October 24, 2025, 10:59 am | Read time: 6 minutes
Vitamin D is one of the most commonly supplemented vitamins—especially in regions and during seasons with little sunlight. It supports numerous processes in the body, including bone metabolism, the immune system, and cell division. What many don’t know: There are two different forms that can be absorbed through food or supplements: Vitamin D2 and Vitamin D3. Although both variants have been used for a long time, their effects in the body are not entirely the same. A new analysis of current studies now provides more precise insights.
Vitamin D Absorption
The human body can produce Vitamin D itself when the skin is exposed to the sun. More specifically, Vitamin D3 is formed in the skin under the influence of UVB rays. Additionally, Vitamin D can be absorbed through diet—either from natural sources like fatty fish, egg yolks, or liver, or from fortified foods and supplements.
It is important to know: There are two main forms of Vitamin D that are processed differently in the body:
- Vitamin D2 (Ergocalciferol): This form comes from plant sources, such as mushrooms exposed to UV light. It is often used in vegan supplements.
- Vitamin D3 (Cholecalciferol): The body produces this form itself when sunlight hits the skin. It is also found in animal foods like fish, eggs, or liver. Since the body utilizes this form particularly efficiently, Vitamin D3 is considered the more effective variant today.
Both forms are first converted into a storage form in the body—the so-called 25-hydroxyvitamin D. This intermediate form, especially 25(OH)D3, is the most important measure in the blood to determine a person’s Vitamin D status.
Is Vitamin D2 Supplementation Perhaps Not So Good?
In some earlier studies, it was observed that taking Vitamin D2 can lead to a decrease in the 25(OH)D3 level in the blood.1 However, these indications were mostly incidental findings in studies with other focuses. A targeted, systematic investigation of this interaction has been lacking until now.
The newly published meta-analysis aimed to fill this gap. It examined whether and to what extent Vitamin D2 influenced the level of Vitamin D3 in the blood, using data from high-quality clinical studies.2
20 Vitamin D Studies in Focus
The research team specifically searched the medical database PubMed for studies from the period 1975 to early 2023. From over 200 results, the researchers selected 20 studies that met key criteria—such as measuring the 25(OH)D3 level before and after taking Vitamin D2.
Of these 20 studies, eleven could be included in the actual meta-analysis. This means: The results of these studies were statistically combined to obtain the most accurate picture possible. The remaining eight were included in the overview but were not re-analyzed statistically.
The studies examined were so-called randomized controlled trials (RCTs)—the gold standard in medical research. In total, over 650 people participated in these studies. The researchers compared how the Vitamin D3 level developed in people who took Vitamin D2 compared to those who received a placebo or no supplements at all.
Vitamin D2 Significantly Lowers the Vitamin D3 Level in the Blood
The most important finding: People who take Vitamin D2 have, on average, less Vitamin D3 in their blood than those without D2 supplementation.
The analysis showed:
- At the end of the studies, the 25(OH)D3 level in D2 users was on average 17.99 nanomoles per liter (nmol/L) lower than in the control group. (This unit is used to indicate concentrations of substances in the blood.)
- Also, over the course of the studies, there was a clear difference: The Vitamin D3 level decreased in the D2 groups by an average of 9.25 nmol/L more than in the comparison groups.
Both differences were statistically significant, meaning: They are very unlikely to have occurred by chance. In research, the so-called P-value is used for this—in this analysis, it was below 0.0001, which is considered very significant.
Of the 20 studies evaluated, 18 reported a decrease in the D3 level after D2 intake. In 16 of them, the decrease was even greater than in the control groups. Only two studies showed a slight increase, but even there, the D3 level remained below the level of the comparison group.
Vitamin D2 lowers the Vitamin D3 level in the body, which is the form of Vitamin D that the body utilizes most efficiently.
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What Does This Mean for Supplementation?
Many people take Vitamin D to support their health—especially in the winter months or when a deficiency has been identified. Some deliberately choose Vitamin D2, for example, for vegan reasons, because D3 usually comes from animal sources.
The results of this analysis now show that Vitamin D2 is not only less effective at increasing the Vitamin D level, but it could even lower the level of Vitamin D3. And this is relevant because this value (25(OH)D3) is crucial for many of the positive effects in the body.
What Could Be the Reason?
Researchers suspect that the body wants to regulate the overall level of Vitamin D. If more Vitamin D enters the system through the intake of D2, the body may respond with an increased breakdown of D3 to maintain balance. It is also discussed whether D2 and D3 displace each other in metabolism.
Whether a decreasing D3 level also has negative health consequences is not yet conclusively clarified—especially if the total storage value of Vitamin D (i.e., 25(OH)D overall) is still within the normal range. Nevertheless, it is clear: Those who want to specifically improve their Vitamin D level should—if possible–opt for Vitamin D3, as it is better utilized by the body, especially when there is an increased need.
Are There Limitations?
As with any scientific work, there are some points to consider here:
- The studies reviewed in the analysis differed significantly in design, duration, dosage, and measurement methods—this can make comparability difficult.
- Not all studies reported how many participants dropped out—this can slightly reduce the significance.
- The researchers did not conduct a systematic analysis of possible publication biases—there were not enough studies available.
Despite various checks, the results remained stable: Even when the researchers removed individual studies, the same trend continued to show: Vitamin D2 lowers the Vitamin D3 level.
The researchers examined the effect almost exclusively in healthy adults. Whether older people or those with pre-existing conditions react similarly needs to be clarified in future studies.