Your Vitamin D Supplement Could Be Working Against You, Scientists Warn

A Surprising Discovery Hidden in Plain Sight

Not all vitamin D supplements are created equal. New research from the University of Surrey has found that taking vitamin D2 can actually lower the body's levels of vitamin D3 — the form naturally produced by sunlight and more effective at boosting overall vitamin D status. For millions of people who pop a daily supplement thinking they're doing their bodies a favor, that's a finding worth paying close attention to.

The findings revealed that the unexpected interaction between the two compounds often led to reduced D3 levels — in some cases dropping below those seen in people taking no supplement at all. In other words, some people taking vitamin D supplements may actually end up worse off than if they had taken nothing.

What the Research Actually Shows

To explore the effects of vitamin D2 supplementation, Emily Brown, a PhD research fellow, analyzed 20 randomized controlled trials. In 18 of them, participants who received vitamin D2 had lower levels of vitamin D3 compared with placebo or control groups. Participants taking vitamin D2 experienced an average reduction of 25-hydroxyvitamin D3 ranging from 9 to 18 nanomoles per liter compared to control groups not receiving D2.

Once vitamin D enters the body, it goes through several processing steps. The liver converts it into 25-hydroxyvitamin D, the form doctors commonly measure in blood tests to estimate a person's vitamin D status. Vitamin D2 produces 25-hydroxyvitamin D2, while vitamin D3 produces 25-hydroxyvitamin D3. It is this D3-derived form that researchers found could fall after people took vitamin D2. Scientists are still working to fully understand why, but one leading hypothesis suggests that when total vitamin D metabolites spike, regulatory pathways speed up the breakdown and clearance of these compounds, leading to the disposal of 25-hydroxyvitamin D3.

The Immune System Angle

Professor Colin Smith added, "We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signaling system in the body — a key part of the immune system that provides a first line of defence against bacteria and viruses." That distinction matters enormously. Vitamin D is widely promoted for immune support, but if D2 doesn't deliver that benefit — and simultaneously suppresses D3 — consumers may be getting far less protection than they bargained for.

Researchers are careful to note that vitamin D2 is not dangerous. "Vitamin D2 still raises your total vitamin D levels, but we think vitamin D3 is preferential," Brown told BBC Science Focus. The nuance here is important: D2 isn't harmful in a direct sense, but it may be the less effective option when a better one exists.

What This Means for Your Supplement Choices

Lead researcher Emily Brown noted, "Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight. 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." Vitamin D2 is commonly found in plant-based and vegan supplements, making it the default choice for a large segment of the population.

The discovery is prompting scientists to rethink whether D3 should become the preferred choice for supplementation. For most people choosing between the two, the researchers suggest that vitamin D3 could have an advantage, although further research is needed to determine how much the differences matter for long-term health. As lead researcher Professor Lanham-New put it, "We were really astonished that when you give D2, the D3 levels drop below even the placebo. There's a lot more work that we now need to do because the long-term impact could be that you don't reach the required vitamin D levels or, dare I say it, you could make the situation worse." That's a sobering thought — and a compelling reason to check the label on your supplement bottle before winter sets in.