Scientists Warn: Common Vitamin D Supplement Has “Previously Unknown” Negative Effect

A new study shows that vitamin D2 may reduce vitamin D3 levels, suggesting D3 is the more effective supplement for immune and overall health.

A new study conducted by the University of Surrey, the John Innes Centre, and the Quadram Institute Bioscience suggests that taking vitamin D2 may actually lower the body’s levels of vitamin D3, which is considered the more effective form of the nutrient. Many people use vitamin D supplements to maintain bone strength, support the immune system, and follow the UK government’s advice of 10 micrograms (µg) per day, particularly during the colder months.

Vitamin D is available in two main supplement forms: D2 and D3. The researchers observed that when individuals take vitamin D2, it can cause a decline in vitamin D3 levels. Unlike D2, vitamin D3 is the type that the body naturally makes when exposed to sunlight, and it is also the form most efficient at boosting overall vitamin D status.

The findings, published in Nutrition Reviews, are based on an analysis of randomized controlled trials. The results showed that people taking vitamin D2 experienced reduced vitamin D3 levels compared with those who did not take D2 supplements. In several cases, vitamin D3 levels dropped even lower than in the study’s control groups.

Emily Brown, PhD Research Fellow and Lead Researcher of the study from the University of Surrey’s Nutrition, Exercise, Chronobiology & Sleep Discipline, said: “Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK. 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.”

Professor Cathie Martin, Group Leader at the John Innes Centre, said: “This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK.”

D2 and D3: Not Interchangeable

This research supports a previous study published in Frontiers in Immunology, led by Professor Colin Smith from the University of Surrey, which suggests that vitamin D2 and D3 do not have identical roles in supporting immune function. Vitamin D3 has a modifying effect on the immune system that could fortify the body against viral and bacterial diseases.

Professor Colin Smith said: “We have shown that vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signalling system in the body – a key part of the immune system that provides a first line of defense against bacteria and viruses. Thus, a healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body.”

Further research into the different functionalities of vitamin D2 and D3 should be a priority in deciding whether vitamin D3 should be the first-line choice of vitamin D supplement, subject to individual requirements.

Professor Martin Warren, Chief Scientific Officer at the Quadram Institute, said: “Vitamin D deficiency represents a significant public health concern, especially during the winter months, with significant deficiency across the UK population. This collaborative research effort aligns well with the Quadram Institute’s 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.”

Reference: “Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials” by Emily I G Brown, Andrea L Darling, Tracey M Robertson, Kathryn H Hart, Jie Li, Cathie Martin, Martin J Warren, Colin P Smith, Susan A Lanham-New and Ruan M Elliott, 18 September 2025, Nutrition Reviews.
DOI: 10.1093/nutrit/nuaf166

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