Vitamin D2 vs D3: 11 Trials Find Ergocalciferol Lowers D3

October 8, 2026 · Science · Supplements

What the Meta-Analysis Pooled

Taking vitamin D2 lowers circulating vitamin D3 levels in pooled randomized clinical trials. A systematic review and meta-analysis published in Nutrition Reviews examined how oral ergocalciferol influences serum 25-hydroxyvitamin D3 [25(OH)D3] in human participants. The research team was led by Emily Brown at the University of Surrey, alongside investigators from the John Innes Centre and Quadram Institute Bioscience. Their literature search screened 202 records in PubMed published between 1975 and 2023. Twenty articles met the criteria for the wider systematic review, and 11 randomized controlled trials (RCTs) provided numerical data suitable for statistical pooling.

Across the 11 pooled trials, 655 participants were randomized to either vitamin D2 or a control group. The vitamin D2 arms enrolled 342 participants, while the control arms enrolled 313 participants. Random-effects models run in Review Manager 5.3 showed that participants taking vitamin D2 ended their interventions with significantly lower serum 25(OH)D3 levels than controls. The pooled mean difference at the end of the trials was negative 17.99 nmol/L, with a 95% confidence interval (CI) of negative 25.86 to negative 10.12 nmol/L and a p value under .00001.

When the authors analyzed absolute changes from baseline across the trials, the effect remained consistent. The pooled mean change difference was negative 9.25 nmol/L, with a 95% CI of negative 14.40 to negative 4.10 nmol/L and a p value of .0004. In United States laboratory reports, serum vitamin D is usually reported in nanograms per milliliter (ng/mL) rather than nanomoles per liter (nmol/L). Applying the standard conversion factor of 2.496 nmol/L per ng/mL yields an end-of-trial reduction of approximately 7.2 ng/mL, and an absolute change reduction of approximately 3.7 ng/mL. These two nanogram numbers represent mathematical conversions of the published figures rather than values reported directly by the trial authors. A University of Surrey press release issued on 8 October 2026 highlighted the findings, describing the average observed reduction as roughly 9 to 18 nmol/L depending on the statistical approach.

Why Vitamin D2 vs D3 Is Not Only About Potency

The central finding of this paper differs fundamentally from the established understanding of vitamin D potency. Clinicians and researchers have long recognized that vitamin D3 (cholecalciferol) increases and sustains total 25-hydroxyvitamin D [25(OH)D] more effectively than vitamin D2 (ergocalciferol). Our guide on the best time to take vitamin D outlines that familiar dynamic. An earlier meta-analysis of 24 trials confirmed that cholecalciferol improves systemic vitamin D status more reliably than ergocalciferol. That standard observation describes a difference in upward efficiency.

This meta-analysis reports an active reduction rather than an efficiency gap. Instead of showing that ergocalciferol simply adds fewer total units to the bloodstream, the pooled data indicate that introducing vitamin D2 causes the pre-existing pool of 25(OH)D3 to fall. Ergocalciferol appears to deplete or displace circulating cholecalciferol metabolites.

The authors hypothesize that a clearance mechanism explains this observed reduction. When total circulating vitamin D concentrations rise following exogenous supplementation, a regulatory mechanism may speed up the disposal of 25(OH)D. If those degradation pathways target 25(OH)D3 while D2 is entering circulation, the D3 fraction drops. The investigators emphasize that this regulatory mechanism remains unconfirmed, and further biochemical work is needed to resolve the exact pathway.

What the Trials Disagreed About

The statistical findings must be interpreted alongside substantial heterogeneity and methodology limitations across the included trials. The 11 meta-analyzed studies exhibited high statistical heterogeneity, with an I-squared value of 83% for the end-of-trial comparison and 69% for the absolute-change comparison. High heterogeneity means the individual trials produced widely divergent effect sizes. Supplement doses in the analyzed protocols ranged from 300 international units (IU) daily up to 50,000 IU administered as single boluses or intermittent therapy.

Methodological quality varied among the underlying studies. Only 8 of the 11 trials used double-blind designs, and 7 of the 11 failed to document participant withdrawals. Most included trials reported vitamin D3 data incidentally rather than as a primary aim. Publication bias could not be assessed because too few studies were available.

Laboratory methods also introduced variability across the dataset. Assay methods varied across studies, which may affect absolute differences. The systematic review identified 20 eligible studies with 1,080 total participants across pediatric and adult cohorts, but excluded nine studies from the primary meta-analysis, including two pediatric trials. These limitations do not invalidate the pooled statistical association, but they caution against treating the exact numerical drop as a universal constant.

