Everyone takes vitamin D. But which one?
Vitamin D is one of the most widely consumed supplements in the world. In northern latitudes with limited winter sun, millions of people take it daily to maintain adequate levels.
What most people don't realize is that there are two main forms of vitamin D in supplements: D2 (ergocalciferol), plant-derived, and D3 (cholecalciferol), animal-derived or synthetic — the form closest to what our skin produces from sunlight. And according to a new study published in May 2026, these two forms don't have the same effect in the body.
What the University of Surrey study found
A team from the University of Surrey, in collaboration with the John Innes Centre and Quadram Institute Bioscience, published a meta-analysis in Nutrition Reviews. The researchers analyzed 11 randomized controlled trials involving 655 adults.
The unexpected finding: people taking vitamin D2 supplements had significantly lower vitamin D3 levels than those not taking D2.
In other words, D2 doesn't just raise your D2 levels. It actively reduces your D3 levels. And that's an effect nobody had documented before.
Why this matters
Vitamin D3 is the form the body naturally produces from sunlight — and the form most efficiently used to raise overall vitamin D status. If taking D2 is simultaneously lowering your D3, the net effect on your health may be very different from what you thought you were getting.
The precise mechanism behind this interaction isn't fully understood yet. But researchers believe D2 and D3 compete in the body — and D2 appears to have the upper hand in that competition at the expense of D3.
What this changes in practice
The researchers are cautious about blanket recommendations, but the direction is clear: for most people (absent personal health considerations), D3 supplements are likely more beneficial than D2.
An important exception: vegans and vegetarians avoiding animal products often turn to D2 because it's the only plant-based option widely available. In that case, the decision remains personal — and is worth discussing with a healthcare provider to assess the actual impact on your blood levels.
Either way, if you're currently taking vitamin D and haven't checked which form you're taking: now would be the time to check the label.
Source: University of Surrey, May 2026