Why vitamin D2 may lower vitamin D3 levels

A new meta-analysis by Brown et al. 2025 in Nutrition Reviews suggests the vitamin D2 versus D3 debate is not just about which form raises blood levels faster. More striking, D2 supplementation may also lower circulating 25-hydroxyvitamin D3, the metabolite many clinicians treat as the more dependable marker of vitamin D3 status. Suddenly, a familiar supplement-label distinction becomes a more practical question: when people buy ergocalciferol instead of cholecalciferol, are they merely choosing a weaker option, or a form that changes the balance of vitamin D metabolites in a less helpful direction?

For years, the market has treated the two forms as near substitutes. D2 had a clean narrative. As a supplement form, it could raise vitamin D status, it appeared in fortified foods and prescriptions, and for vegan readers it often looked like the obvious alternative to animal-derived D3. Earlier evidence already complicated that story. A 2012 systematic review and meta-analysis by Tripkovic et al. in the American Journal of Clinical Nutrition found D3 produced a greater rise in total 25-hydroxyvitamin D than D2, and a 2024 daily-dose meta-analysis still found D2 lagged even when supplementation was given in the steadier, less dramatic daily pattern. Brown’s paper, though, pushes the argument one step further. More specifically, it suggests the downside of D2 is not only underperformance, but displacement.

That distinction matters because Vitalspell readers are not usually asking whether vitamin D exists. Instead, they are asking whether the specific capsule in front of them is the right one. Brown’s group does not prove that D2 is useless, nor does it show that everyone taking D2 is being harmed. What the paper does do is narrow the room for calling D2 and D3 interchangeable. For a site that covers formulation as seriously as ingredients, that is the real news.

What the meta-analysis adds

Brown and colleagues looked specifically at randomized controlled trials that could test whether vitamin D2 changes serum 25(OH)D3. Instead of asking only which form lifts total vitamin D higher, they isolated the question that had been sitting in the background of earlier comparisons: what happens to the body’s D3 pool when D2 enters the picture? Cautious but clear, their answer was the same across the eligible trials: D2 supplementation reduced 25(OH)D3 relative to control conditions.

Not just weaker, then. Potentially subtractive. Seen against the older literature, that framing does not arrive from nowhere. Back in 2012, the Tripkovic meta-analysis estimated that D3 raised 25(OH)D more effectively overall, with the biggest spread in bolus dosing. In the newer daily-dose review, the gap narrowed under everyday supplementation but still did not disappear, with D2 coming in 10.39 nmol/L lower for total 25(OH)D. Brown’s contribution is to say the lost ground may partly reflect a drop in D3 itself, not simply a smaller boost from D2.

In comments carried by a university summary published via ScienceDaily, lead researcher Emily Brown put the implication plainly.

“Vitamin D2 supplements can actually decrease levels of vitamin D3 in the body… D3 supplements may be more beneficial for most individuals over vitamin D2.”

— Emily Brown, via ScienceDaily’s summary of the study

Even so, the paper does not justify the cartoon version of the story. In many settings, D2 still raises vitamin D metabolites, still appears in prescribed formulations, and still matters in fortified foods. Narrower, and more useful, is the stronger claim: if the goal is to preserve or build vitamin D status as efficiently as possible, the evidence base keeps leaning toward D3.

Why dose pattern still matters

One reason the D2 versus D3 literature has stayed muddled is that dosing schedules are not interchangeable. Some studies use large intermittent boluses. Others use daily capsules closer to what readers buy over the counter. That detail alone changes the size of the gap. In Tripkovic et al. 2012, bolus dosing amplified D3’s advantage. In the 2024 daily-dose analysis, the difference looked smaller, but it was still there.

Readers should resist turning that into a dosing tutorial. This is not a case for self-prescribing bigger amounts of anything. Rather, it is a reminder that form and schedule interact. A weekly or monthly prescription built around D2 is not biologically equivalent to a daily D3 softgel just because both sit under the vitamin D umbrella. That helps explain why two people can say they are “taking vitamin D” while getting meaningfully different outcomes in blood work.

