Research published in 2025 indicates that vitamin D2 supplementation may have unintended consequences, specifically reducing levels of vitamin D3, which is the form produced naturally by the body when skin is exposed to sunlight. While vitamin D2 and D3 are often considered interchangeable, evidence suggests that D3 is more effective at elevating and maintaining vitamin D levels in the blood.
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Vitamin D is vital for calcium absorption, bone health, and various bodily functions including muscle, nerve, and immune responses. During the darker months, especially in the UK, individuals are advised to consider taking 10 micrograms (µg) of vitamin D daily, as sunlight is insufficient for natural synthesis.
Vitamin D2, known as ergocalciferol, is typically derived from fungi or yeast exposed to ultraviolet light, whereas vitamin D3, or cholecalciferol, originates from the skin's reaction to UVB radiation from sunlight. Traditionally, vitamin D3 supplements have been sourced from lanolin from sheep's wool, but plant-based alternatives from lichen are now available, catering to vegan and plant-based diets.
The study, conducted by researchers from the University of Surrey, John Innes Centre, and Quadram Institute Bioscience, reviewed data from 20 randomized trials. The findings revealed that participants taking vitamin D2 had significantly lower concentrations of 25-hydroxyvitamin D3 compared to those not receiving vitamin D2, with reductions averaging between 9 to 18 nanomoles per liter.
Emily Brown, PhD Research Fellow and lead researcher, noted that while vitamin D2 can elevate 25-hydroxyvitamin D2 levels, it appears to coincide with a decrease in vitamin D3 levels. She stressed the potential benefits of vitamin D3 over D2 based on their findings.
The precise reasons for the observed reduction in vitamin D3 levels following vitamin D2 supplementation remain uncertain. One hypothesis suggests that the body may respond to increased vitamin D metabolites by enhancing the breakdown of certain compounds, potentially leading to a decrease in 25-hydroxyvitamin D3 levels.
Moreover, earlier studies have indicated differing effects of D2 and D3 on gene activity, particularly regarding immune system responsiveness. Research led by Professor Colin Smith found that only vitamin D3 stimulated type I interferon signaling, crucial for the body’s defense against infections.
Despite findings suggesting D2 might reduce D3 levels, a 2026 systematic review also highlighted D2’s continued significance in reducing parathyroid hormone levels and slightly increasing blood calcium. These findings underscore that while both forms can increase vitamin D status, D3 is more effective at achieving sustained increases.
Current research emphasizes the need for further exploration to determine the health implications of the differences between D2 and D3, particularly in regard to long-term health outcomes. According to Professor Martin Warren from the Quadram Institute, addressing vitamin D deficiency is a crucial public health issue, especially in winter, and the choice of supplement is important for national health.