New Research Suggests Vitamin D2 Supplements May Lower Levels of the More Potent Vitamin D3

What's new

New research suggests that taking vitamin D2 supplements might have an unexpected effect: it could lead to a reduction in the body's levels of vitamin D3 [1]. This finding challenges the long-held assumption that vitamin D2 and D3 are largely interchangeable for boosting overall vitamin D status. While both forms contribute to circulating vitamin D, a meta-analysis published in 2025 indicated that individuals supplementing with vitamin D2 experienced significantly lower concentrations of 25-hydroxyvitamin D3, the primary storage form of vitamin D derived from D3 [1].

This observation is particularly relevant because vitamin D3 is the form naturally synthesized in human skin upon exposure to sunlight and is increasingly recognized for its potential advantages in maintaining vitamin D levels and supporting immune function. Earlier studies have also suggested that D3 may offer stronger immune protection compared to D2 [1].

The science behind it

The human body produces vitamin D3 (cholecalciferol) when skin is exposed to ultraviolet B (UVB) radiation from the sun. Vitamin D2 (ergocalciferol), on the other hand, is commonly derived from fungi or yeast exposed to UV light and is often found in fortified foods and some supplements [1]. Both forms are metabolized in the liver into 25-hydroxyvitamin D, which is the main circulating form measured in blood tests to assess vitamin D status. However, the recent meta-analysis, which reviewed 20 studies (with 11 suitable for detailed analysis), found that vitamin D2 supplementation was associated with a reduction of approximately 9 to 18 nanomoles per liter in 25-hydroxyvitamin D3 levels [1].

Scientists are still investigating the precise mechanisms behind this observed reduction. One hypothesis suggests that when vitamin D metabolites increase in the body, regulatory mechanisms may accelerate the breakdown and removal of these compounds to maintain balance. Therefore, an increase in D2-derived metabolites might inadvertently trigger the disposal of existing D3-derived metabolites [1].

Beyond the impact on D3 levels, other research has explored functional differences between the two forms. A study published in Frontiers in Immunology found that vitamin D3, but not D2, appeared to stimulate activity related to type I interferon signaling, a crucial part of the immune system's initial defense against viruses and bacteria [1]. While these immune findings are intriguing, further clinical research is needed to determine their direct implications for preventing illness.

What it means in practice

For individuals considering vitamin D supplementation, this research suggests that the specific form—D2 or D3—may matter more than previously understood. While vitamin D2 does raise overall vitamin D levels and has demonstrated biological effects, such as reducing parathyroid hormone and slightly increasing blood calcium, the evidence increasingly points to vitamin D3 as being more effective at raising and sustaining vitamin D levels in the blood [1].

Given the potential for D2 to lower D3 levels and the suggested immune benefits of D3, many experts are now advocating for D3 as the preferred form for routine supplementation, especially during darker months when natural sun exposure is limited [1]. This is becoming more feasible with the availability of plant-based and animal-free sources of vitamin D3, such as those derived from lichen, which cater to various dietary preferences [1].

However, it's important to note that the reduction in D3 levels by D2 does not mean D2 is ineffective or harmful. Both forms contribute to overall vitamin D status, and current guidelines from bodies like the U.S. National Institutes of Health acknowledge that D3 generally leads to a greater and more sustained increase in vitamin D levels compared to D2 [1].

Caveats

While the evidence strongly suggests that vitamin D2 can reduce circulating vitamin D3, the long-term health implications of this reduction are not yet fully understood [1]. Researchers emphasize that more studies are needed to determine how D2 and D3 are metabolized, whether their effects on immune function lead to meaningful differences in health outcomes, and if D3 should become the universally preferred form for supplementation [1].

Changes in gene activity and immune signaling, while promising, do not directly translate to fewer illnesses without further clinical validation. Therefore, while the biochemical differences are becoming clearer, the clinical significance for everyday health remains an area of ongoing investigation [1]. Individuals should consult with healthcare professionals to determine the most appropriate form and dosage of vitamin D supplementation for their specific needs.

Source: [1] https://www.sciencedaily.com/releases/2026/10/261007232940.htm