Taking Vitamin D2? It May Be Lowering Your D3 Levels, Review of Trials Finds

As the days shorten, vitamin D goes back onto a lot of shopping lists. Most people grab whatever bottle is on the shelf, on the assumption that vitamin D is vitamin D. Research from the University of Surrey, back in the news this week, suggests the form on the label matters more than that. In randomized trials, people given vitamin D2 ended up with less vitamin D3 in their blood than people who took nothing.

Two Forms, One Name

"Vitamin D" on a label usually means one of two molecules. D3 (cholecalciferol) is the form your skin makes in sunlight, and it's found in oily fish, egg yolks and most standard supplements. D2 (ergocalciferol) comes from plants and fungi, such as UV-exposed mushrooms. It's common in prescription high-dose products, many fortified foods and some vegan supplements.

Both are converted in the liver into 25-hydroxyvitamin D, the marker your doctor checks in a blood test. Standard tests usually report a single total, so a fall in one form can be hidden if the other rises.

What the Review Found

The team, led by Emily Brown with Professor Susan Lanham-New and colleagues from Surrey, the John Innes Centre and the Quadram Institute, searched nearly five decades of trials for a narrow question: what happens to D3 status when people take D2? Twenty randomized controlled trials met their criteria, and 11 had usable data for the pooled analysis, covering 655 participants. Doses ranged widely, from 300 IU to single 50,000 IU boluses, over periods from a few weeks to around six months. The paper appeared in Nutrition Reviews.

The result was consistent in direction. By the end of the trials, people in the D2 groups had D3-derived vitamin D levels about 18 nmol/L lower than controls. Measured as change from each person's own starting point, the gap was about 9 nmol/L. For scale, many guidelines treat 50 nmol/L as the line for adequate status, so a 9–18 point drop is not trivial for someone already near that line.

The researchers don't yet know why this happens. Their leading idea is that when D2 floods the system, the body speeds up the enzymes that break down and clear vitamin D metabolites, and those enzymes don't pick and choose, so D3 is cleared along with D2. A separate study from the same Surrey group, in Frontiers in Immunology, adds a second wrinkle. D3 appeared to switch on type I interferon signalling, part of the body's early antiviral defence, while D2 did not.

D2 still does something. A separate 2026 meta-analysis of 26 trials by Junmei Lai and colleagues, also in Nutrition Reviews, found D2 lowered parathyroid hormone and nudged blood calcium up slightly, which are both expected effects of vitamin D. So the finding isn't that D2 is useless. It's that the two forms aren't interchangeable, and D2 may cost you some of the D3 you already have.

How This Fits With Our August Coverage

If you read our post on vitamin D and cognition, you may remember that the Emory study found D2 and D3 users scored about the same on a cognitive test. The two findings don't really conflict. That study was small (54 adults), observational and based on self-reported supplement use, and it measured test scores rather than blood levels. The Surrey review pools randomized trials and measures the blood marker directly. Its evidence on what D2 does to D3 status is stronger, but it says nothing about focus or memory.

What This Review Can't Tell You

The limits are real. The pooled trials varied a lot in dose, length and population, and the statistical heterogeneity was high (I² of 69–83%), so the exact size of the drop is uncertain even though its direction held up when each study was removed in turn. The authors didn't formally test for publication bias. Most importantly, nobody has yet shown that this drop in D3 leads to worse health outcomes, such as more infections, weaker bones or poorer cognition. The immune gene findings point that way but aren't proof, and larger clinical trials are needed. Also worth noting: the main review was first published online in September 2025. It's newsworthy now because it's being widely shared again at the start of the darker months, not because the data are new.

What to Do With This

The practical step is small: read the label. If your supplement or fortified milk alternative says ergocalciferol or D2, and you have no specific reason to use it, D3 is the better-supported choice. Earlier meta-analyses already found D3 raises and sustains overall vitamin D status more effectively, and this review adds a second reason. If you avoid animal products, lichen-derived D3 is a vegan option. UK guidance suggests 10 micrograms (400 IU) a day from about October to March, and many other countries recommend similar amounts. If you've been prescribed high-dose D2 by a doctor, don't switch on your own. Ask whether D3 would work for you, and whether a follow-up blood test should look at D2 and D3 separately.

Vitamin D is a background factor for brain health, not a focus booster you'll feel by Friday. Getting the basics right this winter, including daylight exposure when you can, steady sleep and a consistent work rhythm (our Focus Timer helps with that), does more than any single capsule.

• D2 lowered D3 in trials. Across 11 randomized trials (655 people), those given vitamin D2 ended with roughly 9–18 nmol/L less D3-derived vitamin D than controls.
• The mechanism is unconfirmed. The leading theory is that extra D2 speeds up the breakdown of all vitamin D metabolites, including D3.
• D3 may matter for immunity. A related Surrey study found D3, but not D2, activated early antiviral gene signalling. That hasn't yet been shown to mean fewer infections.
• D2 isn't useless. It still lowers parathyroid hormone and affects calcium, so it has real biological effects.
• Check your label. Unless you have a specific reason, choose D3 (cholecalciferol). Vegans can use lichen-derived D3. Talk to your doctor before changing a prescribed dose.

This isn't a reason to stop taking vitamin D. It's a reason to check which form you take, since the two aren't as interchangeable as most labels suggest.

Frequently Asked Questions

Health disclaimer: This article summarizes published research and is for educational purposes only. It is not medical advice. If you have a medical condition, take prescribed vitamin D or other medication, or are unsure about your vitamin D status, talk with a qualified healthcare professional before changing your supplements.

FocusWaveHub Editorial Team

Editorial & Research Team

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