4EBP1	4E-binding protein 1
5-HT	Serotonin
AD	Alzheimer’s disease
ASD	Autism spectrum disorder
BBB	Blood–brain barrier
BCAAs	Branched-chain amino acids
BCAT	Branched-chain amino acid aminotransferase
BCKAs	Branched-chain α-keto acids
BCKDH	Branched-chain α-keto acid dehydrogenase complex
BCKDK	Branched-chain ketoacid dehydrogenase kinase
BMI	Body mass index
CNS	Central nervous system
CSF	Cerebrospinal fluid
DA	Dopamine
DALYs	Disability-adjusted life years
EMT	Epithelial–mesenchymal transition
FMT	Fecal microbiota transplantation
GABA	Gamma-aminobutyric acid
GBA	Gut–brain axis
GC-MS	Gas chromatography mass spectrometry
GI	Gastrointestinal
GWAS	Genome-wide association studies
HPLC-FLD	High-performance liquid chromatography
ILE	Isoleucine
KIC	α-ketoisocaproate
KIV	α-ketoisovalerate
KMV	α-keto-β-methylvalerate
L-dopa	Levodopa
LAT1	L-type amino acid transporter 1
LC-MS	Liquid chromatography-mass spectrometry
LEU	Leucine
LID	Levodopa-induced dyskinesia
LPS	Lipopolysaccharide
MCI	Mild cognitive impairment
MPTP	1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine
MSA	Multiple system atrophy
MSUD	Maple Syrup Urine Disease
NMDA	N-methyl-D-aspartate
PD	Parkinson’s disease
PPM1K	Protein phosphatase, Mg2+/Mn2+ dependent 1K
PSP	Progressive supranuclear palsy
RBD	REM sleep behavior disorder
REM	Rapid eye movement
ROS	Reactive oxygen species
S6K	S6 kinase
SCFAs	Short-chain fatty acids
SNP	Single-nucleotide polymorphism
T2DM	Type 2 diabetes mellitus
TCA	Tricarboxylic acid
TRP	Tryptophan
TYR	Tyrosine
VAL	Valine
mTORC1	Mechanistic target of rapamycin complex 1