You probably have a bottle of vitamin D sitting in a kitchen cupboard or on a bathroom shelf. Most of us never flip it over to read what form of vitamin D is actually inside. The label matters more than you might think.
A team at the University of Surrey, the John Innes Centre and the Quadram Institute pooled randomized trials for a meta-analysis in Nutrition Reviews. People taking vitamin D2 ended up with lower levels of their own vitamin D3, the form the body makes from sunlight.
Across 11 trials with 655 adults, the D2 group finished with D3 levels about 9 to 18 nanomoles per liter lower than people who did not take D2. In some trials, D2 takers ended with less D3 than the placebo group.
Why the form on the bottle matters
D2, called ergocalciferol on the label, is made by shining ultraviolet light on yeast or mushrooms. D3, or cholecalciferol, is the version your skin produces in the sun. It is also found in fatty fish and egg yolks.
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The two forms do not act the same way in the body. In work from the same Surrey group, only D3 switched on type I interferon signaling, part of the immune system's first response to viruses and bacteria. D2 did not.
Emily Brown is the study's lead researcher at Surrey. In the university's announcement, she said "vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect."
The authors stress that D2 still raises total vitamin D by lifting the D2 fraction, so this is not the same as saying D2 is harmful.
Where you might run into D2
D2 is cheap to produce, so it turns up in many prescription-strength vitamin D pills, some fortified plant milks and a few breakfast cereals. A 50,000 IU weekly capsule written by a doctor is almost always D2.
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Over-the-counter vitamin D in the United States is most often sold as D3 already. The places to double-check are a prescription vitamin D bottle and the back label of any fortified plant milk or cereal in your fridge or pantry.
If the label says ergocalciferol or vitamin D2, and you are taking it to support bones, mood or immune defense, ask your doctor whether switching to D3 makes sense for you.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.