Scientists warn a popular vitamin D supplement may have a hidden downside

Scientists have cautioned that a popular vitamin D supplement, vitamin D2, may have an unexpected downside. Recent research published in 2025 reveals that taking vitamin D2 can lead to a decrease in vitamin D3, the form of vitamin D that humans naturally produce when their skin is exposed to sunlight. While both forms contribute to overall vitamin D levels, vitamin D3 is generally more effective at raising and maintaining blood vitamin D levels.

The study, conducted by researchers from the University of Surrey, John Innes Centre, and Quadram Institute Bioscience, analyzed data from 20 randomized controlled trials. The findings indicate that vitamin D2 supplementation can lead to a reduction in vitamin D3 levels, ranging from 9 to 18 nanomoles per liter. Emily Brown, the lead researcher from the University of Surrey, noted that vitamin D2 supplements can actually decrease the levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements.

Vitamin D is crucial for absorbing calcium, maintaining healthy bones, and supporting muscle, nerve, and immune function. In the UK, individuals are advised to take 10 micrograms of vitamin D per day during darker months when sunlight exposure is insufficient. Vitamin D supplements usually contain either vitamin D2, also known as ergocalciferol, or vitamin D3, also called cholecalciferol.

While vitamin D2 is typically produced from fungi or yeast, vitamin D3 is traditionally sourced from lanolin in sheep wool but is also available from plant-based sources like lichen for vegan or plant-based diets.

The researchers are still unclear about the exact mechanisms behind the drop in vitamin D3 levels when taking vitamin D2. One possibility is that the body may accelerate the breakdown and removal of vitamin D metabolites when their concentrations rise, leading to a reduction in vitamin D3. Another possibility is that increasing vitamin D2 could enhance the disposal of existing 25-hydroxyvitamin D3. Further research is needed to determine the significance of this balancing effect for overall health.

Despite this finding, vitamin D supplementation remains important, especially during the darker months when sunlight exposure is limited in the UK. The study suggests that vitamin D3 supplements may be more beneficial for most individuals over vitamin D2. Researchers emphasize the need for accessible plant-based and animal-free sources of vitamin D3, such as those derived from lichen, to ensure that individuals have healthy options available.