AMSTAR 2	A measurement tool to assess systematic reviews (version 2)
ACTH	Adrenocorticotropic hormone
BAI	Beck Anxiety Inventory
BDI	Beck Depression Inventory
BDNF	Brain-derived neurotrophic factor
CFU	Colony-forming unit
CI	Confidence interval
CRH	Corticotropin-releasing hormone
DSM-5	Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition
GABA	Gamma-aminobutyric acid
GAD	Generalized anxiety disorder
GI Tract	Gastrointestinal tract
GOS	Galactooligosaccharide
HADS	Hospital Anxiety and Depression Scale
HAM-A	Hamilton Anxiety Rating Scale
HAM-D	Hamilton Depression Rating Scale
HPA Axis	Hypothalamic
Pituitary	Adrenal axis
HDAC	Histone deacetylase
IBS	Irritable bowel syndrome
IL-6	Interleukin-6
IL-10	Interleukin-10
KYN Pathway	Kynurenine pathway
MeSH	Medical subject headings
NE	Norepinephrine
PRISMA	Preferred Reporting Items for Systematic Reviews and Meta-Analyses
PROSPERO	International Prospective Register of Systematic Reviews
QOL	Quality of life
RCT	Randomized controlled trial
SCFAs	Short-chain fatty acids
SCID	Structured Clinical Interview for DSM Disorders
SMD	Standardized mean difference
SNRI	Serotonin–norepinephrine reuptake inhibitor
SSRI	Selective serotonin reuptake inhibitor
STAI	State-Trait Anxiety Inventory
TNF-α	Tumor necrosis factor–alpha
WHOQOL-BREF	World Health Organization Quality of Life–Brief Version