Vitamin D2 Supplements May Quietly Deplete Your Body's More Powerful D3

Millions of people take vitamin D supplements to support bone strength and immune health, especially during winter months when sunlight exposure is insufficient. But new research suggests that choosing the wrong form of vitamin D could quietly undermine your body's own vitamin D stores.

A meta-analysis published in Nutrition Reviews, led by researchers at the University of Surrey, John Innes Centre, and Quadram Institute Bioscience, found that vitamin D2 supplementation consistently reduced circulating vitamin D3 levels compared to controls. In many trials reviewed, D3 concentrations in D2 supplement users fell below those seen in people taking no supplement at all — a finding researchers describe as a previously unknown and concerning effect.

The two forms are not interchangeable. Vitamin D3 is the form the body naturally synthesizes from UVB sunlight and uses most efficiently to raise total vitamin D status. Vitamin D2, derived primarily from plant and fungal sources, has long been used in supplements and fortified foods but appears less effective at sustaining vitamin D levels and may actively displace D3.

Beyond blood levels, D3 appears to have a functionally distinct role in immunity. Complementary research published in Frontiers in Immunology showed that D3, but not D2, stimulates type I interferon signaling — a critical early immune response that helps the body detect and fight off bacterial and viral infections. This immune distinction adds another layer of reason to prefer D3.

The practical implication is clear: for most individuals, vitamin D3 supplements are likely the better choice. Researchers also highlighted the need to expand access to plant-based D3 (typically derived from lichen), which would allow vegans and vegetarians to benefit from D3 without relying on animal-sourced options. Further trials are still needed to fully establish whether D3 should become the universal first-line recommendation.