Scientists Warn A Popular Vitamin D Supplement May Have A Hidden Downside

TL;DR

For listenersOffer from Amazon

Turn quiet afternoons into listening time

• Thousands of audiobooks, podcasts and originals
• Listen on your phone, tablet or Echo — also offline
• Cancel anytime

Try Audible free →Free trial for new members

As an affiliate, we earn on qualifying purchases.Hide

A 2025 review of randomized controlled trials found that people taking vitamin D2 had lower blood concentrations of 25-hydroxyvitamin D3 than people not taking D2. The finding does not show that D2 is harmful or that it reduces total vitamin D status; researchers say the health significance and biological mechanism remain uncertain.

A 2025 review of clinical trials found that vitamin D2 supplements were associated with lower blood levels of 25-hydroxyvitamin D3, a marker derived from the form of vitamin D the body makes in skin exposed to sunlight. The researchers said the finding does not establish that D2 is harmful or lowers total vitamin D status, and its consequences for health are not yet known.

The review, published in Nutrition Reviews by researchers from the University of Surrey, the John Innes Centre and Quadram Institute Bioscience, brought together evidence from randomized controlled trials. The researchers assessed 20 studies; 11 supplied data for the main meta-analysis. Compared with people who did not receive D2, those taking it had significantly lower concentrations of 25-hydroxyvitamin D3. Depending on the analysis, the average difference was about 9 to 18 nanomoles per liter.

That result concerns one vitamin D metabolite, not a finding that D2 strips vitamin D from the body. D2 supplementation also raises 25-hydroxyvitamin D2, and the source report says it can contribute to overall vitamin D status. The review identified an association between taking D2 and lower levels of the D3-derived marker; it did not establish that the change causes illness or show that D2 supplementation is necessarily harmful.

Vitamin D3, also called cholecalciferol, is made in human skin after exposure to ultraviolet B radiation. D2, or ergocalciferol, is commonly produced from fungi or yeast exposed to ultraviolet light. The liver converts both forms into metabolites measured in blood tests. The U.S. National Institutes of Health says D3 tends to raise vitamin D levels more and keep them elevated longer than D2, according to the report.

How the D2 Finding Could Affect Supplement Choices

The finding matters because supplements containing D2 and D3 are often treated as interchangeable ways to increase vitamin D. The review suggests that the forms may interact in the body, so a rise in one blood metabolite may occur alongside a fall in another. It adds a complication to comparisons based only on whether a supplement raises vitamin D overall.

That is not enough evidence to conclude that people should stop taking D2 or switch products. The review did not establish that the measured decrease leads to worse bone, muscle, immune or other health outcomes. Its authors’ suggestion that D3 may be more beneficial for most people is an interpretation of the evidence, not a finding that every individual will benefit more from D3.

The distinction also has practical relevance for people who avoid animal-derived ingredients. Some D3 supplements have traditionally used lanolin from sheep’s wool, while lichen-derived D3 is available as an animal-free option. Researchers cited in the report said making plant-based D3 accessible is relevant, but the study did not compare every supplement product or make individual recommendations.

What the Review Compared

Vitamin D helps the body absorb calcium and supports bone health; it also has roles in muscle, nerve and immune function. Blood tests commonly estimate vitamin D status by measuring 25-hydroxyvitamin D, which includes metabolites derived from both D2 and D3. The review’s particular finding concerned the D3-derived component, rather than a claim that all measures of vitamin D fell.

In the United Kingdom, people are advised to consider taking 10 micrograms daily during darker months, when sunlight may be too weak for the skin to make enough vitamin D. The report describes that as current UK guidance, not as a recommendation arising from this review. The trial review focused on what happened to measured vitamin D metabolites after D2 supplementation, not on whether a specific dose or product is best for each person.

A separate study published in Frontiers in Immunology examined gene activity in blood after D2 and D3 supplementation. The report says it found different responses, including an apparent effect of D3, but not D2, on activity associated with type I interferon signaling. These are biological findings; they do not establish that D3 prevents infections or that D2 weakens immune defenses.

“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.”

— Emily Brown, lead researcher and research fellow at the University of Surrey

Health Effects and Mechanism Remain Open

Researchers do not yet have a definitive explanation for why D2 supplementation was associated with lower 25-hydroxyvitamin D3. One proposed possibility is that the body speeds up the breakdown and removal of vitamin D metabolites as their concentrations rise, potentially increasing disposal of the D3-derived form. The report presents this as a hypothesis, not an established mechanism.

The review also does not settle how clinically important the measured decrease is. The supplied account does not give enough detail to assess how the result varies by dose, length of supplementation, participants’ starting vitamin D levels or other characteristics. Nor does it establish whether the change affects health outcomes or how it relates to a person’s total vitamin D status.

The immune-system findings cited in the report come from separate research and concern gene activity and signaling. Whether those differences translate into meaningful effects on infections or other health outcomes remains unclear. Readers should not treat either study as proof that one form prevents illness or that the other causes harm.

Further Trials Must Clarify the Difference

The next step for researchers is to establish why D2 and D3 supplementation are associated with different metabolite levels and whether the change has measurable consequences for health. Further studies could clarify how dose, duration and participants’ baseline vitamin D status relate to the result; the source report does not announce a specific upcoming trial or timetable.

For now, the review adds evidence that the two forms may not behave identically, while leaving individual supplement choices dependent on personal circumstances. People with questions about supplementation or test results can discuss them with a qualified health professional. The report does not advise people to stop or change a prescribed supplement.

Key Questions

What did the vitamin D2 review find?

It found that people taking vitamin D2 had lower blood concentrations of 25-hydroxyvitamin D3 than people who did not receive D2. The estimated average difference was about 9 to 18 nanomoles per liter, depending on the analysis.

Does the finding mean vitamin D2 is harmful?

No. The review found a change in a D3-derived blood marker; it did not establish that D2 causes harm, reduces total vitamin D status or leads to worse health outcomes.

Why might vitamin D2 affect vitamin D3 levels?

Researchers have proposed that the body may increase the breakdown of vitamin D metabolites when their levels rise. That is a possible explanation, not a confirmed mechanism.

Is vitamin D3 always a better choice?

The report says U.S. guidance notes that D3 tends to raise vitamin D levels more and keep them elevated longer than D2. The review does not show that D3 is best for every person, and its authors qualify their preference by individual considerations.

Can people who avoid animal products take vitamin D3?

Animal-free D3, including D3 derived from lichen, is available, according to the report. Anyone deciding whether a supplement is suitable for them can ask a qualified health professional.

Source: rss