Vitamin D Supplement Study Raises Concerns About Internal Balance

Research published in 2025 has raised concerns about a common vitamin D supplement that may have an unintended consequence on the body's internal balance of this essential nutrient.

The study found that taking vitamin D2 can reduce levels of vitamin D3, which is the form naturally produced when skin is exposed to sunlight. This distinction matters because both forms are often considered interchangeable sources of vitamin D, but they differ in their effectiveness at raising and maintaining healthy levels of the nutrient in the blood.

Vitamin D3 has been shown to be more effective than vitamin D2 in increasing overall vitamin D status, with evidence suggesting it can raise levels more efficiently and sustain them for longer periods. This is reflected in current guidance from the U.S. National Institutes of Health, which highlights the superior benefits of D3 over D2.

Vitamin D plays a crucial role in maintaining strong bones by aiding calcium absorption, as well as supporting muscle, nerve, and immune function. In regions with limited sunlight during winter months, such as the UK, public health guidelines recommend considering daily vitamin D supplements to ensure adequate levels are maintained.

The two main forms of vitamin D found in supplements are D2 and D3. Vitamin D2 is typically produced from fungi or yeast exposed to ultraviolet light, while D3 is naturally synthesized in human skin after exposure to UVB radiation from sunlight.

Traditionally, animal-derived sources such as lanolin obtained from sheep's wool have been used to produce vitamin D3 supplements. However, alternative, plant-based options are now available for individuals following vegan or plant-based diets, including D3 derived from lichen and other non-animal sources.

Once vitamin D enters the body, it undergoes a series of processing steps before its effects can be measured. The liver plays a crucial role in this process by converting the supplement into 25-hydroxyvitamin D, which is commonly used to estimate an individual's vitamin D status through blood tests.

The specific form of 25-hydroxyvitamin D produced depends on whether the individual takes vitamin D2 or D3. Vitamin D2 results in the production of 25-hydroxyvitamin D2, while vitamin D3 produces 25-hydroxyvitamin D3.

A recent study published in Nutrition Reviews shed light on a previously unknown effect of taking vitamin D2 supplements. Researchers from the University of Surrey and other institutions found that individuals who took vitamin D2 experienced a decrease in their levels of 25-hydroxyvitamin D3.

The study's meta-analysis, which combined data from 20 clinical trials, revealed significant differences in 25-hydroxyvitamin D3 concentrations between those taking vitamin D2 and those not receiving the supplement. The average reduction in 25-hydroxyvitamin D3 levels was estimated to be around 9 to 18 nanomoles per liter.

Researchers emphasized that this finding does not necessarily imply that vitamin D2 supplements are harmful or lead to a net loss of vitamin D from the body. Rather, it suggests that increasing one form of vitamin D may coincide with a reduction in the other.

The discovery has important implications for individuals who rely on vitamin D supplements, particularly during periods when sunlight exposure is limited, such as between October and March.

Research into the effects of vitamin D supplements has taken an intriguing turn, with scientists proposing that a popular supplement may have unintended consequences for the body's regulation of this essential nutrient.

One theory is that increasing levels of vitamin D2 could lead to a faster breakdown and removal of existing 25-hydroxyvitamin D3 in the body. This balancing effect might be a natural response to high concentrations of these compounds, but its significance for human health remains unclear.

Studies have shown that the relationship between D2 and D3 supplementation may not be one-way, with some experiments suggesting that D3 can actually reduce levels of D2-derived metabolites.

The implications of this finding are still being explored, but it has sparked debate about the potential risks associated with relying solely on vitamin D supplements.

In related research, scientists have discovered differences in how vitamin D2 and D3 interact with the immune system.

The discovery of type I interferons as signaling proteins has shed light on how the body's immune system responds to infections. These proteins play a crucial role in alerting cells to potential threats and initiating an early defense against viruses and other pathogens.

Research has shown that vitamin D3 stimulates the type I interferon signaling system, but not vitamin D2, which implies that maintaining healthy levels of D3 may help prevent viral and bacterial infections from taking hold. This finding is significant as it highlights a key difference in how these two forms of vitamin D interact with the immune system.

However, it's essential to note that the relationship between gene activity, immune signaling, and actual illness prevention is complex. While changes in immune responses may indicate a potential benefit, larger clinical studies are necessary to establish whether taking D3 will directly translate to fewer infections.

New research has added complexity to this picture, with a 2026 systematic review and meta-analysis examining the effects of vitamin D2 supplementation on parathyroid hormone levels and blood calcium. The findings suggest that D2 still has meaningful biological effects, even though it may reduce circulating levels of D3.

A key takeaway from this study is that the reduction in D3 caused by D2 should not be misinterpreted as evidence that D2 is ineffective. Both D2 and D3 have been shown to increase overall blood levels of vitamin D, but the accumulated evidence suggests that D3 produces a larger increase and maintains it for longer.

The distinction between D2 and D3 is crucial, particularly when considering the information provided by the NIH, which continues to state that both forms can boost overall blood levels of vitamin D. However, the data favors D3 as the more effective choice for achieving this increase.

The latest research on vitamin D supplements has shed light on a crucial distinction between two commonly used forms: D2 and D3. Studies have consistently shown that D3 is more effective than D2 at improving overall vitamin D status.

This disparity holds true even when comparing the two forms directly in meta-analyses. For instance, one analysis of 24 studies found that D3 outperformed D2 in boosting circulating levels of the essential nutrient.

While it's clear that D2 can reduce circulating levels of D3, the significance of this reduction remains unclear. The real question is whether this biochemical difference translates into meaningful differences in health outcomes.

To answer this question, scientists are calling for further research to investigate how D2 and D3 are metabolized by the body. This knowledge will be crucial in determining whether their effects on immune function differ significantly.

The Quadram Institute's Chief Scientific Officer, Professor Martin Warren, emphasizes the importance of tackling vitamin D deficiency, which affects a significant proportion of the UK population during winter months.

Researchers have made significant progress in developing an innovative approach to tackling vitamin D deficiency, which affects millions of people worldwide during winter months.

The findings suggest that a popular supplement may have unforeseen consequences on overall health, underscoring the need for a more nuanced understanding of its effects. As scientists continue to unravel the intricacies of nutrition and wellness, it becomes increasingly clear that no single solution can address the complex needs of individuals.