Effect of nutritional supplements on gut microbiome in individuals with neurodevelopmental disorders: a systematic review and narrative synthesis

The study investigates whether nutritional supplements modulate gut microbiota and, through the gut-brain axis, improve gastrointestinal and behavioral outcomes in individuals with neurodevelopmental disorders (NDDs). NDDs, including ASD, ADHD, specific learning, communication, and motor disorders, have multifactorial etiologies and frequently co-occur with GI symptoms. Evidence links gut microbiota composition and gut-brain axis activity to NDD pathophysiology, with diet and specific nutrients influencing microbial strains and neuroimmune signaling. Prior research has examined supplements’ effects either on gut microbiota or clinical outcomes in isolation; a gap remained in synthesizing their combined effects. This review aims to systematically evaluate clinical trials assessing nutritional supplementation’s impact on both gut microbiome profiles and GI/behavioral symptoms in NDD populations.

The paper contextualizes growing evidence that gut dysbiosis is associated with NDDs, including consistent findings of altered Firmicutes/Bacteroidetes ratios and microbial composition differences compared to neurotypical controls. Trials and reviews have examined probiotics, prebiotics, and dietary approaches (gluten/casein-free diets, ketogenic diets, specific carbohydrate diets) with mixed results on GI and behavioral symptoms. Vitamins and minerals (e.g., vitamin D, iron, zinc, omega-3 fatty acids) show variable efficacy, with some meta-analyses reporting small or modest effects. Probiotic findings are heterogeneous: several reviews support benefits for ASD/ADHD behavior and GI symptoms, while some meta-analyses with small samples found no significant differences versus placebo. Variability in strains, dosing, duration, baseline microbiota, concurrent treatments, and methodological differences likely contribute to inconsistent outcomes. This review integrates both microbiome and clinical endpoints to address prior fragmentation.

Protocol registered in PROSPERO (CRD42023460449). Reporting followed Cochrane Handbook and PRISMA 2020 guidelines. Databases searched up to February 2025 by two independent authors: PubMed, Scopus, Web of Science, Embase, APA PsycINFO; gray literature via Google Scholar (first 100 results) and ProQuest. A PICOS-based strategy defined: Population—individuals with NDDs; Intervention—nutritional supplements (probiotics, vitamins, other nutritional supplementation); Comparators—standard care, placebo, non-intervention, or other interventions; Outcomes—gut microbiome composition and GI symptoms; Study designs—RCTs, single-arm, open-label, and non-randomized trials. No language/date restrictions; reference lists of relevant articles were screened. Records were managed in Endnote X9.3.3, with duplicate removal, and two reviewers independently screened titles/abstracts, then full texts; disagreements were resolved by discussion or third reviewer adjudication. Inclusion criteria: clinical trials in NDD populations assessing short- or long-term effects of nutritional supplements and reporting gut microbiota data. Exclusions: in vitro/in silico/animal studies; ecological/cross-sectional/case-control studies; systematic reviews/meta-analyses; non-intervention studies. Risk of bias was assessed independently by two authors: RoB-1 for RCTs (seven domains; overall judgments of low/unclear/high) and ROBINS-I for non-RCTs (seven domains; overall judgments of low/moderate/serious/critical/no information), with visualization using robvis. Due to substantial heterogeneity in interventions, populations, outcomes, and designs, Synthesis without Meta-analysis (SWIM) was applied for narrative synthesis. Quantitative and qualitative results were summarized by effect estimates, direction of effects, individual study p-values, and combined p-values via Fisher’s method (Corbi packages in R). Outcomes were grouped as primary: GIS, GI symptoms, good microbiome, bad microbiome, Firmicutes/Bacteroidetes ratio; and secondary: social skills (ABC, ABC-2, SRS) and symptom severity (ADHD-IV-RS, ATEC, CARS, CGI, PGI-2). Interventions were categorized into probiotics, vitamins, and other nutritional supplements (ONS). PRISMA flow: 1173 records screened; 49 full texts assessed; 21 clinical trials included.

