Synbiotic Supplements Show Mixed Effects on Oxidative Stress Markers, Meta-Analysis Finds

A large pooled analysis of randomized controlled trials suggests that synbiotic supplements, combinations of live probiotic bacteria and the prebiotic fibers that feed them, may shift several biomarkers of oxidative stress in adults. The updated systematic review and meta-analysis, published in Genes & Nutrition, synthesized data from 18 randomized controlled trials and applied the GRADE framework to rate how trustworthy each pooled result actually is. The verdict is nuanced: several antioxidant and damage markers moved in a favorable direction, but the certainty of that evidence ranged from moderate down to very low depending on the biomarker.

The research team, led by Mohammad Vesal Bideshki, set out to fix two weaknesses in earlier syntheses. Previous meta-analyses had typically examined only a narrow set of biomarkers, such as total antioxidant capacity, glutathione, malondialdehyde, and nitric oxide, and mostly within specific patient groups like people with diabetes or kidney disease. More troubling, recent scrutiny revealed that some trials included in earlier reviews had since been retracted or flagged for data integrity concerns. To address this, the authors rigorously identified and excluded retracted publications and studies with serious concerns before pooling anything, a filtering step that removed ten trials during full-text screening.

The methods were conventional but thorough. Following PRISMA guidance and a protocol registered with PROSPERO, the researchers searched PubMed, SCOPUS, and Web of Science for peer-reviewed, English-language randomized controlled trials published through December 2025. Eligible studies had to administer oral synbiotic supplementation for at least three weeks to adults aged 18 or older, against a placebo group differing only by the supplement. Outcomes of interest were changes in eight biomarkers: malondialdehyde (MDA), total oxidant status (TOS), oxidative stress index (OSI), nitric oxide (NO), total antioxidant capacity (TAC), superoxide dismutase (SOD), glutathione peroxidase (GPx), and reduced glutathione (GSH). Pooled effects were calculated as standardized mean differences using random-effects models, with heterogeneity, sensitivity, subgroup, and publication-bias analyses alongside.

From 3,697 initially identified records, 18 trials survived screening. They were published between 2017 and 2025, and geographically the field is heavily skewed: 13 of the 18 trials were conducted in Iran, with the remainder from Thailand, Poland, Egypt, and China. Most used double-blind, parallel-group designs. The populations were highly varied, spanning healthy individuals and patients with type 2 diabetes, non-alcoholic fatty liver disease, gestational diabetes, obesity and overweight, Parkinson’s disease, metabolic syndrome, migraine, and hemodialysis, plus one study in breast cancer survivors. Supplementation ran from 6 to 24 weeks, most commonly 8 to 12 weeks, with daily probiotic doses ranging from 10^8 to roughly 10^11 colony-forming units, mainly Lactobacillus, Bifidobacterium, and Streptococcus strains plus some Bacillus coagulans, paired with prebiotics such as inulin, fructooligosaccharides, and galactooligosaccharides.

The headline pooled results favored the supplements. Synbiotic supplementation significantly reduced the lipid peroxidation marker MDA (SMD -0.87; 95% CI -1.37 to -0.37) across 13 studies and 791 participants, though with very high heterogeneity (I² = 90.6%). Total oxidant status fell significantly (SMD -0.55; 95% CI -1.03 to -0.08) across four studies, and the oxidative stress index dropped (SMD -0.46; 95% CI -0.78 to -0.15) in just two studies with no detected heterogeneity. On the antioxidant side, TAC rose modestly (SMD 0.25; 95% CI 0.06 to 0.43) across 11 studies with moderate heterogeneity, while SOD (SMD 0.57), GSH (SMD 0.70), and GPx (SMD 1.40) all increased significantly, the GPx estimate coming from only four studies with substantial heterogeneity. Nitric oxide showed no significant change (SMD -0.05; 95% CI -0.48 to 0.38) across three studies.

The GRADE ratings complicate the picture considerably. Only OSI reached moderate certainty. TAC, GSH, SOD, TOS, and NO were rated low certainty, with imprecision and moderate heterogeneity the key limiting factors. MDA and GPx received very low certainty ratings, reflecting marked inconsistency and evidence of publication bias. That bias was not hypothetical: Egger’s regression test indicated small-study effects for MDA (P = 0.004), TAC (P = 0.028), and GPx (P < 0.001). The authors themselves flag that the MDA reduction, despite its statistical significance, should be interpreted cautiously, and that findings for NO and OSI rest on very few trials and small cohorts, making them preliminary.

