A commonly used vitamin D supplement may have an unexpected effect on the body, with research suggesting that vitamin D2 can reduce levels of vitamin D3, the form naturally produced when the skin is exposed to sunlight.
Vitamin D2 and D3 are often considered interchangeable because both can improve overall vitamin D levels. However, evidence suggests vitamin D3 is generally more effective at raising and maintaining vitamin D levels in the blood.
The U.S. National Institutes of Health says vitamin D3 tends to increase vitamin D levels more and keep them elevated for longer than D2.
Vitamin D helps the body absorb calcium and maintain healthy bones. It also supports muscle, nerve and immune functions.
A 2025 study published in Nutrition Reviews reviewed 20 studies, with 11 providing data for its main analysis. Researchers found that people taking vitamin D2 had significantly lower levels of 25-hydroxyvitamin D3, a form measured in blood tests to assess vitamin D status.
The average reduction was about 9 to 18 nanomoles per liter, depending on how the results were analyzed.
Researchers stressed that the finding does not mean vitamin D2 is necessarily harmful. D2 itself increases another form of vitamin D and can contribute to overall vitamin D status.
Emily Brown, lead researcher at the University of Surrey, said the findings suggest vitamin D3 may be more beneficial for most people, depending on individual circumstances.
Scientists do not yet know why D2 lowers D3 levels. One theory is that increasing one vitamin D compound may cause the body to break down existing compounds more quickly.
Earlier research has also suggested that D2 and D3 may affect the immune system differently. Vitamin D3 appeared to increase activity linked to type I interferon signaling, which helps the body respond to infections.
However, researchers cautioned that this does not prove D3 prevents infections.
A separate 2026 review of 26 randomized trials found that D2 still has meaningful biological effects, including lowering parathyroid hormone levels and slightly increasing blood calcium.
Researchers say more studies are needed to determine whether differences between D2 and D3 lead to important long-term health effects.
For now, evidence increasingly suggests that D2 and D3 are not completely interchangeable, with D3 appearing more effective at raising and maintaining vitamin D levels.
Source: Science Daily