Vitamin D2 or D3? What a new study means for your supplement

If you take a vitamin D supplement, there's a good chance you've never checked which kind it is. A recent review from the University of Surrey suggests it's worth a look: one form, vitamin D2, may lower the levels of the other form, vitamin D3, in your blood.

Here's what the study found, what it doesn't tell us, and how to check your own label.

What the study found

Researchers pooled 11 randomised controlled trials with 655 people. Those who took vitamin D2 ended up with less vitamin D3 in their blood than people who didn't take it: on average about 18 nmol/L (roughly 7 ng/mL) lower by the end of the trials.

It was published in the journal Nutrition Reviews in 2025 and funded by the UK's BBSRC, a public research council, not a supplement company.

D2 and D3: the two forms

• Vitamin D3 (cholecalciferol) is the form your skin makes in sunlight. It's also in eggs and oily fish, and in most supplements.

• Vitamin D2 (ergocalciferol) comes from plants and mushrooms. It's used in some supplements and fortified foods.

Both raise your total vitamin D. But an earlier Surrey review of trials found D3 does it more effectively, and the new study adds another reason: D2 may also push down the D3 you already have. The researchers say D3 may be the better choice for most people.

What it doesn't tell us

• It measured blood levels, not health outcomes. It doesn't show that D2 harms your bones, immunity or weight.

• The trials were small and varied a lot in dose, length and who took part.

• The researchers say more work is needed on how the two forms behave in the body.

So this isn't a reason to panic. It's a reason to read your label.

How to check your supplement

1. Read the ingredients. Cholecalciferol means D3. Ergocalciferol means D2. Multivitamins and calcium tablets list it too.

2. Vegetarian or vegan? Many D3 supplements are made from lanolin, which comes from sheep's wool. Plant-based D3 made from lichen is also available, so you don't have to choose D2.

3. Don't change a prescribed dose on your own. If your doctor has put you on a weekly or monthly dose, ask them before switching or stopping. A simple blood test shows where your levels are.

Food and sunlight still count

Supplements fill a gap; they don't replace the basics. Eggs, oily fish and fortified milk add some vitamin D, and short spells of sun on your arms and face help your skin make D3. Many women who work indoors all day get very little of either, which is why it's worth knowing what your supplement actually contains.

Your next step

If you're working on fat loss or your energy and you're not sure your supplements are doing their job, book a call. We'll look at your food, routine and reports together.

Sources

• Brown et al., Effect of vitamin D2 supplementation on 25-hydroxyvitamin D3 status, Nutrition Reviews 2025

• University of Surrey: Vitamin D3 concentrations are lowered by a common vitamin D supplement

• Tripkovic et al., Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D, Am J Clin Nutr 2012

• NIH Office of Dietary Supplements: Vitamin D fact sheet

This article is general information, not medical advice. If you're pregnant, breastfeeding or managing a medical condition, speak to your doctor before changing any supplement.