Vitamin D on the label is not always the same thing. A systematic review and meta-analysis published in Nutrition Reviews in September 2025 found the D2 form of the supplement may actually pull down blood levels of vitamin D3.
Vitamin D2 (ergocalciferol) comes from UV-treated yeast or mushrooms and appears in cheaper products, prescriptions and fortified plant milks. Vitamin D3 (cholecalciferol) is what your skin makes from sunlight, the form the body uses more efficiently.
The practical takeaway is simple. Check the label for vitamin D3 (cholecalciferol) rather than D2 (ergocalciferol).
What the study found
Researchers at the University of Surrey, the John Innes Centre and Quadram Institute Bioscience pooled data from randomized controlled trials, drawing on articles indexed in PubMed between January 1975 and February 2023.
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Eleven trials with 655 participants were included in the meta-analysis, 342 taking vitamin D2 and 313 controls. Participants on vitamin D2 showed statistically significant reductions in blood levels of 25-hydroxyvitamin D3 compared with those who did not.
"Vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect," said Emily Brown, PhD research fellow at the University of Surrey, who led the analysis.
An earlier related study identified a functional difference between the two forms. Professor Colin Smith of the University of Surrey has said vitamin D3, but not D2, appears to stimulate the body's type I interferon signalling system, a key part of the immune system.
D2 or D3, what is on your label
Vitamin D3 is what your body produces when sunlight hits the skin. It occurs naturally in fatty fish, egg yolks, liver and fortified dairy. Most supplements derive it from lanolin, and lichen-based versions are widely available for vegans.
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D2 is produced by exposing yeast or mushrooms to UV light. It shows up in fortified plant milks and cereals, and in many prescription high-dose vitamin D preparations.
Both forms raise blood vitamin D on standard tests, but the new meta-analysis suggests they behave differently inside the body.
How much you actually need
The National Institutes of Health's Office of Dietary Supplements sets the recommended dietary allowance for most adults at 15 micrograms (600 IU) per day, rising to 20 micrograms (800 IU) after age 70.
The tolerable upper intake level for adults is 100 micrograms (4,000 IU) a day. High-dose products above that mark are common on shelves, and long-term intake past the upper limit can cause hypercalcemia and, in extreme cases, kidney damage.
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A 25-hydroxyvitamin D blood test, ordered by a doctor, is the standard way to check whether your levels are low, adequate or high. If you have been taking a D2 product and your numbers are not moving, ask your doctor whether a D3-based supplement is a better fit.
The Nutrition Reviews authors note that longer trials are needed before official recommendations change. For consumers, the immediate step is on the label. Look for cholecalciferol (D3), not ergocalciferol (D2).
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.