Scientists warn a popular vitamin D supplement may have a hidden downside
Not all vitamin D supplements are created equal, and new research suggests that one popular form may have an unexpected downside. Scientists found that taking vitamin D2 can actually lower the body's levels of vitamin D3, the form naturally produced by sunlight and more effective at boosting overall vitamin D. Previous studies also suggest that vitamin D3 may provide stronger immune protection against viruses and…
Key points
- A 2025 study found vitamin D2 supplements can reduce levels of vitamin D3 in the body.
- The meta-analysis of 20 trials showed an average reduction of about 9 to 18 nanomoles per liter of 25-hydroxyvitamin D3.
- Researchers say vitamin D3 may be more beneficial for most people, but more studies are needed on health outcomes.
Research published in 2025 has found that a popular vitamin D supplement may have an unexpected effect: taking vitamin D2 can lower levels of vitamin D3, the form the body naturally produces from sunlight. The study, published in Nutrition Reviews by researchers from the University of Surrey, John Innes Centre and Quadram Institute Bioscience, combined evidence from randomized controlled trials.
The analysis reviewed 20 studies, with 11 providing data for the main meta-analysis. Compared with people who did not receive vitamin D2, those taking D2 had significantly lower concentrations of 25-hydroxyvitamin D3, the form derived from vitamin D3. The average reduction was roughly 9 to 18 nanomoles per liter, depending on how results were analyzed.
What researchers say
Emily Brown, PhD Research Fellow and Lead Researcher from the University of Surrey, said the discovery is a previously unknown effect. She noted that vitamin D supplements are important, especially between October and March in the UK when the body cannot make vitamin D from sunlight, but suggested that vitamin D3 may be more beneficial for most individuals over vitamin D2.
Professor Cathie Martin, Group Leader at the John Innes Centre, said the meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK. Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, said tackling vitamin D deficiency with the most effective form of supplementation or fortification is of the utmost importance to the health of the nation.
Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK. However, we discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements.
— Emily Brown
Immune and other effects
A separate study in Frontiers in Immunology led by Professor Colin Smith from the University of Surrey found substantial differences in gene activity in the blood after D2 and D3 supplementation. Vitamin D3 appeared to stimulate type I interferon signaling, while vitamin D2 did not. Professor Smith said vitamin D3, but not D2, appears to stimulate this key part of the immune system that provides a first line of defense against bacteria and viruses.
A 2026 systematic review and meta-analysis of 26 randomized trials involving vitamin D2 found that D2 supplementation significantly reduced parathyroid hormone levels and produced a small increase in blood calcium. This shows D2 continues to have meaningful biological effects even though it may reduce D3. Current guidance from the U.S. National Institutes of Health states that both D2 and D3 can increase overall vitamin D levels, but evidence favors D3 for raising and maintaining those levels.
