Vitamin D is one of the most important nutrients for human health. It helps the body absorb calcium, supports healthy bones and muscles, and plays an important role in the immune system.
Many people also take vitamin D supplements during winter because the body produces less vitamin D when there is limited sunlight.
In countries such as the United Kingdom, health authorities recommend that people take vitamin D supplements during the colder months, especially between October and March. This is because sunlight exposure becomes too weak for the body to produce enough vitamin D naturally through the skin.
There are two main forms of vitamin D supplements available on the market: vitamin D2 and vitamin D3. For many years, both forms were generally considered helpful ways to improve vitamin D levels.
However, new research from the University of Surrey, the John Innes Centre, and Quadram Institute Bioscience suggests the two forms may not work the same way inside the body.
The researchers discovered that taking vitamin D2 supplements may actually lower levels of vitamin D3 in the body. Vitamin D3 is considered the more effective form because it is the type naturally produced when sunlight reaches the skin.
Scientists believe vitamin D3 raises and maintains overall vitamin D levels more efficiently than vitamin D2.
The study was published in the journal Nutrition Reviews. Researchers analyzed data from several randomized controlled trials, which are considered among the strongest forms of medical research.
They compared people taking vitamin D2 supplements with people who were not taking vitamin D2.
The results showed a clear pattern. People who took vitamin D2 often experienced a drop in their vitamin D3 levels. In some studies, vitamin D3 levels fell even lower than in the control groups who did not take vitamin D2 supplements at all.
Lead researcher Emily Brown from the University of Surrey explained that the findings reveal a previously unknown effect of vitamin D2 supplementation. She noted that vitamin D supplements remain very important, especially during winter months when sunlight exposure is low.
However, the research suggests that vitamin D3 supplements may provide greater benefits for many people.
Vitamin D deficiency is a major public health concern in many countries. Low vitamin D levels have been linked to weak bones, muscle weakness, fatigue, and a higher risk of fractures. Scientists are also studying how vitamin D affects immune function, inflammation, and infection risk.
The new findings also support earlier research published in Frontiers in Immunology. That study suggested vitamin D2 and vitamin D3 may affect the immune system differently.
Professor Colin Smith from the University of Surrey explained that vitamin D3 appears to activate a part of the immune system known as type I interferon signaling. This system acts as one of the body’s early defense mechanisms against bacteria and viruses.
According to Professor Smith, healthy vitamin D3 levels may help stop viruses and bacteria from establishing infections in the body. The researchers did not observe the same immune-related effects with vitamin D2.
Scientists say this growing evidence suggests that vitamin D3 may play a more important role in immune protection than vitamin D2. This could become especially important during winter months when respiratory infections are more common.
The study also raises questions about plant-based vitamin D sources. Traditionally, vitamin D2 has often been used in vegan and vegetarian supplements because it can be produced from fungi and yeast. Vitamin D3 has historically come from animal-based sources such as lanolin from sheep’s wool.
However, researchers now emphasize the importance of expanding access to plant-based vitamin D3 products so that vegetarians and vegans can also benefit from the potentially more effective form of supplementation.
Professor Cathie Martin from the John Innes Centre highlighted the need for better availability of plant-based vitamin D3 options in the UK.
The findings may eventually influence future health recommendations and supplement guidelines. However, researchers caution that individual health needs still matter. Some people may require different supplement types depending on medical conditions, diet, age, or other factors.
Experts also stress that people should not suddenly stop taking prescribed supplements without medical advice. Vitamin D deficiency remains a significant health issue, and both forms of supplementation can still help raise vitamin D intake compared to taking nothing at all.
Professor Martin Warren from Quadram Institute Bioscience said vitamin D deficiency continues to affect large parts of the population, particularly during winter. He added that identifying the most effective form of supplementation is important for improving national health.
The researchers believe more studies are needed to fully understand the biological differences between vitamin D2 and vitamin D3. Scientists hope future research will clarify how each form affects immunity, long-term health, and disease prevention.
Overall, the findings suggest that vitamin D supplements are more complex than previously believed. Although vitamin D2 and D3 may look similar on store shelves, they may behave very differently inside the body.
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