Vitamin D2 supplements linked to lower vitamin D3 levels

New research suggests that taking one type of vitamin D may unexpectedly reduce blood levels of another. A review of studies found that vitamin D2 supplements were linked with lower concentrations of vitamin D3, the form the body makes when skin is exposed to sunlight.

The result is notable because vitamin D supplements are commonly used to help maintain bone health and prevent deficiency. However, the two main forms used in supplements, vitamin D2 and vitamin D3, do not always act the same way in the body.

Scientists from the University of Surrey, the John Innes Centre and the Quadram Institute looked at evidence from earlier trials that compared vitamin D2 supplementation with other approaches, including no supplement at all.

In several of the studies reviewed, people who took vitamin D2 had lower blood levels of vitamin D3 than participants in the comparison groups. The finding raises questions about how the body manages different forms of the nutrient.

Published in Nutrition Reviews, the review does not suggest that vitamin D2 is useless. Both forms can contribute to overall vitamin D status, even if their effects on individual vitamin D compounds are different.

Vitamin D3 is produced in the skin after sunlight exposure and is also available in supplement form. Vitamin D2 is usually derived from plant or fungal sources, which makes it suitable for some fortified foods and dietary products.

Earlier studies have suggested that vitamin D3 may be more effective at raising certain measures of vitamin D status. Researchers have also examined whether the two forms affect immune responses differently, though the health implications are still unclear.

The new analysis focused on changes seen in blood samples. It did not show that lower vitamin D3 levels caused illness or that people taking vitamin D2 had worse health outcomes.

Questions remain about dose, duration, starting vitamin D status and differences among populations. These factors may affect how much vitamin D3 changes after someone begins taking vitamin D2.

Blood tests can help researchers see what supplements are doing inside the body, but they are only one part of assessing health effects. A shift in one vitamin measure does not automatically show whether bones, muscles or immune defenses are better protected or more vulnerable. Studies that look at clinical outcomes, not just lab markers, will be needed to interpret the finding fully.

The findings underline that supplements sharing the same vitamin name should not automatically be considered identical. More research is needed to determine whether the measured differences matter for long-term health and how they should shape supplementation guidance.

For broader related reading, please see articles about How can vitamin D benefit your health, and Why Calcium and Vitamin D Supplements So Important to Older People.

For more related background, please read articles about Vitamin D and cognitive function: What you need to know, and Can vitamin D help prevent or treat cancer?.

Source: University of Surrey, John Innes Centre and Quadram Institute.