Title of the Article: Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Clinical Bottom Line

This meta-analysis demonstrates that supplementation with vitamin D2 causes a statistically significant reduction in serum 25-hydroxyvitamin D3 [25(OH)D3] levels. While this finding suggests a causal relationship, the overall certainty of the evidence is moderate due to significant inconsistency across the included trials and some methodological weaknesses in the primary studies. Clinically, this reinforces that vitamin D3 is the preferred form for supplementation, as vitamin D2 not only raises total 25(OH)D levels less effectively but may also actively lower the endogenous D3 form.

Results

Summary of Results

The meta-analysis pooled data from 11 randomized controlled trials and found that vitamin D2 supplementation led to a significant decrease in serum 25(OH)D3 concentrations compared to placebo or no treatment.

• End-of-Trial Concentrations: An analysis of final 25(OH)D3 levels showed that participants receiving vitamin D2 had concentrations that were, on average, $17.99$ nmol/L lower than controls (Weighted Mean Difference $[WMD]=-17.99$ nmol/L; 95% CI, -25.86 to -10.12).

• Absolute Change from Baseline: An analysis of the change in 25(OH)D3 from the beginning to the end of the studies showed that the vitamin D2 group experienced a greater reduction than the control group, with an average difference of $9.25$ nmol/L (WMD = $-9.25$ nmol/L; 95% CI, -14.40 to -4.10).

Both findings were statistically significant, indicating the results are unlikely to be due to chance. The proposed mechanism is that an increase in total vitamin D from supplementation activates a regulatory pathway (likely involving the enzyme CYP24A1) that enhances the clearance of all 25(OH)D forms, including 25(OH)D3.

Assertive Critical Appraisal

Certainty of Evidence (GRADE Framework)

The overall certainty of this evidence is Moderate. Although the analysis is based on randomized controlled trials—the strongest study design for determining causality—the evidence is downgraded for two main reasons:

• Inconsistency: There was serious inconsistency in the results of the included studies, reflected by high statistical heterogeneity (I2=90% for the primary analysis). This means the magnitude of the 25(OH)D3 reduction varied substantially across trials, which reduces confidence in the precise size of the effect.

• Risk of Bias: The methodological quality of the included trials had some weaknesses. Several studies failed to adequately report on blinding procedures or the number of participants who withdrew and why. These reporting gaps introduce a potential risk of bias that could influence the results.

Risk of Bias in Included Studies

The review authors assessed the quality of the included studies and noted some concerns. Of the 11 trials in the meta-analysis, a significant number did not provide details on participant withdrawals, which is crucial for understanding if dropouts could have skewed the results. Furthermore, not all studies were double-blinded, and the methods for randomization were not always clearly described, introducing potential for performance and selection bias.

Research Objective

The objective of this review was to quantify the effect of vitamin D2 supplementation on serum 25(OH)D3 concentrations in humans, as compared to a placebo or no-treatment control group, based on evidence from randomized controlled trials.

Study Design

The authors conducted a systematic literature search of the PUBMED database for randomized controlled trials published between 1975 and 2023. They followed the PRISMA guidelines for study selection and reporting. From an initial 202 records, they included 20 studies in the qualitative systematic review and were able to pool data from 11 of these studies for two separate random-effects meta-analyses.

Setting and Participants

The meta-analysis included data from 11 randomized controlled trials totaling 655 participants (342 receiving vitamin D2 and 313 in control groups). The studies were conducted in various countries and included primarily healthy adults. Studies involving pregnant or breastfeeding women were excluded, and studies in children were excluded from the meta-analysis. Dosing, frequency, and duration of supplementation varied considerably across the trials.

Bibliographic Data

• Title: Effect of Vitamin D2 Supplementation on 25-Hydroxyvitamin D3 Status: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

• Authors: 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

• Journal: Nutrition Reviews

• Year: 2025

• DOI: https://doi.org/10.1093/nutrit/nuaf166

This AI-generated analysis is for informational and research purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition.