Systematic review of gut microbiota and attention-deficit hyperactivity disorder (ADHD).

BACKGROUND Gut-brain axis (GBA) is a system widely studied nowadays, especially in the neuropsychiatry field. It is postulated to correlate with many psychiatric conditions, one of them being attention-deficit hyperactivity disorder (ADHD). ADHD is a disorder that affects many aspects of life, including but not limited to financial, psychosocial, and cultural aspects. Multiple studies have made a comparison of the gut microbiota between ADHD and healthy controls. Our aims were to review the existing studies analyzing the gut microbiota between human samples in ADHD and healthy individuals. METHODS The literature was obtained using Google Scholar, Pubmed, and Science Direct search engine. The keywords used were "ADHD", "gut microbiota", "stool", "gut", and "microbiota". The selected studies were all case-control studies, which identify the gut microbiota between ADHD and healthy individuals. RESULT We found six studies which were eligible for review. The model and methods of each study is different. Forty-nine bacterial taxa were found, yet none of them can explain the precise relationship between ADHD and the gut microbiota. Bifidobacterium was found in higher amount in ADHD patients, but other study stated that the abundance of this genus was lower in ADHD with post-micronutrient treatment. This may suggest that micronutrient can modulate the population of Bifidobacterium and improve the behavior of ADHD patients. Other notable findings include a significantly lower population of Dialister in unmedicated ADHD, which rose after patients were medicated. A smaller amount of Faecalibacterium were also found in ADHD patients. This may explain the pathogenesis of ADHD, as Faecalibacterium is known for its anti-inflammatory products. It is possible the scarcity of this genera could induce overproduction of pro-inflammatory cytokines, which is in accordance with the high level of pro-inflammatory cytokines found in children with ADHD. CONCLUSION There were no studies that examined which bacterial taxa correlated most to ADHD. This might occur due to the different model and methods in each study. Further study is needed to identify the correlation between gut microbiota and ADHD.