Vitamin D: Are you taking the right type of supplement? Study says the wrong one can actually affect your immunity

Taking the wrong kind of vitamin D could be doing the opposite of what you want

If you’re taking vitamin D supplements to boost your health, here’s something you need to know—not all vitamin D is created equal. A new study from the University of Surrey, John Innes Centre and Quadram Institute Bioscience, found that vitamin D2 supplements can actually lower your vitamin D3 levels, the form your body needs most for strong immunity, bone health, and overall wellness.

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For millions of Americans who take vitamin D for immune support, bone strength, or to combat seasonal vitamin D deficiency, this discovery is big. The study suggests that vitamin D3 (cholecalciferol)—the form your body makes from sunlight—is far more effective than vitamin D2 (ergocalciferol), which is often used in supplements and fortified foods.

Vitamin D2 vs D3: What’s the difference?

Vitamin D2 comes from plants and mushrooms, especially those exposed to UV light. Vitamin D3, on the other hand, is produced naturally in your skin when you’re in the sun and is also found in foods like fish, eggs, and fortified milk.


When either form enters your body, it gets converted into 25-hydroxyvitamin D (also called 25(OH)D or calcidiol)—the form doctors check in your blood tests. Both versions work similarly, but the body seems to prefer D3 for long-term vitamin D balance.


The new study: What researchers found

The study reviewed 20 clinical trials conducted between 1975 and 2023, analyzing how D2 supplements affect D3 levels in the blood. Out of those, 11 studies were combined into a major meta-analysis.


The surprising result: when participants took vitamin D2 supplements, their vitamin D3 levels dropped. That means D2 didn’t just fail to boost D3—it may have actually suppressed it.

Professor Cathie Martin, Group Leader at the John Innes Centre, said: “This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK. ” This research supports a previous study published in Frontiers in Immunology, led by Professor Colin Smith from the University of Surrey, which suggests that vitamin D2 and D3 do not have identical roles in supporting immune function. Vitamin D3 has a modifying effect on the immune system that could fortify the body against viral and bacterial diseases.

Professor Colin Smith said: “We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signalling system in the body – a key part of the immune system that provides a first line of defence against bacteria and viruses. Thus, a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body.”



What this means for you

For Americans who don’t get much sun—especially during the winter months—vitamin D3 supplements are the clear winner. Research shows D3 is more potent, longer-lasting, and more effective at raising total vitamin D levels than D2.


If you’re vegan or plant-based, you may still come across vitamin D2 in certain foods or supplements. While it’s still better than nothing, it’s less efficient and may even lower D3 levels if taken regularly.


"Vitamin D2 supplement production would of course remain necessary to facilitate availability for personal requirements, as nearly all vitamin D3 supplements are derived from animal sources and are therefore not applicable to vegans. However, the production and prescription of vitamin D3 as a first-line recommendation may be a useful policy to implement due to the potential detrimental influence vitamin D2 supplements may have upon serum 25(OH)D3 status," the researchers have said.


When it comes to vitamin D and immunity, D3 is the gold standard. It’s the form your body naturally makes and uses best. Taking vitamin D2 may seem harmless, but research shows it can lower your D3 levels, which could impact everything from your immune system to your bone strength.


“However, further research is needed to establish whether vitamins D2 and D3 elicit different changes in overall vitamin D metabolism that might influence clinical advice to recommend vitamin D3 supplements over vitamin D2 supplements, where appropriate,” the researchers have said.