Scientists warn a popular vitamin D supplement may have a hidden downside — SkimNews
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- University of Surrey researchers published a 2025 meta-analysis in Nutrition Reviews finding that vitamin D2 supplementation lowered 25-hydroxyvitamin D3 levels by roughly 9–18 nanomoles per liter across randomized controlled trials.
- Emily Brown, lead researcher and PhD Research Fellow at Surrey, said D3 supplements may be more beneficial than D2 for most individuals, subject to personal considerations.
- A separate Frontiers in Immunology study led by Professor Colin Smith found D3 — but not D2 — stimulated type I interferon signaling, a key part of the immune system's early defense against viruses and bacteria.
- A 2026 systematic review of 26 randomized trials found D2 still reduced parathyroid hormone and produced a small rise in blood calcium, showing it retains meaningful biological effects even as it lowers D3.
- Current U.S. National Institutes of Health guidance reflects that D3 raises vitamin D levels more and keeps them elevated longer than D2.
- Professor Cathie Martin of the John Innes Centre called for accessible plant-based D3 options, noting lichen-derived D3 is now available for vegans and others avoiding animal-derived sources.
Why it matters: For UK consumers choosing winter supplements — and for public health policy on food fortification — the accumulated evidence now favors D3 over D2 for raising and sustaining vitamin D status. D2 remains common in fortified foods and cheaper supplements, meaning many buyers may be getting a form that is less effective at maintaining the D3 the body actually uses.
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