Recent research from the University of Surrey has uncovered a concerning effect of vitamin D2 supplements, revealing that they may actually reduce the body's levels of vitamin D3, the more effective form of vitamin D. Vitamin D3 is not only better at boosting overall vitamin D status but also plays a vital role in enhancing the immune system's ability to combat infections. The study, published in *Nutrition Reviews*, analyzed randomized controlled trials and found that individuals taking vitamin D2 experienced lower levels of vitamin D3 compared to those not supplementing with D2. Lead researcher Emily Brown emphasized the importance of vitamin D supplements during the winter months when sunlight exposure is limited, but cautioned that vitamin D2 might not be the best choice for many. The findings suggest that vitamin D3 should be considered the preferred supplement, especially given its unique immune-modulating properties. Experts stress the need for greater access to plant-based vitamin D3 in the UK, as vitamin D deficiency poses a significant public health concern. This research calls for further exploration into the distinct roles of vitamin D2 and D3, which could reshape supplementation practices.