The first concern centers on who was actually included in the meta-analysis. According to the commentary, the review stated that eligible participants were individuals with type 1 or type 2 diabetes mellitus, yet studies involving people with gestational diabetes and prediabetes were subsequently included in the analysis. The corresponding PROSPERO registration record, which documents the review protocol in advance, defined the eligible population simply as subjects with diabetes mellitus, without explicitly specifying these additional groups. Fuentes-Barría argues that this drift between the registered protocol and the executed review matters because gestational diabetes and prediabetes differ substantially from established type 1 and type 2 diabetes in pathophysiology, baseline cardiovascular and metabolic risk, and clinical management. Pooling these distinct populations, he contends, inflates clinical heterogeneity and weakens the applicability of the combined estimates to any single patient group.

A second and equally consequential issue involves the nature of the interventions themselves. Not all of the trials included in the meta-analysis tested zinc alone. Some interventions combined zinc with other micronutrients or bioactive compounds, a fact the original authors themselves acknowledged when they conceded that such co-interventions make it difficult to attribute observed effects specifically to zinc. The commentary argues that mixing zinc-only trials with multicomponent interventions introduces another layer of clinical heterogeneity and complicates the interpretation of pooled effects. To resolve this ambiguity, the commentator recommends sensitivity analyses that exclude multicomponent interventions, which would reveal whether the reported benefits survive when the analysis is restricted to trials in which zinc is the sole active agent. Without such analyses, he suggests, readers cannot judge the robustness of the pooled estimates or the extent to which zinc itself deserves credit.

Statistical heterogeneity represents the third pillar of the critique. The commentary highlights that several pooled outcomes showed substantial or even considerable heterogeneity across the included trials. The most striking example is plasma zinc concentration, where the I-squared statistic reached 99 percent, meaning virtually all of the variability between studies reflects real differences rather than chance. Other metabolic and inflammatory outcomes also demonstrated high heterogeneity. Although the original authors applied random-effects models and conducted meta-regression in an attempt to account for this variability, the commentary notes that substantial residual heterogeneity remained unexplained. Under these circumstances, Fuentes-Barría argues, a single pooled estimate carries limited clinical meaning, and its generalizability to diverse patient populations, dosing regimens and treatment durations should be interpreted with pronounced caution.

The fourth concern targets the meta-regression analyses themselves. The original review performed multiple meta-regressions examining age, sex, zinc dose and intervention duration as potential effect modifiers, despite several outcomes being based on a limited number of studies. This is problematic, the commentary explains, because meta-regression generally requires an adequate number of studies per moderator variable to produce reliable results. The Cochrane Handbook, the field’s leading methodological reference, advises caution when fewer than ten studies are available for such analyses. Beyond official guidance, methodological research has shown that meta-regression analyses are frequently undermined by overfitting and other pitfalls that can generate misleading findings. A meta-epidemiological study cited in the commentary found that most published meta-regressions based on aggregate data suffer from such methodological problems. Given the limited number of trials and the multiple moderators examined, the commentator concludes that these dose-response and subgroup associations should be treated as exploratory and hypothesis-generating rather than confirmatory evidence.

Perhaps the most clinically pointed criticism concerns the gap between the review’s title and its actual findings. The meta-analysis did not demonstrate significant improvements in fasting plasma glucose or HbA1c, the two canonical measures of glycemic control, despite the title emphasizing glycemic control as a primary outcome. Instead, statistically significant effects were observed primarily for surrogate markers, including circulating insulin levels, HOMA-IR as a measure of insulin resistance, C-reactive protein as an inflammatory marker, and various oxidative stress biomarkers. The commentary argues that these statistically significant shifts in surrogate endpoints should not automatically be equated with clinically meaningful improvements in diabetes control or with outcomes that matter to patients, such as reduced complications, improved quality of life or decreased mortality. Research on surrogate endpoints in diabetes trials has repeatedly shown that changes in biomarkers do not always translate into tangible clinical benefit, making this distinction far more than a semantic quibble.

The commentary’s overall message is one of measured skepticism rather than outright rejection. Fuentes-Barría explicitly frames his remarks as a constructive contribution to a clinically relevant topic, acknowledging the importance of the question the original review addressed. Zinc is an essential trace element involved in insulin synthesis, storage and secretion, as well as in antioxidant defense mechanisms, which provides a plausible biological rationale for studying its supplementation in diabetes. However, plausibility of mechanism cannot substitute for methodological rigor in the evidence synthesis that is supposed to translate biology into clinical recommendations. The commentary suggests that the enthusiasm generated by statistically significant pooled effects on biomarkers risks outpacing what the underlying trial data can actually support.

To strengthen the evidence base, the commentator proposes a concrete path forward. Stratified and sensitivity analyses by diabetes phenotype would clarify whether zinc exerts different effects in type 1 diabetes, type 2 diabetes, gestational diabetes and prediabetes, populations whose distinct metabolic contexts could plausibly modify any treatment effect. Restricting analyses to zinc-only interventions would isolate the specific contribution of the mineral from that of co-administered compounds. And prioritizing clinically relevant glycemic outcomes, particularly HbA1c and fasting glucose, over surrogate biomarkers would anchor the conclusions in endpoints that directly inform patient care. These refinements, the commentary argues, could substantially improve both the validity and the clinical interpretability of future updates to this body of evidence.

The exchange is a timely reminder of how meta-analyses, often perceived as the pinnacle of the evidence hierarchy, remain only as reliable as the methodological choices embedded within them. Decisions about which populations to pool, which interventions to combine, how to handle heterogeneity and how many moderator analyses to run can each shift the final estimates and the confidence readers place in them. For the growing number of people with diabetes worldwide who may be considering zinc supplements, and for the clinicians who advise them, the commentary underscores that the current evidence supports caution: meaningful effects on the measures that define diabetes control have not yet been demonstrated, and the significant biomarker changes reported so far should be viewed as signals worth further investigation rather than proof of clinical benefit. As the field awaits more rigorously designed and analyzed syntheses, the debate illustrates the self-correcting nature of scientific publishing, where critical commentary serves as an essential quality-control mechanism for evidence that ultimately shapes real-world health decisions.

Subject of Research: Methodological critique of a meta-analysis on zinc supplementation in diabetes

Article Title: Comment on ‘Effects of Zinc Supplementation on Glycemic Control, Insulin Resistance, Inflammation and Oxidative Stress in Diabetes’

Article References: Fuentes‐Barría, H. (2026). Comment on ‘Effects of Zinc Supplementation on Glycemic Control, Insulin Resistance, Inflammation and Oxidative Stress in Diabetes’. Endocrinology, Diabetes & Metabolism, 9(5), Article e70328. https://doi.org/10.1002/edm2.70328

Image Credits: AI Generated

DOI: 10.1002/edm2.70328

Keywords: zinc supplementation, diabetes, meta-analysis, glycemic control, insulin resistance, HbA1c, inflammation, oxidative stress, clinical heterogeneity, meta-regression, Comment, Effects