A common vitamin D supplement may have an unexpected effect inside the body. Research published in 2025 has revealed that taking vitamin D2 can actually reduce levels of vitamin D3, the specific form of the nutrient that humans naturally synthesize when their skin is exposed to sunlight.
This distinction matters significantly because vitamin D2 and vitamin D3 are frequently treated as interchangeable sources of the same essential nutrient. While both variations of the supplement can increase overall vitamin D status within the body, a growing body of scientific evidence indicates that D3 is generally more effective at raising and maintaining the main form of vitamin D measured in the blood. Current clinical guidance from the U.S. National Institutes of Health reflects this expanding evidence base, noting that D3 tends to elevate vitamin D levels more substantially and keep them elevated for a longer duration than D2.
Vitamin D is a critical nutrient essential for helping the body absorb calcium and maintain healthy, strong bones. Beyond skeletal health, it also plays vital regulatory roles in muscle performance, nerve signaling, and immune system function. In the United Kingdom, public health authorities advise the population to consider taking a daily supplement of 10 micrograms of vitamin D during the darker, winter months, when natural sunlight is generally too weak for the skin to manufacture an adequate supply of the vitamin independently.
Vitamin D2 and D3 Are Not Quite the Same
Vitamin D supplements generally contain one of two distinct forms. Vitamin D2, scientifically known as ergocalciferol, is commonly produced commercially from fungi or yeast that have been exposed to ultraviolet light. On the other hand, vitamin D3, or cholecalciferol, is identical to the exact form produced naturally in human skin following exposure to ultraviolet B radiation from the sun.
Traditionally, the vitamin D3 utilized in commercial supplements has frequently been derived from lanolin, a wax obtained from sheep’s wool. However, animal-free versions of D3 manufactured from alternative sources such as lichen are now readily available on the market. This represents an important consideration for individuals who follow strict vegan or plant-based diets and wish to avoid animal-derived ingredients.
Once any form of vitamin D enters the human body, it undergoes several complex processing steps. The liver is primarily responsible for converting the ingested nutrient into 25-hydroxyvitamin D, which is the exact form doctors commonly measure during routine blood tests to accurately estimate a patient’s vitamin D status. Vitamin D2 specifically produces 25-hydroxyvitamin D2, while vitamin D3 yields 25-hydroxyvitamin D3.
It is this specific D3-derived form that researchers discovered could plummet after individuals ingested vitamin D2 supplements.
Vitamin D2 Was Linked to a Drop in Vitamin D3
The 2025 study, published in the academic journal Nutrition Reviews by a collaborative team of researchers from the University of Surrey, the John Innes Centre, and the Quadram Institute Bioscience, brought together comprehensive evidence from prior randomized controlled trials to examine this physiological interaction more closely.
The researchers conducted a meticulous review of 20 separate studies, out of which 11 provided data that were deemed suitable for the primary meta-analysis. When comparing individuals who did not receive vitamin D2 against those who actively took D2 supplements, the researchers found that those taking D2 exhibited significantly lower concentrations of 25-hydroxyvitamin D3 in their blood. Depending on the specific statistical methods used to analyze the results, the average reduction ranged roughly from 9 to 18 nanomoles per liter.
Crucially, researchers emphasize that this finding does not mean vitamin D2 simply flushes or removes vitamin D from the body entirely, nor does it imply that taking D2 is inherently harmful. Vitamin D2 itself successfully raises levels of 25-hydroxyvitamin D2 and can undeniably contribute to a person’s total vitamin D status. Instead, the surprising scientific takeaway is that artificially increasing one form of the vitamin appears to coincide directly with a measurable reduction in the other.
Dr. Emily Brown, a PhD Research Fellow and the lead researcher of the study from the University of Surrey’s Nutrition, Exercise, Chronobiology & Sleep Discipline, explained the context of 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."
Why Would Vitamin D2 Lower Vitamin D3?
Despite uncovering this clear biochemical interaction, scientists do not yet possess a definitive explanation for the exact mechanisms driving it.
One primary hypothesis involves the body’s sophisticated feedback mechanisms for regulating vitamin D homeostasis. When concentrations of various vitamin D metabolites rise sharply in the bloodstream, the body may accelerate the metabolic breakdown and elimination of these compounds in an effort to keep overall concentrations under strict control. Researchers have proposed that artificially increasing vitamin D2 levels could therefore inadvertently trigger an accelerated disposal of existing 25-hydroxyvitamin D3.
