One form of vitamin D could cancel out the other, more active form |
Nearly 1 in 4 people in the United States has a moderate to severe vitamin D deficiency, yet the supplements many take could be exacerbating the issue.
A meta-analysis of 11 randomized controlled trials reveals that vitamin D2 (ergocalciferol) supplementation actively reduces circulating vitamin D3 (cholecalciferol) levels in the bloodstream.
“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,” explained Emily Brown, PhD research fellow at the University of Surrey and the study’s author.
The distinction matters because emerging research specifically links vitamin D3 to reduced cancer mortality, a stronger immune response against viruses and bacteria, and a reduced risk of depressive symptoms. Vitamin D2 shows no such associations, and it may even correlate with increased depression risk.
The mechanism behind this interference remains unclear. Both forms are converted in the liver to their respective active compounds, but researchers don’t yet understand why D2 suppresses D3 levels. What is certain is that groups taking D2 supplements showed greater declines in serum D3 than those taking no supplements at all, suggesting D2 doesn't simply fail to boost D3, but actively reduces it.
For more on how to choose the right supplement, what dosage experts recommend, and when D2 might still be appropriate, jump to “Vitamin D supplements: D3 may be more beneficial than D2.”
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