New research shows vitamin D2 supplements can lower levels of vitamin D3—the form that strengthens the immune system. Scientists say vitamin D3 may be the more effective and protective option for overall health.
Many people take vitamin D supplements to stay healthy during the darker months—but new research suggests that choosing the wrong type might actually weaken your immune system.
A study by scientists from the University of Surrey, the John Innes Centre, and the Quadram Institute Bioscience reveals that vitamin D2 supplements can reduce levels of vitamin D3, the form the body uses most effectively. The findings, published in Nutrition Reviews, raise important questions about which supplement people should prioritize.
D2 vs D3: Not All Vitamin D Is Equal
There are two main types of vitamin D available in supplements:
- Vitamin D2 (ergocalciferol) – usually derived from plants and fungi.
- Vitamin D3 (cholecalciferol) – the form the human body naturally produces from sunlight and uses to support bones, muscles, and the immune system.
Researchers found that people taking vitamin D2 actually experienced a drop in vitamin D3 levels, suggesting that D2 may interfere with the body’s preferred form of the vitamin.
“Vitamin D is essential, especially between October and March when sunlight is limited,” said Dr. Emily Brown, lead researcher from the University of Surrey. “But our analysis shows that vitamin D2 can lower vitamin D3 levels in the body, which is something we didn’t know before. This suggests that vitamin D3 may be the more beneficial option for most people.”
Why Vitamin D3 Matters More for Immunity
Vitamin D3 doesn’t just support bones—it plays a vital role in the body’s first line of immune defense. Previous research by Professor Colin Smith from the University of Surrey found that only vitamin D3—not D2—activates the type I interferon system, which helps fight off bacteria and viruses before they spread.
“We’ve shown that vitamin D3 can strengthen immune responses,” Smith said. “Maintaining healthy levels of D3 could help prevent infections from taking hold.”
A Call for Better Supplementation Policies
Experts argue that these findings highlight a need to make plant-based vitamin D3 more widely available, especially for those who avoid animal-derived supplements.
“Vitamin D deficiency is a major public health concern, particularly in winter,” said Professor Martin Warren of the Quadram Institute. “Ensuring access to the most effective form of vitamin D—whether through supplements or food fortification—is vital for national health.”
The researchers emphasize that while vitamin D2 isn’t harmful, it may not deliver the same immune and health benefits as vitamin D3. Further studies are needed to confirm whether D3 should become the standard supplement recommendation.