By Hendra Lukman 
A groundbreaking study published in 2025 has revealed a surprising interaction within the human body concerning common vitamin D supplements. Research indicates that taking vitamin D2, a synthetic form derived from fungi or yeast, may inadvertently lead to a reduction in vitamin D3 levels. Vitamin D3 is the form that humans naturally synthesize when their skin is exposed to sunlight. This distinction is crucial, as D2 and D3 are often perceived as interchangeable sources of the same essential nutrient. While both can elevate overall vitamin D status, a growing consensus among scientists suggests that D3 is generally more effective at both increasing and sustaining the primary form of vitamin D measured in blood tests. Current guidelines from the U.S. National Institutes of Health (NIH) reflect this growing body of evidence, noting that D3 tends to raise vitamin D levels more robustly and maintain them for a longer duration compared to D2.
Understanding the Vitamin D Dichotomy: D2 vs. D3
Vitamin D plays a vital role in numerous bodily functions, most notably facilitating calcium absorption for the maintenance of strong bones. Its influence extends to muscle, nerve, and immune system health. In regions like the United Kingdom, where sunlight is insufficient during autumn and winter months, individuals are advised to consider a daily intake of 10 micrograms (µg) of vitamin D to compensate for the lack of natural synthesis.
The difference between vitamin D2 and D3 lies in their origins and how they are processed by the body. Vitamin D2, chemically known as ergocalciferol, is primarily produced through the exposure of fungi or yeast to ultraviolet light. In contrast, vitamin D3, or cholecalciferol, is the identical compound synthesized in human skin following exposure to ultraviolet B (UVB) radiation from the sun.
Historically, vitamin D3 found in supplements was often derived from lanolin, a waxy substance extracted from sheep’s wool. However, the increasing demand for animal-free alternatives, particularly among individuals following vegan or plant-based diets, has led to the development of D3 synthesized from sources such as lichen.
Once ingested or synthesized, vitamin D undergoes a two-step metabolic process. The liver converts both D2 and D3 into their respective 25-hydroxyvitamin D forms. Specifically, vitamin D2 is converted to 25-hydroxyvitamin D2, and vitamin D3 is converted to 25-hydroxyvitamin D3. It is this D3-derived metabolite, 25-hydroxyvitamin D3, that the 2025 study identified as potentially decreasing following vitamin D2 supplementation.
The 2025 Study: Unveiling the D2-D3 Interaction
The pivotal research, published in the esteemed journal Nutrition Reviews, was a collaborative effort involving scientists from the University of Surrey, the John Innes Centre, and Quadram Institute Bioscience. This comprehensive study employed a meta-analysis approach, pooling data from multiple randomized controlled trials to rigorously examine the effect of vitamin D2 supplementation on vitamin D3 levels.
The researchers meticulously reviewed 20 distinct studies, with 11 of these providing data of sufficient quality for inclusion in the primary meta-analysis. The findings were striking: individuals who received vitamin D2 supplementation exhibited significantly lower concentrations of 25-hydroxyvitamin D3 compared to those who did not receive D2. The magnitude of this reduction, depending on the analytical method employed, ranged from approximately 9 to 18 nanomoles per liter (nmol/L).
It is crucial to emphasize that this finding does not imply that vitamin D2 actively removes vitamin D from the body or that its consumption is inherently detrimental. Vitamin D2 effectively increases levels of 25-hydroxyvitamin D2 and contributes to the overall vitamin D status. The unexpected revelation is the apparent inverse relationship: an increase in one form of vitamin D appears to coincide with a decrease in the other.
Dr. Emily Brown, PhD Research Fellow and Lead Researcher from the University of Surrey’s Nutrition, Exercise, Chronobiology & Sleep Discipline, commented on the significance of these 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."
Exploring the Underlying Mechanisms: Why the Reduction?
