A 9 to 18 nmol/L fall in one component is a meaningful slice of that range, although the same sheet cautions that results vary by assay, antibody-based or chromatography-based, and by laboratory, so a single reading can come out falsely high or low, and that serum 25-hydroxyvitamin D has a circulating half-life of about 15 days, which is why one result is a snapshot rather than a verdict on long-term status for any individual patient.
Parathyroid hormone, calcium and the D2 marker
None of this means D2 strips the body of vitamin D. D2 raises its own marker, 25-hydroxyvitamin D2, and the Surrey summary points to a second 2026 Nutrition Reviews analysis of 26 randomized trials in which D2 lowered parathyroid hormone and slightly raised blood calcium, evidence that the compound remains biologically active. The NIH sheet likewise says both D2 and D3 raise serum 25-hydroxyvitamin D and both cure rickets.
The mechanism is unsettled. The summary mentions one proposal, that higher D2 speeds the breakdown of metabolites derived from existing D3, and says the clinical meaning of the lower D3 is unknown.
The case made for D3 supplements
The same NIH fact sheet states that D3 raises serum 25-hydroxyvitamin D to a greater extent than D2, and the new meta-analysis adds a reason beyond potency. Brown said vitamin D3 supplements may be more beneficial than D2 for most individuals, with personal considerations applying. The summary cites a separate analysis of 24 studies that also judged D3 more effective than D2 at improving overall vitamin D status.
Prof. Colin Smith, also of Surrey, linked D3 to immunity through a separate 2022 Frontiers in Immunology study of blood gene activity, in which D3 but not D2 appeared to stimulate the type 1 interferon signalling system. That work was not part of the meta-analysis, and the summary stresses it does not prove D3 prevents infections. Smith said a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold.
The plant-based supply problem
D2 is made by irradiating yeast ergosterol, which is why it is the form usually sold as vegan, while D3 is typically produced from sheep’s wool lanolin, the NIH sheet explains, with a lichen-derived vegan D3 also existing. That is the context for the comment from Prof. Cathie Martin of the John Innes Centre, quoted in the Quadram Institute announcement: the meta-analysis stresses the importance of ensuring plant-based vitamin D3 is accessible in the UK.
Prof. Martin Warren, chief scientific officer at the Quadram Institute, framed it as a winter public health matter, noting that vitamin D deficiency is a significant concern in the UK. The NHS advises everyone to consider a daily supplement of 10 micrograms during autumn and winter, since sunlight between October and early March is too weak for the body to make enough, though its page does not distinguish between D2 and D3. Anyone choosing between the two forms is left reading the product label, and the Surrey authors have not yet shown what the lower D3 means for bone or immune outcomes.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
More from Morning Overview
- Regulators cleared the first U.S. small modular reactor, 4 months early
- Lake Powell sank to a record 3,517 feet, nearing the level that stops Glen Canyon Dam’s turbines
- Seven of Earth’s nine planetary boundaries are breached, and all seven are worsening
- The NSA says three phone features should be off whenever you aren’t using them
