Vitamin D2 comes from mushrooms and fortified foods, D3 from sunlight and animal sources — same vitamin, different packaging. Most people asking which is better stop right there.
But the difference between vitamin D2 and D3 isn't about where they come from. It's about what one of them does to the other after you swallow it.
Vitamin D2 vs D3: Which Is Actually Better
Vitamin D2 supplementation actively reduces existing D3 levels in the bloodstream. Across twenty randomized controlled trials, eighteen found that D2 pulled D3 levels down — with an average reduction of approximately 18 nmol/L compared to placebo. D3 is the recommended form for anyone who can take it; vegan D3 from lichen is available for plant-based eaters.
— Brown et al. 2025 · Nutrition Reviews · n=1,080
D2 doesn't just raise your vitamin D levels less effectively than D3. It actively lowers the D3 already in your bloodstream.
A 2025 meta-analysis of twenty trials found something close to unanimous: eighteen showed that D2 supplementation reduced existing D3 levels — not raised them less, not left them unchanged, but actively pulled them down. People taking D2 ended up with less D3 in their blood than people taking nothing at all.
Your supplement wasn't underperforming. It was undoing the thing you were trying to build.
Inside your body, here's what happens. D2 arrives and total vitamin D goes up — which sounds exactly right. But that rise trips a cleanup response. A disposal enzyme kicks in, speeding up the breakdown of all vitamin D forms. And it clears D3 faster than D2.
D2 gets in. The alarm fires. The D3 your skin made from last weekend's sun gets swept out as collateral.
How exactly this happens isn't fully confirmed — the disposal-enzyme explanation is the strongest theory so far, but the precise mechanism hasn't been locked down. What IS confirmed is the direction. Twenty trials. Eighteen showing the same result. Whatever drives it, D2 supplementation and falling D3 levels travel together.
Blood levels might not even tell the full story. Only D3 triggered the body's interferon response — part of the frontline defense your immune system uses against infections. D2 didn't. If that holds up, D2 and D3 aren't different speeds of the same vitamin. They're different tools entirely — and what your muscles lose when D3 doesn't show up might matter more than any number on a blood test.
If you're taking D2, it may be reducing the D3 your own skin produces from sunlight.
Plant-based eaters face a genuine bind here. Nearly all D3 supplements come from animal sources — sheep's wool lanolin, usually — which makes D2 the default for anyone avoiding animal products. The plant-based alternative could be quietly undoing what your body makes for free.
Vegan D3 exists — sourced from lichen, a plant-like organism that produces D3 naturally — though it costs more and shows up in fewer stores. Worth checking, especially if your multivitamin is your only source of vitamin D. Most contain D2, not D3.
For anyone who can choose: D3. Not because it's marginally more effective — the answer every other page on the internet gives. Because D2 actively fights the D3 already in your blood.
Two bottles. Same shelf. Same word on the label. One of them raises what matters. The other tears it down while the label promises it's helping.