If you have already heard how vitamin D can optimize your immunity but are taking the wrong form of this multi-function vitamin, you may not be seeing the benefits promised on the bottle. Not everything that claims to optimize actually does, as D2 shows. Research out of England strongly suggests that the D2 form of Vitamin D may actually lower your active vitamin (D3) levels rather than raise them, throttling the boost to your immune system.
The Differences
If the D3 (cholecalciferol) version is what our bodies make, then why not just take this form? D3 even has a longer duration of action in the body and a longer shelf-life in the store. Well, the D2 form comes from plant or fungi sources, so for some it is a vegetarian or meat allergy issue.
Now, though, we can say with confidence that D2 is also not able to raise the blood levels as much as D3 and that its immune benefits are much smaller. In this study, the D2 version (ergocalciferol) even lowered the active D3 form in some (compared to those not taking it). For some people, taking D2 may actually be worse than taking no Vitamin D at all.
Direct Immune Effects
The researchers also noted that the activation of the type 1 interferon system in our immune cascade was lower for those taking D2 versus those taking D3. With less activity in this arm of the immune system, we could see weaker resistance to viral infections.
In a day when most health care providers are busy keeping up with charts and insurance contract requirements, they may not know the difference between D2 and D3, much less the potential futility of using D2 in this research. Do your own research, and ask questions. At Sanctuary, helping our patients achieve healthier, more abundant lives requires taking the time to answer patient’s questions. We want you to become an expert in your health choices.
Original Article:
Emily I G Brown, Andrea L Darling, Tracey M Robertson, Kathryn H Hart, Jie Li, Cathie Martin, Martin J Warren, Colin P Smith, Susan A Lanham-New, Ruan M Elliott. Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrition Reviews, 2025; DOI: 10.1093/nutrit/nuaf166
Thanks to Science Daily:
University of Surrey. “Scientists warn popular vitamin D supplement may have a “previously unknown” downside.” ScienceDaily. ScienceDaily, 23 May 2026. <www.sciencedaily.com/releases/2026/05/260523021820.htm>.
Sanctuary Functional Medicine, under the direction of Dr Eric Potter, IFMCP MD, provides functional medicine services to Nashville, Middle Tennessee and beyond. We frequently treat patients from Kentucky, Alabama, Mississippi, Georgia, Ohio, Indiana, and more... offering the hope of healthier more abundant lives to those with chronic illness.
Dr. Eric Potter graduated from Vanderbilt Medical School and then went on to specialize in internal medicine (adult) and pediatric care, spending significant time and effort in growing his medical understanding while caring for patients from all walks of life.
