Millions of people take vitamin D supplements to support bone and immune health, especially during the darker winter months. But new research suggests that not all forms of vitamin D act the same in the body.
In a study published in Nutrition Reviews, researchers from the University of Surrey, John Innes Centre and Quadram Institute analyzed data from multiple randomized controlled trials comparing vitamin D2 and vitamin D3. They found that when participants took vitamin D2, levels of vitamin D3, the form our bodies naturally make from sunlight, actually went down.
“Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK,” said lead researcher Emily Brown of the University of Surrey. “However, 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. This study suggests that subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2.”
For those in the United States, the distinction matters too. Government guidelines recommend 600 IU (15 micrograms) of vitamin D per day for most adults up to age 70, but many Americans fall short, especially in northern states during winter. Vitamin D2 is the form commonly added to fortified foods such as milk, orange juice and breakfast cereals.
The findings build on earlier research showing that vitamin D3 may better support immune function than vitamin D2. Previous studies suggest that D3, but not D2, activates certain immune pathways that help the body defend itself against bacteria and viruses.
Still, researchers emphasize that both forms matter. Vitamin D2, often found in fortified foods and plant-based supplements, remains an important source, especially for people following vegan diets. The new study highlights that D2 and D3 aren’t identical and that public health guidance may eventually favor plant-based D3 options where available.
“Vitamin D deficiency represents a significant public health concern, especially during the winter months,” said Professor Martin Warren of the Quadram Institute. “Tackling this with the most effective form of vitamin D supplementation or fortification is of the utmost importance to the health of the nation.”
For now, experts agree the priority is ensuring adequate vitamin D intake. Whether through safe sun exposure, fortified foods or supplements, keeping levels in a healthy range supports bone strength and immune health.
This research was funded by the UK’s Biotechnology and Biological Sciences Research Council (BBSRC) through its Food Microbiome and Health program and a partner project at the University of Surrey. Lead researcher Emily Brown also received support from a BBSRC doctoral training fellowship.