By Hendra Lukman 
A groundbreaking study published in 2025 has unveiled a surprising interaction within the human body concerning vitamin D supplements. Research indicates that vitamin D2, a commonly available form, can paradoxically lead to a reduction in vitamin D3 levels, the form naturally synthesized by the skin upon sun exposure. This distinction is critical, as D2 and D3 are often perceived as interchangeable sources of vitamin D, yet emerging evidence suggests a significant difference in their efficacy.
Vitamin D: An Essential Nutrient with Two Primary Forms
Vitamin D plays a pivotal role in numerous bodily functions, most notably in facilitating calcium absorption, which is crucial for maintaining strong bones and preventing conditions like osteoporosis. Beyond skeletal health, vitamin D is implicated in the proper functioning of muscles, nerves, and the immune system. In regions with limited sunlight, such as the United Kingdom during its darker months, public health bodies like the National Health Service (NHS) recommend daily supplementation of 10 micrograms (10 µg) of vitamin D to ensure adequate levels.
The two primary forms of vitamin D are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is typically derived from fungi or yeast exposed to ultraviolet light, while vitamin D3 is produced endogenously in human skin when exposed to ultraviolet B (UVB) radiation from the sun. Supplements historically have utilized both forms. For vitamin D3, a traditional source has been lanolin, extracted from sheep’s wool. However, advancements in biotechnology have led to the development of animal-free vitamin D3 derived from sources like lichen, offering a vital alternative for individuals adhering to vegan or plant-based diets.
Upon ingestion or synthesis, both vitamin D2 and D3 undergo a two-step metabolic process. In the liver, they are converted into 25-hydroxyvitamin D, the primary circulating form of vitamin D and the one commonly measured in blood tests to assess an individual’s vitamin D status. Specifically, vitamin D2 is converted to 25-hydroxyvitamin D2, and vitamin D3 is converted to 25-hydroxyvitamin D3. It is this D3-derived metabolite that the recent research highlights as being potentially reduced by vitamin D2 supplementation.
The Unexpected Discovery: Vitamin D2’s Impact on Vitamin D3
The 2025 study, published in the esteemed journal Nutrition Reviews, was a collaborative effort by researchers from the University of Surrey, the John Innes Centre, and Quadram Institute Bioscience. This meta-analysis meticulously reviewed evidence from randomized controlled trials to quantify the effect of vitamin D2 supplementation on vitamin D metabolite levels.
The researchers analyzed data from 20 studies, with 11 providing sufficient data for the core meta-analysis. The findings were significant: individuals who received vitamin D2 supplementation exhibited considerably lower concentrations of 25-hydroxyvitamin D3 compared to those who did not take vitamin D2. The magnitude of this reduction, depending on the analytical approach, ranged from approximately 9 to 18 nanomoles per liter.
It is crucial to emphasize that this discovery does not imply that vitamin D2 actively removes vitamin D from the body or that its consumption is inherently detrimental. Vitamin D2 demonstrably increases levels of 25-hydroxyvitamin D2 and contributes to the overall vitamin D status. The surprising revelation lies in the apparent inverse relationship: an increase in one form of vitamin D metabolite seems to be accompanied by a decrease in the other.
Dr. Emily Brown, PhD Research Fellow and Lead Researcher from the University of Surrey’s Discipline of Nutrition, Exercise, Chronobiology & Sleep, commented on the findings: "Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK. However, we discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements. This study suggests that subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2."
Unraveling the Mechanism: Why Does Vitamin D2 Lower D3?
The precise biological mechanism behind this observed reduction in 25-hydroxyvitamin D3 levels due to vitamin D2 supplementation remains a subject of ongoing scientific investigation. However, researchers have proposed several plausible hypotheses.
One leading theory centers on the body’s intricate regulatory systems for vitamin D metabolism. When the concentrations of vitamin D metabolites rise, the body may initiate accelerated breakdown and excretion processes to maintain homeostasis. It is theorized that an elevated intake of vitamin D2 could, in turn, stimulate the disposal of existing 25-hydroxyvitamin D3.
Furthermore, earlier experimental research has hinted at a bidirectional relationship, suggesting that vitamin D3 supplementation can sometimes lead to a reduction in vitamin D2-derived metabolites. The clinical significance and health implications of this balancing effect are yet to be fully elucidated.
