A British group reported recently that administration of vitamin D2, which is converted to vitamin D3 by the body, may be disadvantageous. A review of random controlled trials showed that taking vitamin D2 can lead to a decrease in vitamin D3 levels, the more active form generated by sunlight exposure. The research indicates that D2 supplementation may not only be less effective, but can also lead to lower concentrations of D3 in the circulation. [Emily I G Brown, et al. Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutr Rev, 2025 Sep 18:nuaf166.doi: 10.1093/nutrit/nuaf166]. Is this an accurate and meaningful assessment? What is most important for us to know?

This, in fact, is accurate information as far as it goes. It does not reveal the mechanism, the intricate interplay of dietary forms of the vitamin and the differing effects within the body. There is a feedback mechanism that helps to regulate levels of the active D3 form, and this is a way to provide a source for counterbalancing situations where someone might have quite intense and prolonged sunlight exposure much of the day for months at a time, and thus provides a way that natural means can be encouraged to keep things in balance. We cannot lay this out for you biochemically as that would require leading. But, as far as it goes, the publication is not misrepresenting the existence of the interplay it describes, between vitamin D2 in vitamin D3. So it has a practical implication immediately, in showing that if one desires to have adequate biological vitamin D benefits through supplementation, they need to take vitamin D3 and not vitamin D2.