Where Prescription and Dietary Vitamin D2 Appear

While vitamin D3 is common in over-the-counter supplements, vitamin D2 remains widespread in specific clinical and dietary niches. In medical practice, high-dose prescription capsules frequently contain 50,000 IU of ergocalciferol. A patient completing a prescription course of 50,000 IU weekly may find that their circulating level change is smaller than expected while their 25(OH)D3 fraction drops.

Dietary sources also introduce vitamin D2 into daily life. Vitamin D2 is commonly used in plant-based supplements and vegan formulations. Many fortified foods also incorporate vitamin D2.

In the United Kingdom, guidance suggests considering 10 micrograms of vitamin D each day during darker months. That guidance represents general national advice rather than a trial protocol.

What a 25(OH)D Result on a Panel Does and Does Not Tell You

Standard clinical blood tests typically report a single number for total 25-hydroxyvitamin D, masking the underlying distribution of D2 and D3 metabolites. Commercial longevity panels and routine primary care tests rely on total 25(OH)D as the primary benchmark for sufficiency. When a longevity panel reports total 25(OH)D, it does not separate the D2 and D3 fractions. Without tests that separate the fractions, the displacement remains invisible.

In the guides we publish here, the recurring gap sits between a biomarker that shifts on paper and a clinical outcome that improves. A lower 25(OH)D3 fraction sounds alarming in isolation, but the Surrey paper evaluated a biochemical marker rather than a functional medical outcome. The meta-analysis did not evaluate clinical endpoints like fractures, infections, or mortality.

Other physiological markers show complex responses to ergocalciferol. A separate 2026 meta-analysis of 26 randomized trials found that vitamin D2 supplementation lowered parathyroid hormone (PTH) levels and caused a minor increase in circulating blood calcium, though specific effect sizes were not disclosed in the press release. Furthermore, an immune study published in Frontiers in Immunology and led by Colin Smith documented distinct patterns of gene expression following D2 versus D3 intake. Those genetic shifts demonstrate biological divergence, but they do not prove that D3 prevents infection or that changes in gene activity translate into fewer illnesses. Our biomarkers hub and our guide on the best vitamins for energy examine how blood concentrations interact with physiological performance.

What Would Change This Vitamin D2 vs D3 Read

Several specific research milestones would clarify whether the suppression of 25(OH)D3 requires changes to everyday clinical practice. First, randomized controlled trials would need to be designed around fractionated 25(OH)D3 and 25(OH)D2 kinetics as primary endpoints. Existing trials collected fractionated data incidentally, using heterogeneous dosing protocols and varying laboratory techniques.

Second, the biological clearance mechanism needs resolving. If experimental models confirm a regulatory mechanism that speeds up 25(OH)D disposal, the biochemical rationale will be confirmed. The authors of the Nutrition Reviews paper specifically noted that more investigation is required before clinical authorities can officially recommend D3 over D2.

Finally, trials would need to measure hard endpoints rather than blood concentrations. Demonstrating that D2 lowers circulating D3 by 9 to 18 nmol/L does not prove that patients taking D2 suffer higher rates of fractures or infections. Until longitudinal trials compare clinical outcomes between the two forms, patients should not alter medically indicated therapies. Anyone evaluating their own vitamin D2 vs D3 intake should share both their supplement label and their full lab panel with their prescribing clinician before adjusting doses.

Sources

• 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, Ruan M Elliott, "Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials," Nutrition Reviews, advance article nuaf166, published September 18, 2025, DOI 10.1093/nutrit/nuaf166 (accessed October 8, 2026). Source of the 202 records screened, the 20 studies in the review and 11 in the meta-analysis, the 655 participants split 342 to 313, the end-of-trial mean difference of 17.99 nmol/L with its interval and p value, the absolute-change difference of 9.25 nmol/L with its interval and p value, the 300 IU to 50,000 IU dose range, the I-squared values of 83% and 69%, the blinding and withdrawal counts, the unassessable publication bias, the varied assays, and the authors' proposed disposal mechanism.
• Oxford University Press advance-article record for nuaf166 (accessed October 8, 2026). Source of the 1,080 participants across the whole review and the author list.
• University of Surrey, "Scientists warn a popular vitamin D supplement may have a hidden downside," release carried by ScienceDaily, October 8, 2026 (accessed October 8, 2026). Source of the "roughly 9 to 18 nanomoles per liter" summary, the UK guidance of 10 micrograms daily in darker months, the separate 26-trial meta-analysis on parathyroid hormone and blood calcium, the earlier 24-study comparison, and the Frontiers in Immunology gene-activity work led by Colin Smith.