Scientists study vitamins D2 vs. D3 and make a strange conclusion about which is the better supplement

Most people link vitamin D to sunshine. When ultraviolet rays touch the skin, they turn a protein into vitamin D3. That version fuels bones, muscles, and the immune system.

The process works well in summer, but in autumn and winter it slows down. In countries like the UK, sunlight isn’t strong enough to produce much vitamin D. Health officials therefore suggest supplements during the darker months.

Deficiency can creep in quietly. Fatigue, bone pain, frequent illness, and muscle weakness often appear, but people blame stress or ageing.

The lack of obvious warning signs means vitamin D deficiency remains one of the most underdiagnosed problems in public health.

Two kinds of vitamin D

Supplements come in two forms: vitamin D3, or cholecalciferol, and vitamin D2, or ergocalciferol. D3 usually comes from lanolin, a waxy coating on sheep’s wool.

D2 mainly comes from mushrooms. For years, experts believed the body treated them the same. That assumption shaped public advice and personal choices, especially for people who prefer plant-based diets.

Many pharmacies and health stores continue to stock both types side by side.

Until recently, people assumed choosing between them was a matter of cost or personal values. The latest research, however, suggests the decision is more complex.

Vitamin D2 lowers D3

Emily Brown at the University of Surrey and her colleagues examined 11 trials with 655 participants. Their analysis showed something surprising: vitamin D2 supplements lowered the concentration of vitamin D3 in the body.

Why this happens is still unknown. Researchers also don’t yet know if D3 supplements affect D2 levels in the same way.

Furthermore, in several studies, people who took D2 ended up with lower D3 levels than those who took no supplements at all.

This is a previously unknown effect. The discovery flips long-standing assumptions and shows that supplement interactions deserve closer attention.

Vitamin D3 and immunity

A 2022 study hinted at an explanation. D2 and D3 share some functions, but they don’t act identically.

Only vitamin D3 appears to trigger the type-I interferon system, which acts as a first barrier against infections. That suggests D3 might carry extra benefits for immune health.

The immune angle makes this debate more than academic.

Respiratory infections peak in the colder months when sunlight is already scarce. Choosing the right supplement could influence how well the body fights off seasonal illness.

Choosing between D2 and D3

Brown believes D3 may suit most people better than D2. But she warns against dismissing D2 completely.

Your overall vitamin D levels will stay adequate with D2 supplements, but the immune benefits may be weaker, and some extra protective functions could be missing.

The takeaway is that supplements matter, but the type you pick may shape the benefits. D2 is not useless, yet it may not offer the same immune boost as D3.

Older adults and D2

The picture changes for older adults. Research shows the body becomes less efficient at turning D3 into its active form, calcitriol, with age. In those cases, D2 might help fill the gap.

That means the best supplement could depend on age as much as diet. This adds another layer to public health advice, which often paints vitamin D supplementation as one-size-fits-all.

Vegan options for vitamin D3

People who avoid animal products face another factor.

“In the absence of any evidence for negative effects, if someone who is vegan is deficient in vitamin D, opting for a D2 supplement over no supplement remains the likely prudent choice,” says Bernadette Moore at the University of Liverpool.

Plant-based D3 is starting to appear. Scientists have even engineered tomatoes that make vitamin D3, though trials are still underway.

These developments could shift the market. If reliable plant-based D3 becomes common, it may resolve the tension between dietary ethics and immune effectiveness.

Until then, many vegans will continue relying on D2, knowing it at least prevents deficiency.

Why supplements matter

Team member Susan Lanham-New, also at the University of Surrey, wants the findings to serve as a reminder.

In the United Kingdom and other northern regions, many people develop mild osteomalacia during winter due to vitamin D deficiency.

It often shows up as tiredness, bone pain, muscle aches, or frequent infections, but most people don’t realize the cause.

Public health campaigns often highlight iron, calcium, or vitamin C, but vitamin D receives less attention. Yet it remains a cornerstone of health, quietly shaping immunity, mood, and energy levels.

As science untangles the differences between D2 and D3, one lesson is already clear: ignoring vitamin D is not an option.

The study is published in the journal Nutrition Reviews.

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