Policy context slips in here too. The UK recommendation cited in the Surrey release remains 10 micrograms a day through autumn and winter for many adults. But population guidance usually talks about amount, not form. Brown’s meta-analysis suggests that split may matter more than consumer advice has often acknowledged. Quantity is only part of the formulation story.

The immune signal is interesting, not settled

Mechanism is where the D2 story becomes more interesting, and easier to overread. A 2022 paper by Durrant et al. in Frontiers in Immunology found that vitamins D2 and D3 had overlapping but distinct effects on blood transcriptome markers, with D3 appearing to stimulate type I interferon signalling and D2 not showing the same pattern. That does not mean D3 has been proven to prevent infections better in real life. It does mean the two forms may not be acting as simple plug-in replacements at the immune-signalling level.

Colin Smith, one of the Surrey researchers, leaned on exactly that point in the same ScienceDaily summary.

“We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signalling system in the body.”

— Colin Smith, via ScienceDaily’s summary of the study

Interesting is not the same as outcome-proven. Transcriptome data can clarify mechanism long before it settles clinical practice. No one should read the interferon finding and jump straight to claims that D3 is an antiviral supplement or that D2 leaves people immunologically exposed. Vitalspell’s standard here is stricter. Mechanistic work earns attention when it helps explain a human trial pattern. It does not automatically cash out into a recommendation on its own.

Still, the mechanistic angle makes Brown’s new review harder to dismiss as a minor blood-test curiosity. If D2 lowers D3 status and D3 also seems to have distinct signalling effects, the case for treating the forms as materially different becomes broader than a single lab value.

Why the plant-based defence looks weaker now

Historically, D2’s clearest practical advantage was dietary. It offered an easy answer for vegetarians and vegans who wanted to avoid lanolin-derived D3. That argument is less secure than it once was because lichen-derived, plant-based D3 products are now widely available in the supplement market. Readers do not need to choose between efficacy and dietary preference in the way they often did a decade ago.

Cathie Martin, the John Innes Centre scientist quoted in the Surrey write-up, pointed directly at that implication.

“This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK.”

— Cathie Martin, via ScienceDaily’s summary of the study

Access, price, and local regulation still matter. Not every shopper will find vegan D3 in the same aisle as cheap ergocalciferol, and not every clinician will switch a prescription simply because the formulation literature has shifted. But the old ethical defence of D2 as the only realistic non-animal option is plainly weaker now. Once plant-based D3 exists at scale, the argument returns to evidence.

What changes for readers now

For an over-the-counter buyer with no special reason to choose ergocalciferol, the present literature increasingly favours D3 as the default form for maintaining vitamin D status. That is the throughline connecting Tripkovic et al. 2012, the 2024 daily-dose synthesis, and now Brown et al. 2025. Different papers ask slightly different questions, but they keep landing in the same neighbourhood.

If a clinician has prescribed D2, that is not a cue to improvise a switch after reading one article. It is a cue to ask a more precise question at the next appointment: why this form, at this schedule, for this goal? The answer may still favour D2 in some contexts. Brown’s meta-analysis does not erase clinical nuance. It simply makes the burden of proof for calling D2 and D3 interchangeable much harder to meet.

Supplement marketing has long treated vitamin D as a category first and a formulation question second. This paper, by contrast, suggests that order should probably be reversed. Next comes the harder work of research on clinical endpoints, not just metabolite shifts. Until then, the safest reading is also the least flashy one: vitamin D2 may work differently enough from vitamin D3 that the label on the bottle deserves more scrutiny than it usually gets.

References

1. Brown EIG, 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. 2025. DOI link
2. Durrant LR, Bucca G, Hesketh A, et al. Vitamins D2 and D3 have overlapping but different effects on the human immune system revealed through analysis of the blood transcriptome. Frontiers in Immunology. 2022. Paper link
3. Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition. 97(6):1357-1364. 2012. PubMed
4. Comparison of the effect of daily vitamin D2 and vitamin D3 supplementation on serum 25-hydroxyvitamin D concentration. PMC. 2024. Full text