By Gabriel Paijo 
A collaborative study led by researchers from the University of Surrey, the John Innes Centre, and Quadram Institute Bioscience has uncovered a significant and previously overlooked interaction between different forms of Vitamin D. The findings, published in the journal Nutrition Reviews, suggest that the consumption of Vitamin D2 (ergocalciferol) may actively deplete the body’s levels of Vitamin D3 (cholecalciferol), which is the more effective form of the nutrient for supporting the human immune system. This discovery challenges long-standing assumptions about the interchangeability of these two supplements and carries profound implications for public health guidelines, particularly in regions with limited winter sunlight like the United Kingdom.
Vitamin D is a critical micronutrient required for the regulation of calcium and phosphate in the body, which are essential for keeping bones, teeth, and muscles healthy. However, recent scientific inquiries have shifted focus toward its role in the immune system. In the UK, government health advice recommends that every citizen consider taking a daily supplement of 10 micrograms (µg) of Vitamin D during the autumn and winter months, as the sun is not strong enough for the body to synthesize the nutrient naturally through the skin. While both Vitamin D2 and D3 are marketed as effective supplements, this new research indicates that they are far from equal in their biological impact.
The Scientific Distinction Between Vitamin D2 and Vitamin D3
To understand the weight of the study’s findings, it is necessary to distinguish between the two primary forms of Vitamin D. Vitamin D2 is largely derived from plant sources, such as fungi and yeast exposed to ultraviolet (UV) light. Because it is plant-based, it has historically been the primary choice for vegans and vegetarians. Vitamin D3, on the other hand, is the form produced naturally by the human body when skin is exposed to sunlight. It is also found in animal-sourced foods, such as oily fish, red meat, and egg yolks, and is commonly synthesized for supplements from lanolin, a wax secreted by wool-bearing animals.
While both forms can be converted by the liver and kidneys into the active hormone calcitriol, the efficiency of this process has long been a subject of debate. The latest meta-analysis of randomized controlled trials (RCTs) provides compelling evidence that Vitamin D3 is significantly more effective at raising total Vitamin D concentrations in the blood. More strikingly, the data revealed that Vitamin D2 supplementation actually led to a decrease in circulating Vitamin D3 levels. In several of the analyzed studies, participants taking Vitamin D2 ended up with lower Vitamin D3 levels than those in the control group who took no supplement at all.
A New Understanding of Supplement Interaction
The lead researcher of the study, Emily Brown, a PhD Research Fellow at the University of Surrey’s Nutrition, Exercise, Chronobiology & Sleep Discipline, highlighted the unexpected nature of these results. According to Brown, the discovery that Vitamin D2 could potentially "crowd out" or reduce the body’s natural Vitamin D3 stores is a paradigm-shifting observation.
"Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK," Brown stated. "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."
This metabolic interference suggests that the body may prioritize the processing of Vitamin D2 when it is introduced in high supplemental doses, potentially accelerating the breakdown or inhibiting the synthesis of Vitamin D3. Given that Vitamin D3 is the form the body evolved to use, its depletion could have cascading effects on various physiological systems.
The Link to Immune Function and Interferon Signalling
The implications of this research extend beyond bone health. The study builds upon a foundation of work previously published in Frontiers in Immunology, led by Professor Colin Smith of the University of Surrey. That earlier research suggested that Vitamin D2 and D3 do not play identical roles in supporting the human immune system.
The human immune response relies on complex signaling pathways to identify and neutralize pathogens. Professor Smith’s work specifically identified that Vitamin D3—but not D2—appears to stimulate the type I interferon signaling system. This system serves as the body’s "first line of defense" against viral and bacterial invasions. Interferons are proteins released by host cells in response to the presence of viruses, signaling nearby cells to heighten their anti-viral defenses.
"We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signalling system in the body," Professor Smith explained. "Thus, a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body."
By potentially lowering Vitamin D3 levels, the intake of Vitamin D2 might inadvertently weaken this interferon response, making individuals more susceptible to seasonal illnesses such as influenza or other respiratory infections. This finding is particularly relevant in the wake of the global COVID-19 pandemic, which spurred renewed interest in how Vitamin D status affects viral resistance.
