Vitamin D2 supplements may actually lower your body's vitamin D3 levels

Most people assume vitamin D is vitamin D — but a growing body of research says the form you take really does matter. A 2025 meta-analysis published in Nutrition Reviews by researchers from the University of Surrey, John Innes Centre, and Quadram Institute Bioscience found that taking vitamin D2 supplements significantly lowers circulating levels of 25-hydroxyvitamin D3, the D3-derived metabolite doctors measure to assess vitamin D status. The average reduction ranged from roughly 9 to 18 nanomoles per liter.

This doesn't mean D2 is useless — it still raises D2-derived metabolites and contributes to total vitamin D status. But the evidence increasingly favors D3 for producing a larger, longer-lasting boost. A separate 2026 systematic review confirmed D2 retains meaningful biological effects (reducing parathyroid hormone and slightly raising blood calcium), yet prior meta-analyses consistently show D3 outperforms D2 at improving overall vitamin D levels. Plant-based D3 derived from lichen is now widely available, making it accessible even for vegans.

Key Takeaways

• D3, but not D2, appears to stimulate type I interferon signaling — a frontline immune defense against viruses and bacteria
• The mechanism behind D2 lowering D3 is still unclear; one theory is that rising D2 metabolites accelerate the breakdown of D3
• Larger clinical trials are needed to determine whether these biochemical differences translate into real-world health outcome differences

Why it matters: Vitamin D deficiency is a significant public health concern, especially during winter months. If D2 and D3 aren't metabolically interchangeable — and evidence suggests they aren't — clinicians and patients may want to reconsider which form they're recommending or reaching for on the shelf.