Sensitivity analyses showed which pooled estimates wobble when individual trials are removed. TAC, MDA, and NO estimates were robust to dropping any single study. But TOS results shifted materially with or without the trials by Zolghadrpour et al. (2024) and Bakhshimoghaddam et al. (2018); GSH estimates depended heavily on three studies; GPx varied with two; SOD with three; and OSI hinged on a single trial by Ghavami et al. (2021). Risk-of-bias assessment with the Cochrane tool classified ten studies as good quality, four as fair, and four as poor, and subgroup analyses found significant effects for several biomarkers even within good-quality studies.

Subgroup patterns hinted at timing and format effects. TAC increased more in trials lasting at least 12 weeks, in participants under 50, and in good-quality studies, a pattern the authors interpret as benefits accumulating progressively through persistent shifts in gut microbiota composition. In contrast, GSH, MDA, and SOD showed significant improvements in interventions shorter than 12 weeks, which the authors attribute tentatively to synbiotics delivered via enriched foods potentially driving faster short-chain fatty acid production and quicker antioxidant responses. Mechanistically, the paper outlines plausible routes: probiotic bacteria may suppress NF-κB activation and reduce lipopolysaccharide concentrations, specific strains such as Lactobacillus casei and L. fermentum can elevate SOD activity and GSH production, and short-chain fatty acids like butyrate appear to facilitate GSH regeneration and limit lipid peroxidation. Crucially, however, none of the included trials measured fecal microbiota composition, so no direct causal link between microbial changes and biomarker shifts could be established.

The authors are candid about the limitations that temper any clinical enthusiasm. Beyond the geographic skew toward Iranian participants, which limits generalizability given how strongly diet and genetics shape gut microbiota, the populations mixed healthy adults with heterogeneous pathologies, and formulations varied widely in strains, doses, and prebiotic types, all feeding the substantial statistical heterogeneity. Only a subset of trials reported TOS, NO, OSI, and GPx, reducing precision for those markers. Long-term safety and tolerability data were absent. The discussion and conclusion sections do venture into clinical implications, suggesting adjunctive use in oxidative stress-related conditions particularly in adults under 50 with regimens of at least 12 weeks, but they pair this with an explicit call for caution, noting that evidence for NO and related outcomes is still emerging and that microbiota-guided personalization would be needed before widespread therapeutic adoption.

The bottom line for readers tracking the supplement literature: this analysis strengthens the signal that synbiotics are associated with favorable shifts in several oxidative stress biomarkers, and its exclusion of retracted or compromised trials makes the estimates more conservative than those in earlier reviews. But the same analysis demonstrates why pooled supplement findings deserve calibrated skepticism. With certainty of evidence rated moderate at best for a single outcome and very low for two others, publication bias detected for three biomarkers, and no microbiome measurements to confirm mechanism, the authors’ own conclusion stands: well-controlled, multicenter randomized trials with standardized formulations and integrated microbiome profiling are needed before these biomarker changes can be translated into confident clinical recommendations.

Subject of Research: Effects of synbiotic supplementation on oxidative stress biomarkers in adults

Article Title: Effects of synbiotic supplementation on oxidative stress biomarkers in adults: an updated GRADE-assessed systematic review and meta-analysis of randomized controlled trials

Article References: Bideshki, M. V., Nojoumi, M., Behzadi, M., Arefinia, N., Askarpour, H., Sohrabi, Z., Esmailian, M., & Ali-Hassanzadeh, M. (2026). Effects of synbiotic supplementation on oxidative stress biomarkers in adults: an updated GRADE-assessed systematic review and meta-analysis of randomized controlled trials. Genes & Nutrition, 21(1), Article 21. https://doi.org/10.1186/s12263-026-00809-9

Image Credits: AI Generated

DOI: 10.1186/s12263-026-00809-9

Keywords: synbiotics, oxidative stress, meta-analysis, GRADE, probiotics, prebiotics, malondialdehyde, total antioxidant capacity, randomized controlled trials, gut microbiota, antioxidants, systematic review