The precise biological mechanisms driving this observed reduction in vitamin D3 levels due to vitamin D2 intake are not yet fully elucidated. However, researchers have proposed several plausible hypotheses. One leading theory centers on the body’s sophisticated regulatory systems for vitamin D. When concentrations of vitamin D metabolites rise, the body may activate pathways to accelerate their breakdown and elimination, thereby maintaining homeostasis. It is conceivable that an increased intake of vitamin D2 could trigger an enhanced disposal rate 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 metabolites derived from vitamin D2. The precise clinical significance and extent of this balancing effect on overall health remain areas requiring further investigation.
Professor Cathie Martin, Group Leader at the John Innes Centre, highlighted the broader implications: "This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK." This call for increased accessibility is particularly relevant given the growing availability of plant-based and animal-free vitamin D3 sources, such as those derived from lichen, catering to a wider consumer base.
Beyond Blood Levels: Differential Effects on the Immune System
The implications of the 2025 study extend beyond mere biochemical interactions. Emerging research suggests that vitamin D2 and D3 may not exert identical biological effects, even beyond their capacity to raise overall vitamin D levels.
A study published in Frontiers in Immunology, led by Professor Colin Smith from the University of Surrey, investigated the impact of vitamin D supplementation on gene expression in blood cells. This research identified significant differences in the cellular responses to D2 and D3, notably concerning the immune system. Specifically, vitamin D3 appeared to stimulate activity associated with type I interferon signaling, a pathway not demonstrably influenced by vitamin D2.
Type I interferons are critical signaling proteins that act as an early warning system for cells, alerting them to the presence of developing infections. They form a vital component of the innate immune system’s defense against viral and bacterial pathogens.
Professor Colin Smith elaborated on these immune-related 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 immune-related findings are compelling, it is essential to interpret them cautiously. Changes in gene activity and immune signaling do not automatically translate into a reduced incidence of illness. Larger-scale clinical trials are imperative to definitively establish the real-world health implications of these biochemical differences.
Nuances and Further Evidence: A Developing Picture
Subsequent research published since the 2025 analysis has added further layers of complexity to the understanding of vitamin D2 and D3. A separate systematic review and meta-analysis conducted in 2026, examining 26 randomized trials involving vitamin D2, revealed that D2 supplementation still significantly reduced parathyroid hormone levels and induced a modest increase in blood calcium. These findings underscore that vitamin D2 continues to exert meaningful biological effects, even if it may lead to a reduction in circulating vitamin D3.
This recent evidence is critical because the observed reduction in D3 should not be misconstrued as evidence of vitamin D2’s ineffectiveness. Current NIH guidance continues to affirm that both D2 and D3 can enhance overall blood levels of vitamin D. The key distinction remains that the accumulated scientific evidence consistently favors D3 for achieving a greater increase and sustaining elevated levels for a longer period.
This conclusion aligns with previous meta-analyses that directly compared the two forms. One such analysis, encompassing 24 studies, found D3 to be more effective than D2 in improving overall vitamin D status. Similarly, another study focusing on frequent dosing regimens arrived at a comparable conclusion.
The Unanswered Question: Clinical Significance of the Differences
The central unresolved question in this field is not whether vitamin D2 can reduce circulating vitamin D3 levels – the evidence from randomized controlled trials strongly suggests it can. The true uncertainty lies in determining whether this reduction translates into clinically significant differences in health outcomes.
Scientists are actively pursuing additional research to unravel the intricate metabolic pathways of D2 and D3, to ascertain if their effects on immune function are indeed meaningfully distinct, and to determine if D3 should be preferentially recommended for routine supplementation in the absence of specific contraindications for using D2.
Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, emphasized the public health dimension: "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."
For the present, the accumulating scientific evidence increasingly points to the importance of the label on a vitamin D supplement. While both D2 and D3 contribute to vitamin D status, they are not metabolically interchangeable. Vitamin D3 appears to offer an advantage when the primary objective is to raise and sustain vitamin D levels. The critical next step for researchers is to establish whether these biochemical distinctions translate into tangible differences in long-term health and well-being. The ongoing pursuit of this knowledge is vital for optimizing public health strategies related to vitamin D intake and supplementation.
