A large effect, but it rests on two small trials
STUDY: Zhang S et al, Nutr Rev 2026
STUDY TYPE: Network meta-analysis
FUNDING: Ministry of Science and Technology of China
Background
Many families of children with autism try probiotics, drawn by theories that gut bacteria shape behavior. Options include probiotics (live bacteria), prebiotics (fiber that feeds them), and synbiotics (both together). This analysis compares them.
The Study
- 17 randomized controlled trials with 904 children and young people with autism, ages 17 months to 20 years; duration 4 to 28 weeks.
- Main outcome: core autism symptoms on standardized scales, such as the Social Responsiveness Scale (SRS) and Aberrant Behavior Checklist (ABC).
Results
Synbiotics came out on top, with a very large effect over placebo (effect size, SMD 1.90). In the indirect comparisons of the analysis, symbiotics also beat prebiotics (SMD 2.23), multistrain probiotics (SMD 1.59), and single-strain probiotics (SMD 1.52).
Those numbers imply that probiotics and prebiotics alone did little better than placebo, which is true to the trials: several found no benefit.
Synbiotics did no better than the other supplements for gastrointestinal symptoms.
Limitations
- Only 2 synbiotic trials, with about 34 children combined. One enrolled 8 children and had a high risk of bias.
- Certainty of evidence was moderate to low, and direct and indirect comparisons disagreed.
- Products, doses, ages, and rating scales varied widely.
Practice Implications
- An effect this large should raise suspicion, and is a common false-signal in small trials.
- Still, the finding that synbiotics are better than either alone makes biological sense, and is in line with studies of these agents for metabolic side effects of antibiotics.
- Learn how to use synbiotics in The Complementary and Alternative Medicine in Psychiatry Fact Book. The products I recommend here are synbiotics, with prebiotics in them. Fiber supplements and a healthy diet also serve as prebiotics.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