- Study characteristics: 21 clinical trials (11 RCTs; 5 open-label; 3 non-randomized; 1 double-blind; 1 controlled), published 1992–2025, across USA, Europe, Asia, New Zealand, Egypt, Australia. Total N=884 participants (age range children to adults); one male-only trial. Populations: ASD (majority), ADHD, ASD+ADHD, intellectual disability (ID). Interventions: probiotics (most frequent), vitamins, and ONS. - Primary outcomes: • Gastrointestinal Severity Index (GIS): 12 studies assessed GIS; probiotics (mostly in ASD, one ADHD), vitamins, and ONS all reduced GIS scores. Individual p-values ranged from <0.0001 to 0.05; combined p-value <0.0001. • GI symptoms: 15 studies assessed GI symptoms. Probiotics generally improved GI symptoms in ASD (and one ADHD study). ONS showed reductions in GI symptoms in ASD and ID, with two studies showing no significant change (ADHD and ID). One vitamin study in ASD reported GI symptom improvement. Combined p-value <0.0001. • Good microbiome: 17 studies evaluated increases in beneficial taxa (e.g., Bifidobacterium, Lactobacillus, Bacteroidetes). Probiotics and vitamins significantly increased beneficial bacteria, with narrative synthesis indicating strong significance (often p<0.05 to <0.0001). • Bad microbiome: 16 studies reported reductions in putatively harmful taxa or dysbiosis markers; most showed significant decreases (p<0.05 to <0.0001), though three ASD studies reported no significant change. Combined p-value <0.0001. • Firmicutes/Bacteroidetes (F/B) ratio: 9 studies measured F/B; significant changes after probiotics and vitamins in ASD and ADHD in several studies (others non-significant). Combined p-value <0.0001. - Secondary outcomes: • Symptom severity: Significant improvements reported in ATEC, CGI-I, CARS, and reductions in ADHD-IV-RS associations with microbial changes; specific carbohydrate diet and Juvenil showed ATEC reductions; synbiotic interventions reduced ABC. • Social skills: Improvements in SRS subdomains, ABC-2 total/stereotypy, and post-intervention gains in language/cognition/social behaviors in more than half of participants in a large open-label synbiotic study. - Mechanistic interpretation: Nutritional supplementation likely modulates gut microbial composition, reduces dysbiosis, enhances gut barrier integrity, and influences neurotransmitter activity and neuroinflammation along the gut-brain axis, contributing to improvements in GI and behavioral outcomes.

The review demonstrates that nutritional supplements, particularly probiotics and vitamins, can beneficially reshape gut microbiota in NDD populations, leading to reduced dysbiosis and improved gut barrier function. These microbiome shifts—such as increases in Bifidobacterium and Lactobacillus and modulation of the F/B ratio—align with decreased GI burden (GIS and GI symptoms) and correlate with improvements in behavioral and social measures (ATEC, ABC, CARS, CGI, SRS). Mechanistically, microbial metabolites and altered community structures likely reduce gut inflammation, improve permeability, and modulate neurotransmitter systems and neuroimmune pathways within the gut-brain axis, providing a biologically plausible route to symptom relief. Clinically, the findings support gut microbiota-targeted nutritional approaches as adjunct therapies in ASD and ADHD management. However, heterogeneity in strains, dosing, durations, baseline microbiota, and methodological rigor contributes to variability across studies. Standardization, stratification by baseline microbiome, and longer follow-ups are needed to refine efficacy estimates and identify responders.

This systematic review addresses a key evidence gap by jointly synthesizing the effects of nutritional supplements on both gut microbiota and clinical outcomes in NDDs. Across 21 trials, supplementation generally improved GI indices, increased beneficial bacteria, reduced harmful taxa, and, in several studies, favorably altered the F/B ratio. Behavioral and social outcomes also improved in multiple trials. These results highlight the gut-brain axis as a therapeutic target and support incorporating microbiota-focused nutritional strategies (e.g., probiotics, vitamins) as adjunctive care for NDDs. Future research should employ larger, multicenter RCTs with standardized supplement protocols, placebo controls, stratification by baseline microbiota, longer follow-up, and unified outcome measures to determine optimal regimens and long-term benefits.

- High heterogeneity across studies in intervention type, dosing, duration, populations, outcomes, and methodologies, precluding meta-analysis and necessitating SWIM narrative synthesis. - Small number of trials and limited sample sizes reduce robustness and generalizability; combined p-values (Fisher’s method) were used to enhance statistical synthesis of reported significances. - Many studies exhibited high or unclear risk of bias, yielding overall low-quality evidence despite formal RoB assessments. - Short follow-up durations limit conclusions about long-term effects on microbiome and clinical outcomes. - Several trials lacked placebo controls, potentially affecting reliability. - Inter-individual variability (baseline microbiota, diet, medications, genetics) may confound responses to supplementation. Future RCTs should address these limitations with larger samples, multicenter designs, standardized protocols, longer follow-up, and rigorous controls.