By Hendra Lukman 
A significant discovery in vitamin D research, published in 2025 by a collaborative team of scientists from the University of Surrey, John Innes Centre, and Quadram Institute Bioscience, has revealed a potentially counterintuitive effect of vitamin D2 supplementation: it may lead to a reduction in the body’s levels of vitamin D3. This finding is particularly relevant for the vast number of individuals who rely on vitamin D supplements to maintain adequate levels, especially during the prolonged periods of low sunlight experienced in many regions, such as the UK.
Vitamin D, a crucial nutrient for human health, plays a multifaceted role in maintaining well-being. Its most well-known function is its essentiality in bone health, facilitating the absorption of calcium and phosphorus, thereby preventing conditions like rickets in children and osteomalacia and osteoporosis in adults. Beyond skeletal integrity, vitamin D is integral to the proper functioning of the muscular system, contributing to muscle strength and preventing weakness. Furthermore, it plays a vital role in supporting a robust immune system, helping the body to defend against pathogens and regulate immune responses. In the United Kingdom, current government guidelines recommend that individuals consider a daily intake of 10 micrograms (µg) of vitamin D throughout the autumn and winter months. This recommendation stems from the fact that during these seasons, solar ultraviolet B (UVB) radiation, the primary trigger for endogenous vitamin D3 production in the skin, is significantly diminished, rendering it insufficient for meeting the body’s requirements.
Understanding the Two Forms of Vitamin D
Vitamin D supplements are predominantly available in two distinct forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is typically derived from plant or fungal sources, such as mushrooms exposed to ultraviolet light. In contrast, vitamin D3 is the form that the human body naturally synthesizes when the skin is exposed to sunlight. While both forms are recognized as contributing to an individual’s overall vitamin D status, scientific consensus, supported by numerous prior studies, has consistently indicated that vitamin D3 is generally more efficacious in raising and sustaining blood concentrations of vitamin D compared to vitamin D2. This differential efficacy has been attributed to variations in their metabolic pathways and how effectively they are utilized by the body.
The groundbreaking study, meticulously detailed in the journal Nutrition Reviews, involved a comprehensive meta-analysis of data extracted from multiple randomized controlled trials. These trials specifically compared the outcomes of participants receiving vitamin D2 supplementation with those in control groups who did not receive any vitamin D2. The analysis revealed a consistent pattern: individuals who supplemented with vitamin D2 tended to exhibit a decrease in their circulating vitamin D3 levels. In a substantial proportion of the examined trials, vitamin D3 concentrations in the vitamin D2 supplemented groups fell below the levels observed in their respective control groups.
A Shift in the Vitamin D Balance
This intriguing finding does not necessarily negate the potential benefits of vitamin D2. Instead, it strongly suggests that the introduction and metabolism of vitamin D2 may actively influence the delicate balance of different vitamin D metabolites circulating within the body. It appears to trigger a mechanism that leads to a reduction in the body’s own produced or supplemented vitamin D3.
Dr. Emily Brown, a PhD Research Fellow and the Lead Researcher on the study from the University of Surrey’s Nutrition, Exercise, Chronobiology & Sleep Discipline, elaborated on the significance of these findings. "Vitamin D supplements are undeniably important, particularly during the months between October and March, when our bodies are largely unable to synthesize vitamin D from sunlight in the UK," Dr. Brown stated. "However, our research has uncovered a previously unrecognized effect: vitamin D2 supplements can, in fact, lead to a decrease in the body’s levels of vitamin D3. This study strongly indicates that, taking into account individual health considerations, vitamin D3 supplements may offer a more advantageous option for the majority of individuals over vitamin D2."
Implications for Plant-Based Vitamin D Preferences
The research also carries significant implications for individuals who prioritize supplements derived from non-animal sources. The increasing availability of plant-based vitamin D3 options, often sourced from lichen, presents a compelling alternative for those seeking vegan or vegetarian-friendly supplements.
Professor Cathie Martin, a Group Leader at the John Innes Centre, highlighted the growing importance of this accessibility. "This meta-analysis underscores the critical need to ensure that plant-based vitamin D3 is readily available in the UK," Professor Martin remarked. The expansion of these vegan-friendly D3 sources could empower consumers to make informed choices that align with their dietary preferences without compromising on the potentially superior efficacy of vitamin D3.
Distinct Roles in Immune Function
These latest findings build upon a growing body of earlier evidence suggesting that vitamin D2 and D3 may not be biologically equivalent in their functions within the human body. Prior research, notably a study published in Frontiers in Immunology and led by Professor Colin Smith from the University of Surrey, had already identified distinct ways in which the two vitamin D forms influenced immune activity.
Specifically, vitamin D3 demonstrated an ability to modulate the immune system in ways that vitamin D2 did not appear to replicate. A key observation in this earlier work was the impact of vitamin D3 on the type I interferon signaling pathway. Interferons are a group of signaling proteins crucial for the immune system’s initial defense mechanisms against viral and bacterial infections. They act as critical communicators, enabling cells to detect and respond to threats from pathogens, thereby initiating an appropriate immune response.
Professor Colin Smith further explained the implications of his team’s earlier findings. "We have demonstrated that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signaling system within the body," Professor Smith stated. "This system is a cornerstone of the immune response, providing the first line of defense against invading bacteria and viruses. Therefore, maintaining a healthy vitamin D3 status may contribute to preventing infections by limiting the ability of pathogens to establish themselves in the body." While these findings do not establish a direct causal link between vitamin D3 supplementation and infection prevention, they add substantial weight to the hypothesis that vitamin D2 and D3 possess fundamental biological differences that extend beyond their simple contribution to overall vitamin D levels.
Is Vitamin D3 the Preferred Choice?
The scientific community acknowledges that further rigorous research is imperative to fully elucidate the precise mechanisms by which vitamin D2 and D3 function differently and to comprehensively understand the long-term health implications of these distinctions. A pivotal question emerging from this research is whether vitamin D3 should be universally recommended as the primary choice for supplementation, particularly when there is no specific medical indication for the use of vitamin D2. Recognizing that individual nutritional needs and metabolic responses can vary considerably, the researchers strongly emphasize that decisions regarding vitamin D supplementation should always be made in consideration of personal health circumstances and in consultation with healthcare professionals.
Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, underscored the public health significance of vitamin D deficiency. "Vitamin D deficiency represents a considerable public health concern, particularly during the winter months, with widespread deficiency observed across the UK population," Professor Warren commented. "This collaborative research effort aligns perfectly with the Quadram Institute’s mission to promote healthier lives through food innovation, aiming to enhance the nutrient density of the foods we consume. Addressing this deficiency effectively, whether through the most efficacious form of vitamin D supplementation or through fortification strategies, is of paramount importance to the health and well-being of the nation."
In summation, the collective findings from these recent studies contribute compelling evidence that the specific form of vitamin D included in a supplement can indeed matter. While both vitamin D2 and D3 can contribute to an individual’s vitamin D intake, they may not be interchangeable within the body’s complex physiological processes. For the majority of individuals seeking to supplement their vitamin D intake, particularly during periods of reduced sunlight exposure, the current research suggests that vitamin D3 may offer a distinct advantage. However, ongoing scientific inquiry is essential to fully quantify the extent of these differences and their long-term impact on human health. The pursuit of optimal vitamin D status remains a critical public health objective, and understanding the nuances between vitamin D2 and D3 is a vital step in achieving this goal.
