You probably reach for a vitamin D pill once the days start getting shorter.
Most people barely glance past the big "D" on the front. But there is a small letter right next to it that can change how much good the supplement actually does.
A new review of 20 trials, published in Nutrition Reviews in September 2025, found that taking vitamin D2 can lower the body's levels of vitamin D3.
Across 11 of those trials with 655 people, D2 supplements dropped circulating D3 by 9 to 18 nanomoles per liter compared with people who did not take D2.
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The research team came from the University of Surrey, the John Innes Centre and the Quadram Institute Bioscience.
"Vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect," said Emily Brown, the Surrey PhD research fellow who led the work.
Why the two forms are not interchangeable
D3, called cholecalciferol, is made in the skin by sunlight and is also found in oily fish and egg yolks.
D2, called ergocalciferol, comes from plants and fungi. It is often made from UV-treated yeast or mushrooms and is the form used in many vegetarian and vegan supplements.
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Both raise overall vitamin D in the blood. But the body appears to handle them differently once they are inside you.
Earlier work from the same team suggests only D3 switches on an immune pathway called type I interferon signaling. The body uses that pathway as an early alarm against viruses.
The researchers stress that D2 is not harmful. The concern is that people who choose D2 for ethical or dietary reasons may miss out on part of the immune benefit. Some may even see their D3 drop.
What to check before buying your next bottle
Turn the bottle around and look at the ingredients panel. The active form will be listed as either "cholecalciferol (vitamin D3)" or "ergocalciferol (vitamin D2)."
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If the front only says "vitamin D," the ingredient list on the back will spell out which one is in the pill.
Plant-sourced D3 does exist for people on a vegan or vegetarian diet. It is made from lichen and sold by several brands. It tends to cost more than yeast-derived D2.
Anyone unsure which form they take can check the ingredient list on the back of the bottle today, before heading into the darker months.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.