Study finds vitamin D2 supplements may lower vitamin D3 levels, prompting review of plant-based dosing

A new meta-analysis finds that vitamin D2 supplements may lower circulating vitamin D3 levels, potentially undermining efforts to raise overall vitamin D status. Vitamin D3, produced by the body in response to sunlight and found in animal-based foods such as oily fish, egg yolks and liver, is the form most efficiently absorbed to raise vitamin D levels.

The study, published in Nutrition Reviews, analyzed data from randomized controlled trials and found that taking vitamin D2 supplements led to a reduction in vitamin D3 levels compared with non-supplemented controls. Lead author Dr. Emily Brown, a nutrition expert at the University of Surrey, said: 'We discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect. This study suggests that subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2.'

The findings echo earlier work from the same research team, which suggested vitamin D3 provides a stronger immune-modulating effect. Professor Colin Smith of the University of Surrey said: 'A healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body.'

The study comes as public health officials warn that vitamin D deficiency is common in the UK, especially during winter. NHS data indicate that around one in six adults may not get enough vitamin D, and some estimates show up to two in five adults could be deficient in winter. In the winter months when daylight is limited, government guidelines recommend supplementation for many people.

Cancer specialist Dr. Justin Stebbing has suggested that vitamin D could help protect against bowel cancer; while its exact role remains unclear, tests show the nutrient supports the immune system and regulates cell growth, including in the bowel. He noted that while the precise cancer-prevention mechanism is not settled, the nutrient appears to play a role in maintaining cellular balance and immune defense.

Professor Martin Warren, chief scientific officer at the Quadram Institute, said deficiency represents a significant public health concern, especially during the winter months. He added that the collaborative research aligns with efforts to improve nutrient density in the diet and to deliver healthier lives through food innovation, stressing that tackling vitamin D deficiency with the most effective form of supplementation or fortification is essential for the nation's health. 'Vitamin D deficiency represents a significant public health concern, especially during the winter months,' Warren said. 'This collaborative research effort aligns with our mission to deliver healthier lives through food innovation to enhance the nutrient density of the food we eat. Tackling this with the most effective form of vitamin D supplementation or fortification is of the utmost importance to the health of the nation.'

The latest data suggest as many as two in five adults may be vitamin D deficient during the winter months when there are fewer daylight hours. A lack of the vitamin can lead to bone problems, including rickets, a condition that causes bone pain and deformation. But people should also be careful not to have too much of the nutrient. As vitamin D helps to absorb calcium, too much can cause hypercalcaemia, which can damage the kidneys and heart. As such the NHS recommends people don't take more than 100 micrograms of vitamin D per day unless specifically advised to by their doctor.

The researchers stressed that not all vitamin D is created equal; vitamin D3—produced in the skin in response to sunlight and found in animal foods—appears to have a more beneficial profile for bone health and immune function than vitamin D2, which is plant-based and more common in fortified foods due to cost. The study’s authors also noted that vitamin D2 and D3 appear to work differently in the body, reinforcing the need for careful consideration of supplementation strategy. In light of these findings, experts say it may be important to ensure accessible plant-based vitamin D3 options in the UK, in addition to traditional D3 supplements, to support broader public health goals. The researchers concluded that, where possible, choosing the most effective form of vitamin D supplementation could help optimize vitamin D status and potentially reduce health risks associated with deficiency.