Emily Brown had been studying vitamin D for years when her team at the University of Surrey made an unexpected discovery: the supplement millions of people reach for during winter might actually be working against them. Taking vitamin D2—one of two common supplement forms—appears to lower the body's levels of vitamin D3, the more efficient form that our bodies naturally produce from sunlight and use most effectively to raise overall vitamin D levels. This finding, published in Nutrition Reviews after analyzing data from randomized controlled trials, challenges a widespread assumption about how these popular supplements work.
The research matters because vitamin D deficiency is a significant public health concern, particularly across the UK population during the winter months from October to March, when sunlight alone cannot provide enough vitamin D production. The UK government recommends 10 micrograms per day, and many people depend on supplements to meet that target and support bone and immune health. With roughly two forms of vitamin D supplements widely available, understanding which one actually works better is crucial.
The study's findings were striking: vitamin D2 supplementation resulted in reduced vitamin D3 levels compared to people not taking vitamin D2 at all. In many cases, the vitamin D3 levels dropped even below those in the control group. "We discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements," Emily Brown explained. This wasn't a minor detail but a fundamental shift in how scientists understand these supplements' impact on the body.
The implications became clearer when Brown's findings aligned with earlier research from Professor Colin Smith, also at the University of Surrey, published in Frontiers in Immunology. That work revealed that vitamin D2 and D3 don't function identically in supporting immune health. Vitamin D3 has a distinctive modifying effect on the immune system, particularly in stimulating the type I interferon signalling system—a crucial first line of defence against bacteria and viruses. "Vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signalling system in the body," Smith noted, "a key part of the immune system that provides a first line of defence against bacteria and viruses."
The research also raised an important equity question. Professor Cathie Martin from the John Innes Centre highlighted that "plant-based vitamin D3 is accessible in the UK," pointing to the need for options that work for people following vegan and vegetarian diets. Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, echoed this concern, emphasizing that tackling vitamin D deficiency "with the most effective form of vitamin D supplementation or fortification is of the utmost importance to the health of the nation."
The researchers stopped short of declaring a blanket recommendation, noting that further research into the different functionalities of vitamin D2 and D3 should be a priority before determining whether vitamin D3 should become the first-line choice—particularly given individual health considerations. But the direction is clear: for most people seeking to support bone and immune health during winter months, vitamin D3 may be the more effective choice. The next chapter in this research will likely reshape how millions of people approach their winter supplementation routine.