Vitamin D2 may lower vitamin D3 levels, study warns

A new meta-analysis published in Nutrition Reviews finds that vitamin D2 supplements may decrease circulating vitamin D3 levels, potentially undermining bone and immune health. The researchers analyzed data from randomized controlled trials and found that people taking vitamin D2 showed lower vitamin D3 levels than those not supplementing.

Lead author Dr. Emily Brown, a nutrition expert at the University of Surrey, said the findings reveal a previously unknown effect of vitamin D2 on the body's stores. "We discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body," Brown said. "This meta-analysis suggests that, subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2." The study builds on earlier work indicating that vitamin D2 and D3 behave differently in the body.

In the body, vitamin D is often called the sunshine vitamin because the body makes it in response to sunlight, and it is found in foods such as oily fish, eggs and liver. The researchers note that vitamin D3 is generally more efficiently absorbed and used to raise overall vitamin D status than vitamin D2, a difference that helps explain why D3 is more closely linked to calcium absorption and bone health.

Experts say that millions rely on supplements to maintain adequate levels during the winter, when sunlight is scarce. NHS guidelines advise supplementation for many people in winter, and current guidance typically targets about 10 micrograms per day, with a safety limit of 100 micrograms per day unless instructed otherwise by a clinician. Recent data estimate that as many as two in five adults may be vitamin D deficient in winter, underscoring the public health relevance of supplementation strategies. It's important to avoid excessive intake, since too much vitamin D can trigger hypercalcemia, which can harm the kidneys and heart.

The research complements earlier work by Professor Colin Smith at the University of Surrey, who found that D2 and D3 exert different effects in the body, including on immune function. Some researchers say that higher vitamin D status might help protect against infections, but the overall evidence remains nuanced.

Proponents of targeted fortification and supplementation note the practical implications. Vitamin D3 is widely available as a supplement and is often preferred when aiming to raise serum levels, while vitamin D2 is commonly used in fortified foods because it is cheaper to produce. The study suggests that, for many individuals, D3 may offer greater benefit, but personal circumstances will determine the best approach. As part of broader public-health efforts, experts encourage balancing intake with sun exposure, diet and medical guidance.

Dr Justin Stebbing, an oncologist who has commented on the potential cancer-related role of vitamin D, has suggested that sufficient vitamin D might help protect against bowel cancer, though researchers caution that the exact mechanisms and clinical implications are not settled. The latest study from Surrey and collaborators adds that a healthy vitamin D3 status may help support immune function and regulate cell growth, reinforcing the case for careful selection of supplementation forms where needed.

Professor Martin Warren, chief scientific officer at the Quadram Institute, said the deficiency issue remains urgent, especially in winter. He noted that the collaborative research aligns with efforts to improve the nutrient density of the food supply and to ensure that the most effective form of vitamin D supplementation and fortification is accessible to the public. The findings could influence policy and consumer choices in the coming years.

Taken together, the evidence underscores that the form of vitamin D matters. While D2 can raise intake, it may not provide the same benefit as D3 for many people and could even lower D3 stores in some cases. Health authorities stress a balanced approach, combining prudent supplementation with safe sun exposure, dietary sources and regular medical advice, particularly for those at risk of deficiency or bone-related disorders.