Public Health Implications and the Need for Plant-Based D3
The findings present a particular challenge for the vegan and vegetarian communities. Historically, Vitamin D2 was the only widely available plant-based option for supplementation. If Vitamin D2 is less effective and potentially detrimental to Vitamin D3 stores, there is an urgent need for the diversification of Vitamin D3 sources.
Professor Cathie Martin, a Group Leader at the John Innes Centre and an expert in plant biology, emphasized the necessity of innovation in this area. "This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK," she noted. In recent years, scientists have begun extracting Vitamin D3 from lichens—a symbiotic partnership of fungi and algae—which provides a vegan-friendly alternative to lanolin-based D3. Professor Martin’s team is also exploring bio-fortification techniques to increase the Vitamin D3 content in crops like tomatoes.
The Quadram Institute, which focuses on the intersection of food science and health, also weighed in on the study’s significance. Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, underscored the scale of the deficiency problem in the UK.
"Vitamin D deficiency represents a significant public health concern, especially during the winter months with significant deficiency across the UK population," Professor Warren said. "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."
Chronology of Vitamin D Research and Policy
The evolution of Vitamin D science in the UK has been a decades-long process. For much of the 20th century, Vitamin D research focused almost exclusively on preventing rickets in children. It wasn’t until the early 2000s that the broader "non-skeletal" benefits of Vitamin D—including its role in autoimmune diseases, cardiovascular health, and cancer prevention—began to gain mainstream scientific traction.
In 2016, Public Health England (now the UK Health Security Agency) updated its guidelines to recommend 10µg of Vitamin D daily for everyone over the age of four during the winter. Prior to this, the recommendation was primarily targeted at "at-risk" groups, such as the elderly or those with dark skin.
The 2022 study in Frontiers in Immunology marked a turning point by specifically examining the gene expression changes caused by D2 versus D3. The latest meta-analysis in Nutrition Reviews serves as a critical follow-up, providing the statistical weight of multiple trials to confirm that the differences between the two forms are not merely theoretical but have measurable impacts on blood chemistry.
Analysis of Broader Impacts
The revelation that Vitamin D2 might be "counterproductive" by lowering D3 levels could lead to a significant shift in the supplement industry. Manufacturers may face increased pressure to phase out Vitamin D2 in favor of D3, or at the very least, provide clearer labeling regarding the superior efficacy of D3 for immune support.
Furthermore, the study may prompt a revision of national fortification strategies. Many foods, such as breakfast cereals and plant-based milks, are currently fortified with Vitamin D2 because it is cheaper and easier to source as a vegan ingredient. If the goal of fortification is to improve the nation’s immune resilience, switching these additives to plant-based Vitamin D3 could become a public health priority.
From a clinical perspective, the research suggests that doctors and nutritionists should be more specific when prescribing Vitamin D. Rather than a generic "Vitamin D" recommendation, the distinction between D2 and D3 could become a standard part of patient care, especially for those with compromised immune systems or those living in northern latitudes.
Future Research Directions
While the meta-analysis provides strong evidence of the interaction between D2 and D3, researchers agree that more work is needed to understand the "why" behind the phenomenon. Future studies will likely focus on the metabolic pathways of the liver to see if Vitamin D2 competes for the same enzymes required to hydroxylate Vitamin D3.
There is also a call for longer-term studies to determine if the suppression of Vitamin D3 by D2 leads to clinical outcomes, such as a higher frequency of bone fractures or increased rates of infection over several years. For now, the scientific consensus among the Surrey, John Innes, and Quadram teams is clear: when it comes to Vitamin D, the form you choose matters significantly.
As the UK enters future winter seasons, the message to the public is evolving. While the 10µg daily goal remains, the emphasis is shifting toward Vitamin D3 as the gold standard for maintaining not just bone density, but a robust and responsive immune system. In the words of the research team, further investigation into these functionalities should be a priority for policy makers as they decide on the first-line choice for national supplementation.
