Vitamin D supplements are usually treated as a single, interchangeable commodity: a small capsule delivering the "sunshine vitamin" in whatever form happens to be on the shelf. Research published in 2025 and highlighted by the University of Surrey suggests that assumption deserves a second look. Scientists found that taking vitamin D2 can actually reduce levels of vitamin D3 — the form the human body produces naturally when skin is exposed to sunlight, and the one generally considered more effective at raising overall vitamin D status.
The finding matters because D2 and D3 are routinely described as two routes to the same destination. Both can lift a person's vitamin D status, and both are sold over the counter in many countries. But a growing body of evidence indicates they do not behave identically once inside the body.
A Supplement That Appears to Work Against Itself
The headline result is counterintuitive: a nutrient taken to raise vitamin D may, in one specific form, suppress a related version of the same nutrient. The research indicates that vitamin D2 supplementation is associated with lower levels of vitamin D3 circulating in the body.
That is not simply a curiosity of laboratory measurement. Vitamin D3 is the form humans synthesise endogenously, and it is also the form that official guidance increasingly favours for sustaining blood levels over time. If a supplement popular precisely because it raises vitamin D is instead pushing down the body's own preferred version, the practical implications for consumers and clinicians alike are worth examining.
Previous studies add another dimension to the picture. That earlier work suggests vitamin D3 may offer stronger immune protection against viruses and bacteria than D2 — a distinction with obvious relevance for anyone taking a supplement with the aim of supporting immune function.
Vitamin D2 and Vitamin D3 Are Not Quite the Same
The two forms differ in where they come from and, to a meaningful degree, in how the body handles them.
Different origins, different chemistry
- Vitamin D2, or ergocalciferol, is typically manufactured by exposing fungi or yeast to ultraviolet light.
- Vitamin D3, or cholecalciferol, is the same molecule the skin produces after ultraviolet B radiation from sunlight triggers its synthesis.
- Historically, supplement-grade D3 has been derived from lanolin obtained from sheep's wool. Animal-free D3 produced from sources such as lichen is now also available — a significant consideration for people following vegan or plant-based diets.
These are not trivial labelling details. The source of a supplement determines whether it fits a person's dietary or ethical requirements, and it can also shape how the finished product performs once ingested.
What Happens Once Vitamin D Enters the Body
Vitamin D does not act immediately after it is swallowed. It passes through a series of processing steps, beginning with the liver, which converts it into 25-hydroxyvitamin D. This is the metabolite doctors commonly measure in blood tests to estimate a person's vitamin D status, and it is the number most patients see on a lab report.
Crucially, the two forms yield different versions of that marker. Vitamin D2 produces 25-hydroxyvitamin D2, while vitamin D3 produces 25-hydroxyvitamin D3. The two are related but not identical, and they do not necessarily behave the same way in the body or persist for the same length of time.
This is where official assessments have already drawn a line. Guidance from the U.S. National Institutes of Health notes that D3 tends to raise vitamin D levels more effectively and to keep them elevated for longer than D2. In other words, the evidence favouring D3 predates the new finding about D2 lowering D3 — the latest research adds a further reason for caution rather than overturning a settled question.
Why Vitamin D Matters in the First Place
None of this would carry much weight if vitamin D were optional. It is not. The nutrient is essential for helping the body absorb calcium and for maintaining healthy bones. It also plays recognised roles in muscle function, nerve function and the immune system.
In the UK, people are advised to consider taking 10 micrograms (µg) of vitamin D each day during the darker months, when sunlight is generally too weak for the skin to produce enough of the vitamin. That seasonal advice reflects a straightforward biological constraint: for much of the year at higher latitudes, the sun simply cannot do the job.
That makes supplement choice a genuine public health question rather than a niche concern for specialists. When large numbers of people take a daily dose through winter, small differences in how the two forms perform can accumulate across a population.
What This Means for Anyone Taking a Supplement
The research does not suggest that vitamin D supplementation is pointless — far from it. Both forms can raise overall vitamin D status. What it does suggest is that the two are not interchangeable in the way they are often marketed as being.
- Check which form you are taking. Supplement labels list either vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol). The distinction is worth reading.
- Weigh the existing evidence on D3. Official guidance indicates D3 raises and sustains blood levels more effectively than D2.
- Vegan and plant-based consumers still have options. Lichen-derived D3 provides an animal-free route to the form that guidance favours.
- Potency is not the only question. The new finding concerns the direction of effect — D2 appearing to lower D3 — not merely how much vitamin D a dose delivers.
Questions That Remain Open
As with any single study, the results raise as many questions as they answer. How large is the effect on vitamin D3 levels, and how quickly does it appear? Does it reverse when a person stops taking D2? Are some groups — older adults, people with darker skin, those living at high latitudes, or individuals with diagnosed deficiency — more affected than others?
The practical takeaway is narrower but still useful: the two forms of vitamin D are not equivalent, and the evidence increasingly points toward D3 as the better-supported choice for raising and maintaining vitamin D status. For consumers standing in front of a supplement shelf, that is information worth having.
This article is based on reporting by Science Daily. Read the original article.
Originally published on sciencedaily.com
