ACE	Abundance-based Coverage Estimator
ADHD	Attention-Deficit/Hyperactivity Disorder
ADHD-IV-RS	ADHD Rating Scale IV
ASV	Amplicon Sequence Variant
BBB	Blood-Brain Barrier
BDNF	Brain-Derived Neurotrophic Factor
Bf-688	Bifidobacterium bifidum strain Bf-688
CBCL	Child Behaviour Checklist
C-GAS	Child Global Assessment Scale
CNS	Central Nervous System
CPT	Continuous Performance Test
DNA	Deoxyribonucleic Acid
DSM-IV	Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition
DSM-IV-TR	Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision
DSM-5	Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition
FFQ	Food Frequency Questionnaire
GABA	Gamma-Aminobutyric Acid
GWAS	Genome-Wide Association Studies
HDACs	Histone Deacetylases
ICD-10	International Classification of Diseases, 10th Revision
ITS	Internal Transcribed Spacer
JBI	Joanna Briggs Institute
K-ARS	Korean ADHD Rating Scale
K-SADS	Kiddie Schedule for Affective Disorders and Schizophrenia
MR	Mendelian Randomization
MeSH	Medical Subject Headings
OTUs	Operational Taxonomic Units
PRISMA	Preferred Reporting Items for Systematic Reviews and Meta-Analyses
RDP	Ribosomal Database Project
RoB	Risk of Bias
RoB 2	Revised Cochrane Risk of Bias tool for randomized trials
ROBINS-I	Risk of Bias In Non-randomized Studies–of Interventions
ROB-MR	Risk of Bias in Mendelian Randomization
RCT	Randomized Controlled Trial
SCFAs	Short-Chain Fatty Acids
SDQ	Strengths and Difficulties Questionnaire
SNAP-IV	Swanson, Nolan and Pelham Questionnaire IV
UHGG	Unified Human Gastrointestinal Genome