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Scientists continue to hunt for safe ways to ease the day-to-day challenges faced by children with autism spectrum disorder and attention-deficit/hyperactivity disorder. A new randomized, placebo-controlled trial published in Research on Child and Adolescent Psychopathology reports that a 12-week course of two probiotic strains was linked to lower hyperactivity-impulsivity ratings in children aged five to nine and to modest gains in comfort-related quality-of-life scores among children with autism.
Autism spectrum disorder describes a set of lifelong developmental differences marked by persistent social-communication difficulties and a preference for repetitive or highly focused behaviours. Attention-deficit/hyperactivity disorder (ADHD), in contrast, is characterized by age-inappropriate levels of inattention, motor restlessness, and impulsive actions.
Both conditions emerge early in life, show high rates of overlap, and often involve delays in executive functions that help a child plan, inhibit urges, and regulate emotions. Over recent years, researchers have turned their attention to the community of microbes that lives inside the gut.
Some intestinal bacteria can manufacture or modulate neurotransmitters such as dopamine and gamma-aminobutyric acid, chemicals that also shape reward, movement, and self-control within the brain. If the mix of microbes differs in children with neurodevelopmental conditions, supplying strains that boost those messenger molecules could conceivably ease everyday symptoms.
To examine this possibility, a team of researchers in Spain conducted a randomized, double-blind, placebo-controlled clinical trial involving 80 children aged 5 to 16 years. The study included 38 children diagnosed with ADHD and 42 children diagnosed with autism, some of whom had both diagnoses. Participants were randomly assigned to receive either a daily probiotic supplement or a placebo for 12 weeks. The probiotic mixture contained two strains of bacteria—Lactiplantibacillus plantarum and Levilactobacillus brevis—which are known to promote the synthesis of dopamine and GABA.
Before the first dose and again after twelve weeks, parents completed standard rating scales that capture inattentive behaviours, hyperactivity-impulsivity, peer relations, and other challenges. For children with autism, parents also filled out the Social Responsiveness Scale, which gauges social awareness, communication, and repetitive patterns.
Each child took a computerized continuous-performance task that records omission errors (a sign of inattention) and commission errors (a sign of impulsivity). Parents answered questionnaires about executive function in everyday life, recorded their child’s sleep patterns, and judged five domains of health-related quality of life.
Statistical analyses included every child who started the trial, whether or not the family missed a few doses, reflecting real-world use. Across the whole sample, average scores on parent checklists and performance tasks shifted only slightly and did so to a similar degree in the probiotic and placebo groups. When the team separated the data by age, a clearer pattern emerged. Among children aged five to nine, parent ratings of hyperactivity-impulsivity fell more sharply in the probiotic condition than in the placebo condition.
The difference equaled a very large effect size in the autism group and a moderate-to-large effect size in the attention-deficit group. Within-group comparisons told a similar story: young recipients of the probiotic moved from the “elevated” into the “high-average” range on that subscale, while their peers on placebo showed no reliable change.
Performance on the continuous-performance task largely mirrored those parent reports. Children with autism who swallowed the probiotic made fewer commission errors—an index of hurried responses—by the end of the study, whereas the placebo group showed a smaller drop. Detectability scores, which reflect how well a child can distinguish target from non-target stimuli, nudged upward only in the placebo arm. Reaction-time variability, a hallmark of attention-deficit/hyperactivity disorder, was unchanged in both arms.
When the researchers looked beyond core symptoms, they found one additional glimmer of benefit. On the Child Health and Illness Profile, a broad inventory of perceived well-being, children with autism in the probiotic arm reported a higher sense of physical comfort—covering aches, energy, and stomach complaints—than they had at baseline, and the effect size again reached the large range. Sleep ratings, executive-function scores, and social-communication scales stayed level for most participants, regardless of the supplement they received.
How might a bacterial drink calm hyperactivity in younger children yet leave inattention untouched? One possibility is that the strains used in the trial release gamma-aminobutyric acid, an inhibitory neurotransmitter that tempers motor activity. Earlier work shows lower gamma-aminobutyric acid levels in parts of the brain that coordinate movement and impulse control in both neurodevelopmental conditions.
If a probiotic raises the supply of that transmitter—either directly or by stimulating host cells to produce more—it could dampen fidgety behaviour without necessarily sharpening focus. Alternatively, the microbes could influence dopamine signalling, which plays out differently in circuits that support controlled attention versus those that regulate action.
The modest scope of the changes tempers enthusiasm. The trial lasted only three months, a brief window in the long developmental arc of these conditions. The participating families were, on average, socially advantaged and reported few severe behaviour challenges, limiting room for improvement. The sample size—just over eighty—was sufficient to detect large effects but not the nuanced gains that might unfold in smaller subgroups, such as children with pronounced gastrointestinal complaints or those who do not respond well to medication.
Most importantly, the investigators did not profile each child’s gut microbiome before and after treatment, so it remains unclear whether the probiotic actually took hold and shifted microbial balance in a way that mattered to the brain.
Future studies could address those gaps by enrolling larger cohorts across multiple clinics, extending the intervention to six months or a year, and adding stool sequencing to confirm bacterial colonisation and neurotransmitter output. Researchers might also explore whether combining probiotics with traditional behavioural therapies or with dietary fibre that feeds beneficial microbes produces a stronger signal.
Trials that begin during infancy, before hyperactivity or social-communication problems are fully established, may reveal whether microbial support can alter developmental trajectories rather than merely quiet symptoms that have already appeared.
While the current findings are preliminary, they add weight to the idea that the gut-brain connection offers a promising, low-risk target for supporting children with neurodevelopmental differences.
The study, “Effect of Probiotics on the Symptomatology of Autism Spectrum Disorder and/or Attention Deficit/Hyperactivity Disorder in Children and Adolescents: Pilot Study,” was authored by Meritxell Rojo-Marticella, Victoria Arija, and Josefa Canals-Sans.