Many people take vitamin D supplements to keep their bones strong and their immune system healthy, especially during the darker winter months when sunlight is scarce.
But new research suggests that not all vitamin D supplements are equally effective—and one type may even lower the levels of the form our bodies use most efficiently.
The study, led by scientists at the University of Surrey, the John Innes Center, and the Quadram Institute Bioscience, examined the effects of two different types of vitamin D: vitamin D2 and vitamin D3.
Both are available as supplements, and both can raise overall vitamin D levels, but the researchers found that vitamin D2 has an unexpected downside.
When people took vitamin D2 supplements, their blood levels of vitamin D3 dropped.
Vitamin D3 is the form that our bodies naturally produce when skin is exposed to sunlight, and it is considered the most effective for boosting vitamin D status.
In many of the trials analyzed, participants taking vitamin D2 actually ended up with lower vitamin D3 levels than those who did not take any supplement at all.
The findings, published in Nutrition Reviews, highlight the importance of understanding the differences between the two forms.
“Vitamin D supplements are important, especially between October and March, when people in the U.K. can’t make enough vitamin D from sunlight,” said Dr. Emily Brown, lead researcher from the University of Surrey.
“However, we discovered that vitamin D2 supplements can decrease levels of vitamin D3 in the body. This suggests that vitamin D3 may be more beneficial for most people.”
This research supports earlier studies showing that vitamin D2 and D3 do not act in the same way in the body.
In work published previously in Frontiers in Immunology, Professor Colin Smith and colleagues at Surrey found that vitamin D3—but not vitamin D2—appears to stimulate the type I interferon signaling system, a crucial part of the immune system that helps fight off bacteria and viruses.
Maintaining healthy vitamin D3 levels, they suggested, may help prevent infections from taking hold.
Experts say this distinction is especially important given widespread vitamin D deficiency, particularly in winter. Professor Martin Warren of the Quadram Institute noted that tackling this problem with the most effective form of supplementation or food fortification could make a big difference for public health. Professor Cathie Martin of the John Innes Center added that ensuring plant-based vitamin D3 is widely available would help make it accessible to everyone.
Altogether, the findings point toward vitamin D3 as the preferred choice for most people, though individual needs may vary.
More research is needed to fully understand the differences between the two forms. For now, the evidence suggests that if you are taking a supplement to protect your health, vitamin D3 may be the better option.
