Letter: Without a Broader Understanding of the Variables Involved in the Brain–Gut Axis,Probiotics Use Is still an Open Question

To the Editor:

The article “Therapeutic Effects of Probiotics on Symptoms of Anxiety in Children and Adolescents: A Systematic Review and Meta-Analysis of Placebo-Controlled Trials” (Wen et al., 2026) can offer important information concerning the possible benefits of probiotics for anxiety symptom reduction in children and adolescents. However, some features of the study still deserve additional consideration.

Important factors regarding the brain–gut axis modulation are not adequately addressed, including concomitant medication use (e.g., psychostimulants, antibiotics, anti-inflammatory agents), nutritional deficiencies, glucose metabolism, thyroid function, the existence of previous or concomitant gastrointestinal diseases, body mass index, and several biochemical variables, especially the level of high-sensitivity C-reactive protein. Without taking these variables into account, there is a serious risk of overestimating or underestimating the effect attributed to the use of probiotics.

The studies included in the meta-analysis had a treatment duration ranging from 4 to 8 weeks. According to Lin et al. (2023), one of the studies included in the meta-analysis, probiotic treatment over the course of 6 months demonstrated significant improvements in behavioral symptoms compared with placebo (Wen et al., 2026). Given that changes in gut microbiota induced by probiotics may be transient, longer or sustained administration may be necessary to achieve enduring effects on anxiety. Thus, this short duration of the intervention may compromise the possible beneficial results of probiotics. Also, the meta-analysis should have considered possible moderators such as the probiotic dosage, strain composition, and mode of administration that could interfere with the outcome (Walden et al., 2023).

The rationale for including the studies by Jacobs et al. (2017) and Slykerman et al. (2018) is unclear. The first administered probiotics to very preterm infants who are more susceptible to developing attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) (Song, 2023). The second initiated the administration of probiotics at 35 weeks of pregnancy, thereby introducing a potential confounding factor that may have influenced the outcomes.

Another limitation observed in the meta-analysis is that the level of impairment caused by ASD, ADHD, and anxiety is not considered. Additionally, four of the seven included studies permitted psychostimulant use. This factor warrants careful consideration in this meta-analysis, as psychostimulant treatment has been shown to significantly alter microbiota profiles in individuals with ADHD (Boonchooduang et al., 2025).

In conclusion, the meta-analysis by Wen et al. (2026) adds important evidence in the blooming field of research on the interaction between the brain and gut axis. Future studies should address the methodological issues outlined above to provide a more accurate assessment of the effects of probiotics on anxiety symptoms.