You see a headline like “Common Vitamin D Supplement Has Previously Unknown Negative Effect”, and your alarm bells go off. After all, we’re so used to hearing that vitamin D is good for you. Specifically for your bones, immune support and heart. But as it often goes in science, reality is messier.
A new meta-analysis just published in Nutrition Reviews is causing ripples because researchers are saying that vitamin D2 (also known as ergocalciferol) may actually lower levels of vitamin D3 (cholecalciferol) in the body. In some cases, the D3 levels dropped even below levels seen in control groups!
Surprise! There are TWO Types of Vitamin D
Let me walk you through what this means, and what you need to do. First: a refresher. There are two main supplement forms of vitamin D. There is D2 (ergocalciferol) which is used to fortify foods, and used in prescribed vitamin D drugs. Then there’s vitamin D3 (cholecalciferol), this is the natural form our skin produces in response to sunlight. It’s also available at health food stores and online in supplement form.
In the US, when a doctor prescribes “vitamin D,” it’s written as ergocalciferol (D₂) in a 50,000 IU and taken as a capsule once a week. This is the legacy form that got FDA approval decades ago, so it’s still the standard in most pharmacies. D₃, even though it’s more effective for raising blood levels, is usually sold over-the-counter and not covered by insurance. That’s why so many patients end up on prescription D₂ — not because it’s better, but simply because it’s the form that’s officially “on the books.”
Traditionally, they’ve been treated somewhat interchangeably in public health and I don’t know why. There’s a “take either it’s fine” attitude, especially with physicians. But this new meta-analysis upends that simplifying assumption.
There was a paper written in 2006 (which is when I first told my readers of this) entitled, The Case Against Vitamin D2 as a Vitamin Supplement.
The challenge with a title like this is people don’t realize what D2 is… and they just stop taking their vitamin D supplement. That’s the problem. D3 is not D2. And D2 is what doctors prescribe.
For decades, doctors assumed vitamin D2 and D3 were basically twins, so one “unit” of D2 was thought to work just like one “unit” of D3. But newer research shows that’s not the case. D2 is less stable, it doesn’t bind as tightly to your vitamin D–binding protein, and it’s less effective at raising your blood levels of vitamin D.
In fact, a well-known clinical review argued that D2 should probably be retired as a supplement altogether because it simply doesn’t perform as well as D3. That doesn’t mean D2 is useless, but if you’re looking to truly optimize your vitamin D status, D3 is the form your body recognizes and uses more efficiently.So in short, D2 is weaker and less reliable, while D3 is the form your body knows best and uses most efficiently. It’s also the form of D that is in my supplement HERE, and many others sold nationwide at health food stores!
What the Study Found
The key finding: taking D2 appears to suppress or displace D3 levels. So if you’re taking prescription vitamin D (often found in 50,000 IU doses), you might inadvertently push down a more biologically useful form of vitamin D, which ironically you could have just bought from the health food store or gotten from lying out in the sun.
More importantly. Vitamin D3 is not just passive filler. The paper (and related earlier research) suggests D3 has specific immune-modulating effects that D2 doesn’t have, particularly in activating certain interferon pathways and supporting defense against viral or bacterial threats. In short, D3 works better.
D2 is NOT Interchangeable with Natural D3
So: they’re not interchangeable. They play different roles, and D2 might even undermine the other natural form D3.
We already live in a world where vitamin D deficiency is common, particularly during months with low sun exposure. Now imagine someone picks up a D2 supplement (maybe cheaper, or because it’s prescribed), thinking “it’s just vitamin D.” But instead of boosting total vitamin D status, they’re suppressing the more effective D3 form. That could mean lower immunity, less bone benefit, and weaker muscles.
This whole story reminds me of what happens with folic acid supplements. It’s the same story where they don’t work because they’re inactive until they’re converted to methylfolate (which is available now from high-quality supplement formulators). I have more information about all of this at my website, suzycohen.com.
The point of this meta-analysis is clear, but don’t panic. Yes, D2 seems to suppress D3, but the impact may vary with dose, duration, your starting vitamin D level, or your unique metabolism. We also don’t yet know exactly how much this reduction changes real-world health outcomes. And remember, very high doses prescribed for medical reasons are a different situation.
Here’s What You Should Do – 5 Important Tasks
Here are my considered suggestions. Take with a grain of salt (or 15 minutes of sunshine):
- Check what form of vitamin D you are taking (on your bottle). If it says D2 or “ergocalciferol,” consider switching to D3 (cholecalciferol). The D3 can be vegan or sheep-derived, that part doesn’t matter so long as it says “D3.”
Most of the time you’ll hear that vitamin D2 is “plant-based” and vitamin D3 is “animal-based.” That used to be true when D3 came only from lanolin in sheep’s wool. But today, things have changed. Scientists now extract a clean, vegan form of D3 from lichen and certain algae.
This means you can get all the potency and stability of D3 — the form your body actually makes from sunlight — without it being animal-derived. So if you’re vegan, you don’t have to settle for the less effective D2 anymore. You can buy THIS VEGAN D3 if you want to support my brand.
- Monitor your vitamin D levels (25-hydroxyvitamin D total) via blood test, ideally before and 90 days after changing regimens.
- Don’t overdo it with high doses. Too much can cause cardiac rhythm abnormalities due to the fluctuations in calcium, magnesium and K2.
- Talk with your practitioner. If they’ve prescribed D2, this new research is worth bringing up. You can buy natural D3 over-the-counter at any health food store and online. Because it’s more active, you’ll be able to take much lower dosages compared to the 50,000 IU D2 that’s usually prescribed.
- Find out if you take a drug mugger. Go to drugmuggers.com and type in your medication. I’ve built a database of information that is impossible to find otherwise. See if vitamin D is quietly drained from your body due to a drug-nutrient depletion. My website will tell you exactly what nutrients are depleted by the medications you take. Save the site in case you start something new. CLICK HERE to get started.
This isn’t just a minor tweak in understanding. It’s a reminder that biology is rarely simple, and even nutrients we think of as “safe” can have surprises. The idea that all vitamin D is equal is under challenge. If you’re serious about optimizing your health, it’s worth digging into the details and acknowledging that the form of a molecule can change everything.
Suzy Cohen, has been a licensed pharmacist for over 30 years and believes the best approach to chronic illness is a combination of natural medicine and conventional. She founded her own dietary supplement company specializing in custom-formulas, some of which have patents. With a special focus on functional medicine, thyroid health and drug nutrient depletion, Suzy is the author of several related books including Thyroid Healthy, Drug Muggers, Diabetes Without Drugs, and a nationally syndicated column.
