Vitamin D2 may lower vitamin D3 levels: What new research says about the two supplements Vitamin D supplements are often spoken about as though they all work in the same way. But new research suggests there may be an important difference between the two main forms found in supplements. A 2025 analysis found that people taking vitamin D2 had lower levels of vitamin D3 in their blood, raising fresh questions about whether the two forms should be considered interchangeable.The finding, reported by ScienceDaily based on research from the University of Surrey, John Innes Centre and Quadram Institute Bioscience, comes as interest in vitamin D supplementation remains high, particularly in countries where sunlight exposure can be limited during parts of the year.Vitamin D2 and D3 are different forms of the same vitaminVitamin D2, known as ergocalciferol, is commonly produced from fungi or yeast. Vitamin D3, or cholecalciferol, is the form the human body produces naturally when the skin is exposed to ultraviolet B radiation from sunlight.Both forms are converted by the body into compounds that help maintain vitamin D status. Vitamin D is important for calcium absorption and healthy bones, and it also has roles in muscle, nerve and immune function.However, the body does not appear to handle D2 and D3 in exactly the same way. Guidance from the US National Institutes of Health notes that D3 generally raises blood vitamin D levels more effectively and keeps them elevated for longer than D2.Study found a drop in vitamin D3 after D2 supplementationResearchers looked at evidence from randomised controlled trials to investigate whether taking vitamin D2 had any effect on levels of vitamin D3.The analysis included 20 studies, with 11 providing data for the main meta-analysis. Compared with people who did not take vitamin D2, those who received D2 had significantly lower concentrations of 25-hydroxyvitamin D3, the form derived from D3 that is measured in standard blood tests to assess vitamin D status.The average reduction varied depending on the analysis, ranging from roughly 9 to 18 nanomoles per litre.That does not mean vitamin D2 simply removes vitamin D from the body or that taking D2 is inherently dangerous. D2 itself increased levels of its corresponding metabolite and contributed to overall vitamin D status. The unexpected finding was that an increase in D2 appeared to coincide with a reduction in circulating D3.Scientists are still working out why this happensThere is no definitive explanation yet for the drop in vitamin D3.One possibility is that the body responds to changes in vitamin D metabolites by increasing the processes that break them down and remove them. Researchers have suggested that increasing D2 could therefore influence the metabolism or clearance of existing D3-derived compounds.Exactly how this interaction works, and whether it has any meaningful consequences for health, remains unclear.Emily Brown, lead researcher from the University of Surrey, told ScienceDaily that the findings suggest vitamin D3 may be a more beneficial choice for many people, while also stressing the importance of supplementation during periods when sunlight cannot provide enough vitamin D.D3 has also shown different immune effectsThe question becomes more interesting because earlier research has suggested that D2 and D3 may affect the immune system differently.A study led by University of Surrey researchers and published in Frontiers in Immunology examined changes in gene activity following vitamin D supplementation. The researchers reported differences between people taking D2 and those taking D3, including changes related to type I interferon signalling.Type I interferons are proteins involved in the body’s early immune response to infections. The researchers found that D3 appeared to stimulate this signalling pathway, while D2 did not show the same effect.However, this should not be interpreted as evidence that taking vitamin D3 will prevent infections. Changes in immune-related gene activity do not automatically translate into fewer illnesses, and further clinical research is needed to establish whether the difference has a meaningful effect on health.Does this mean D2 should be avoided?Not necessarily.The evidence does suggest that D3 is generally more effective than D2 at raising and maintaining blood vitamin D levels. But D2 is still biologically active and can improve vitamin D status.A separate 2026 review of 26 randomised trials also found biological effects from vitamin D2 supplementation, including changes in parathyroid hormone and blood calcium levels.The bigger question for researchers is whether the difference between D2 and D3 at the biochemical level leads to important differences in long-term health.For people considering a vitamin D supplement, the findings underline why the type of vitamin D on the label matters. D2 and D3 are related forms of the nutrient, but they do not appear to behave identically in the body.Researchers now want to understand that difference more clearly and determine whether D3 should generally be preferred when supplementation is needed. For anyone taking supplements because of a diagnosed deficiency or a specific health condition, the appropriate form and dose should be discussed with a doctor or qualified healthcare professional.
