Why Taking Vitamin D2 Might Actually Lower Your Body's D3 Levels

As sunlight hours decline across northern latitudes, public health bodies routinely advise people to supplement their vitamin D intake. However, researchers warn that the specific form of the vitamin on the label matters far more than previously assumed.

A systematic review and meta-analysis led by researchers at the University of Surrey, the John Innes Centre, and the Quadram Institute shows that taking vitamin D2 supplements can actively diminish circulating levels of vitamin D3 in the bloodstream. The findings evaluate decades of clinical trials and challenge the long-held assumption that the two compounds function interchangeably in human health.

A Measurable Drop in 25(OH)D3

Published in the journal Nutrition Reviews, the analysis examined randomized controlled trials published between January 1, 1975, and February 1, 2023. Researchers retrieved 202 papers from PubMed, identifying 20 eligible studies, with 11 trials meeting the criteria for statistical meta-analysis.

The synthesized data revealed that participants who took vitamin D2 supplements experienced a statistically significant reduction in serum 25-hydroxyvitamin D3 [25(OH)D3] compared with non-supplemented control groups. Clinicians measure 25-hydroxyvitamin D as the primary circulating marker of vitamin D status.

Across the trials, the random weighted mean difference in serum 25(OH)D3 at trial completion between the D2 groups and controls was -17.99 nmol/L. When assessing absolute change over the course of the trials, the mean difference was -9.25 nmol/L.

"Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK," said Emily Brown, lead researcher and PhD research fellow at 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."

Distinct Biological Roles

Vitamin D3 (cholecalciferol) is produced naturally in human skin during exposure to solar ultraviolet B radiation. Commercial D3 supplements are typically extracted from lanolin found in sheep's wool, though plant-based formulations derived from lichen also exist. Vitamin D2 (ergocalciferol) is predominantly manufactured by exposing fungi or yeast to ultraviolet light, making it a common choice for vegan fortified foods and tablets.

While both forms raise total measurable vitamin D levels to varying degrees, research indicates that D3 is substantially more effective at sustaining total 25-hydroxyvitamin D concentrations over time. The exact mechanism behind the D2-induced decline in D3 remains unconfirmed, though researchers theorize that high D2 intake activates metabolic pathways that accelerate the clearance and degradation of existing 25(OH)D3.

Functional differences may also extend to immune signaling. Research led by Professor Colin Smith and published in Frontiers in Immunology observed that vitamin D3 stimulated the type I interferon signaling system, a primary immune pathway defending against viruses and bacteria. Vitamin D2 did not show the same activity.

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

What the Evidence Means for Consumers

Health guidelines in the UK recommend a daily intake of 10 micrograms of vitamin D during autumn and winter months, when sunlight is insufficient for cutaneous synthesis. For consumers, the findings point toward checking ingredient formulations closely.

Researchers emphasize that the practical outcomes remain unproven: whether the biochemical drop in 25(OH)D3 translates into clinically meaningful adverse health effects is still unknown. Even so, the distinct biological pathways indicate that relying solely on D2 may not support optimal D3 retention.

The findings also draw attention to supply options for those avoiding animal products. "This meta-analysis highlights the importance of ensuring plant-based vitamin D3 is accessible in the UK," noted Professor Cathie Martin, group leader at the John Innes Centre.