Earlier experimental work has also hinted that this regulatory relationship might operate in both directions, with D3 supplementation occasionally reducing D2-derived metabolites under certain conditions. Exactly how important this balancing effect is for overall human health remains an unresolved question requiring further study.
Professor Cathie Martin, a Group Leader at the John Innes Centre, highlighted the practical implications of these discoveries:
"This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK."
That goal has become increasingly practical in recent years because high-quality plant-based and animal-free sources of vitamin D3 are now widely available to consumers, including D3 derived specifically from lichen.
D3 May Affect the Immune System Differently
The newly published findings also align neatly with earlier scientific work suggesting that vitamin D2 and D3 may not possess identical biological effects beyond their shared basic ability to raise overall vitamin D levels in the blood.
A separate study published in the journal Frontiers in Immunology and led by Professor Colin Smith, also from the University of Surrey, closely examined how supplementation with the two different forms affected gene activity within human blood cells. The research team uncovered substantial differences between the body’s cellular responses to D2 and D3, including notable changes involving the immune system. Specifically, vitamin D3 appeared to actively stimulate biological activity associated with type I interferon signaling, whereas vitamin D2 did not show this effect.
Type I interferons are specialized signaling proteins that help alert neighboring cells when a viral infection is developing, forming a vital component of the body’s early frontline defenses against viruses and other pathogens.
Professor Colin Smith noted the significance of these immunological differences:
"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 undeniably intriguing, researchers caution that they should not be interpreted as definitive proof that taking D3 will automatically prevent any specific infection. Changes observed in gene activity and immune signaling do not necessarily translate directly into fewer illnesses in everyday life, and much larger clinical studies will be required to establish what these molecular differences truly mean for human health.
Newer Evidence Adds an Important Caveat
Research published in the wake of the 2025 analysis has continued to add nuance to the scientific picture surrounding these supplements.
A separate 2026 systematic review and meta-analysis closely examined 26 randomized trials involving vitamin D2 supplementation. That review discovered that D2 supplementation significantly reduced parathyroid hormone levels in patients and produced a small, measurable increase in blood calcium. These outcomes demonstrate that vitamin D2 continues to exert meaningful biological effects in the human body, even if it simultaneously reduces circulating levels of D3.
This finding is important because the observed reduction in D3 should not be misinterpreted as evidence that vitamin D2 is entirely ineffective. Current informational resources from the National Institutes of Health continue to confirm that both D2 and D3 are capable of increasing overall blood levels of vitamin D. The core difference lies in the fact that the accumulated body of scientific evidence consistently favors D3 for producing a larger initial increase in blood concentrations and maintaining that elevation for a longer duration.
This conclusion is also fully consistent with previous meta-analyses that directly compared the two forms against one another. One extensive analysis of 24 studies concluded that D3 was significantly more effective than D2 at improving overall vitamin D status, while another targeted analysis focusing specifically on frequent dosing schedules reached a nearly identical conclusion.
Scientists Still Need to Know What the Difference Means
The central unanswered question facing researchers is not whether vitamin D2 can successfully reduce circulating D3 levels in the blood, as the randomized trial evidence strongly suggests that it can. What remains highly uncertain is whether that specific reduction produces any meaningful, measurable differences in long-term health outcomes for patients.
Researchers emphasize that additional clinical studies are urgently needed to determine precisely how D2 and D3 are metabolized differently by human physiology, whether their distinct effects on immune function have practical clinical implications, and whether D3 should officially become the preferred form for routine supplementation when there is no specific individual medical reason to utilize D2.
Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, emphasized the broader public health context of the work:
"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 now, the growing body of scientific evidence increasingly suggests that the exact details written on a vitamin D supplement label genuinely matter. While both D2 and D3 can contribute meaningfully to an individual’s overall vitamin D status, they are not necessarily metabolically interchangeable inside the human body. Vitamin D3 consistently appears to hold a clear physiological advantage when the primary clinical goal is raising and sustaining optimal vitamin D levels. What scientists still need to establish moving forward is whether those biochemical differences ultimately translate into important, tangible differences in long-term human health.
