When was the last time you flipped your vitamin D bottle over and checked which kind was inside? Maybe never. A vitamin is a vitamin, right?
A meta-analysis out of the University of Surrey will make you want to look. If the vitamin D you’re taking is D2, it might be lowering the D3 you already have.
I’ll give you a quick refresher on the two forms. Vitamin D3, cholecalciferol, is what your skin makes when UVB light hits it. You also get it from fatty fish, egg yolks, and most good supplements. Vitamin D2, ergocalciferol, comes from fungi and yeast that sit under UV light. Your liver turns both into 25-hydroxyvitamin D, the number your doctor sees on a blood test. D3 becomes 25(OH)D3. D2 becomes 25(OH)D2.
For years doctors and supplement companies treated the two as interchangeable. The standard blood test doesn’t even separate them. It reports a total, which tells you whether you have enough vitamin D overall but not which form.
The Surrey team pulled together every randomized controlled trial they could find that gave people D2 and measured what happened to their D3. Doses ranged from 300 IU all the way up to 50,000 IU, and the trials were anywhere from 3 to 25 weeks. In 18 of the 20, D3 levels dropped after people took D2. In 16 of those, D3 fell further than it did in people who got a placebo or no supplement at all.
Eleven of the studies had clean enough data to combine, covering 655 people. By the end of the trials, the people taking D2 had about 18 nmol/L less D3 in their blood than the people who weren’t. In the units most US labs use, that’s around 7 ng/mL. If you look at how much each person’s D3 changed from where they started, the drop comes out smaller, about 3.7 ng/mL. Either way, your vitamin D supplement shouldn’t be lowering your vitamin D3 levels.
D2 still raises your total vitamin D, because it adds its own D2 to the pile. So your lab report can look fine while the form your body evolved to run on decreases. Kind of like getting a raise the same week your landlord bumps the rent.
The authors have a theory as to why this happens. When vitamin D levels rise, your body ramps up an enzyme called CYP24A1 that breaks vitamin D down, and that cleanup crew might take some of your D3 with it. It makes sense, but nobody has proven it yet.
To be fair to D2, it still does something. A 2026 meta-analysis of 26 trials found D2 lowered parathyroid hormone and increased blood calcium a little. And nobody has shown yet that lower D3 with normal total vitamin D leads to worse health. The authors say that plainly.
I still pick D3, and this study isn’t even my main reason. Researchers have found for years that D3 raises total vitamin D more than D2 does and keeps it up longer. A 2012 meta-analysis of randomized trials in the American Journal of Clinical Nutrition concluded D3 does a better job of raising your levels. The Surrey finding just puts another point on D3’s side of the scoreboard.
There’s an interesting immune angle too. In a separate Surrey trial, D3 changed immune gene activity in ways D2 didn’t, including turning up genes tied to interferon signaling, part of your first line of defense against viruses.
When one form raises your levels better, lasts longer, and doesn’t drag down what you already have, the choice gets easy.
Here’s what’s coming up:
The prescription vitamin D a lot of US doctors write that’s usually D2, and where else D2 hides
The blood test that splits your D2 from your D3
The vitamin D level I recommend
Why I don’t take D3 by itself
The vitamin D supplement I take