Professor Cathie Martin, Group Leader at the John Innes Centre, underscored the practical implications of these findings: "This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK." The increasing availability of plant-derived and animal-free vitamin D3 sources, such as those derived from lichen, addresses this need and provides greater dietary flexibility.
Beyond Levels: Divergent Effects on the Immune System
The research also aligns with previous studies suggesting that vitamin D2 and D3 may exert distinct effects on the body, extending beyond their capacity to elevate overall vitamin D levels.
A study published in Frontiers in Immunology, led by Professor Colin Smith from the University of Surrey, explored the impact of vitamin D supplementation on gene expression in blood cells. This investigation revealed substantial differences in the cellular responses to D2 and D3, particularly concerning immune system pathways. Notably, vitamin D3 appeared to stimulate activity related to type I interferon signaling, a response not observed with vitamin D2.
Type I interferons are a critical class of signaling proteins that act as early warning systems for cells, alerting them to the presence of pathogens and initiating the body’s initial defense mechanisms against viral and bacterial infections.
Professor Colin Smith elaborated on these findings: "We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signaling system in the body — a key part of the immune system that provides a first line of defense against bacteria and viruses. Thus, a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body."
While these immunological findings are compelling, it is important to interpret them cautiously. Changes in gene activity and immune signaling do not automatically translate into a direct reduction in illness. Larger-scale clinical trials are required to definitively establish the health consequences of these observed differences.
Nuances Emerge: Vitamin D2 Still Has Biological Effects
Subsequent research published since the 2025 meta-analysis has introduced further nuance to the understanding of vitamin D metabolism. A separate systematic review and meta-analysis in 2026, which examined 26 randomized trials involving vitamin D2, found that D2 supplementation effectively reduced parathyroid hormone levels and induced a modest increase in blood calcium. These findings demonstrate that vitamin D2 continues to exert meaningful biological effects, even while potentially reducing circulating levels of vitamin D3.
This new evidence is significant because it clarifies that the reduction in D3 should not be misconstrued as an indication of vitamin D2’s ineffectiveness. Current guidelines from organizations like the U.S. National Institutes of Health (NIH) continue to acknowledge that both vitamin D2 and D3 can elevate overall vitamin D levels in the blood. The key differentiator, supported by a growing body of evidence, is that D3 is generally more effective at achieving a greater increase and maintaining elevated levels for a longer duration.
This observation is consistent with previous meta-analyses that directly compared the two forms. One comprehensive analysis of 24 studies concluded that vitamin D3 was superior to D2 in enhancing overall vitamin D status, a finding echoed by another analysis that focused on more frequent dosing regimens.
The Unanswered Question: Clinical Significance of the D2-D3 Interaction
Despite the accumulating evidence, a central unanswered question persists: what is the clinical significance of vitamin D2’s impact on circulating D3 levels? While randomized trial data strongly suggests that vitamin D2 can indeed reduce D3, the long-term health consequences of this reduction remain uncertain.
Scientists are calling for additional research to thoroughly investigate the comparative metabolism of D2 and D3, to ascertain whether their differential effects on immune function translate into meaningful health outcomes, and to determine if vitamin D3 should be universally recommended as the preferred form for routine supplementation in the absence of specific contraindications for vitamin D2.
Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, emphasized the public health imperative: "Vitamin D deficiency represents a significant public health concern, especially during the winter months with significant deficiency across the UK population. This collaborative research effort aligns well with the Quadram Institute’s mission to deliver healthier lives through food innovation to enhance the nutrient density of the food we eat. Tackling this with the most effective form of vitamin D supplementation or fortification is of the utmost importance to the health of the nation."
Implications for Consumers and Public Health
The evolving scientific landscape underscores the importance of paying attention to the specific form of vitamin D listed on supplement labels. While both vitamin D2 and D3 contribute to an individual’s vitamin D status, they are not metabolically identical. Current evidence points towards a distinct advantage for vitamin D3 in terms of its ability to raise and sustain vitamin D levels. The critical next step for the scientific community is to definitively establish whether these biochemical distinctions translate into tangible differences in long-term health outcomes for the general population. This ongoing research has profound implications for dietary recommendations, public health strategies, and consumer choices regarding vitamin D supplementation.
