TABLE 1.

Characteristics of eligible studies.

Author, year	Study design	Location (City, Country)	Participants and interventions	Measurement	Type of metabonomics	Presentation of results
Tan et al., 2021	Clinical observational study	Kuala Lumpur, Malaysia	Patients with PD (n = 104) their spouse (n = 91) or sibling (n = 5) controls living in the same community. For PD patients: anti-parkinsonian medication initiation within the preceding 3 months or adjustment within the preceding month.	NMR spectroscopy and LC-MS	Untargeted	Increased: Long-chain saturated fatty acids (17-octadecynoic-acid and cis-9, 10-epoxystearic acid), sphingolipids (ceramide [Cer(d14:1(4E)/22:0(2OH)] and dehydrophytosphingosine), and glycolysis products (ethanol and scyllo-inositol); Decreased: Ubiquinones (coenzymes Q6 and Q9), ceramide (Cer(d18:0/14:0)), butyrate, amino acids (glutamate and tyrosine), and substrates of choline metabolism [choline, phosphocholine, trimethylamine, trimethylamine N-oxide (TMAO)] and energy metabolism (pyruvate and fumarate).
Vascellari et al., 2020	Clinical observational study	Cagliari, Italy	64 patients with diagnosed idiopathic PD and 51 healthy controls, selected among spouses and family members of study patients. Patients were received stable doses of dopaminergic treatment for at least 4 weeks before enrollment.	GC-MS	Untargeted	Increased: Cadaverine, ethanolamine, hydroxypropionic acid, isoleucine, leucine, phenylalanine, and thymine; Decreased: Linoleic acid, oleic acid, nicotinic acid, glutamic acid, pantothenic acid, pyroglutamic acid, succinic acid, and sebacic acid.
Unger et al., 2016	Clinical observational study	Homburg, Germany	34 PD patients and 34 age-matched controls. All 34 PD patients were on dopaminergic drugs	GC	Targeted	Decreased: Acetate, propionate and butyrate.
Yan et al., 2021	Clinical observational study	Harbin, China	20 PD patients and 20 age, body mass index and gender-matched healthy controls. All PD patients were using anti-parkinsonian medications	GC-MS	Targeted	Decreased: Isoleucine, valine, phenylalanine, tyrosine and tryptophan.

GC-MS, Gas chromatography-mass spectrometry; NMR, nuclear magnetic resonance; LC-MS, liquid chromatography-mass spectrometry; SCFA, Short chain fatty acids; PD, Parkinson’s disease.