Common Vitamin D Supplementation May Lower Vitamin D3 Levels

Recent research conducted by the University of Surrey, in collaboration with the John Innes Center and Quadram Institute Bioscience, reveals that taking vitamin D2 supplements can decrease the body's levels of the more effective form of vitamin D, vitamin D3. This finding is significant because vitamin D3, which our bodies primarily produce from sunlight, plays a crucial role in supporting immune function and maintaining bone health.

Typically, many people supplement with vitamin D to meet the UK government recommendation of 10 micrograms (µg) daily, especially during the winter months when sunlight exposure is limited. However, the study highlights that while there are two common forms of vitamin D supplements—D2 and D3—only vitamin D3 may provide the most benefit for increasing overall vitamin D status.

The research, published in Nutrition Reviews, analyzed data from multiple randomized controlled trials. The results showed that vitamin D2 supplementation was associated with a reduction in circulating vitamin D3 levels, sometimes even dropping below levels observed in control groups that did not take any vitamin D supplements. This is a crucial insight, as it suggests that vitamin D2 might counteract some of the benefits of supplementation.

Emily Brown, Ph.D., a lead researcher from the University of Surrey, emphasized the importance of the findings: "Vitamin D supplements are particularly vital between October and March in the UK when sunlight-induced synthesis is not possible. Our study indicates that vitamin D2 supplements could unintentionally reduce vitamin D3 levels, which are more effective and important for overall health. Therefore, individuals might benefit more from vitamin D3 supplements, tailored to their personal needs."

Further insights from the research point to the immune benefits associated with vitamin D3. Professor Colin Smith from the University of Surrey highlighted that vitamin D3 uniquely stimulates the immune system's type I interferon signaling, which provides a first line of defense against bacteria and viruses. This suggests that maintaining sufficient vitamin D3 levels could help prevent infections.

Professor Cathie Martin from the John Innes Center also noted the importance of making plant-based vitamin D3 accessible in the UK to combat deficiency. Overall, the study underscores the need for public health strategies to focus on supplying the most effective forms of vitamin D, particularly during months of limited sunlight, to support overall health and immune resilience.

For further details, the full study can be accessed in Frontiers in Immunology (Brown et al., 2025).