Table 1.

Main characteristics of the included studies in this review

Ref.	Picca et al[19], 2020	Van Tongeren et al[16], 2005	Xu et al[17], 2021	Lim et al[18], 2021		Zhang et al[22], 2020	Margiotta et al[20], 2020	Ticinesi et al[26], 2017	Haran et al[24], 2018	Haran et al[23], 2021	Larson et al[25], 2020	Zhang et al[21], 2022
Study design	CSS	CSS	CSS	CSS		CSS	CSS	CCS	CHS	CHS	CHS	CSS
Participants	Samples	35	23	94	176	27	64	76	23	166	47	181
	Age (years)	> 70	70-100	80.7 ± 5.7	74.7	81.63 ± 7.90	≥ 65	83.3 ± 7.5	≥ 65	86.2 ± 9.1	> 65	≥ 65
	Male/female	20/15	4/19	44/50	54/122	17/10	43/21	39/37	23	30/136	47	72/109
Genetic analysis	16S rRNA V3-V4	Fluorescence in situ hybri dization	16sRNA V3-V4	16S rRNA		16S rRNA	16sRNA V3-V4	16S rRNA	metagenomic sequencing	metagenomic sequencing	metagenomic sequencing	16sRNA V3-V4
Frailty diagnosis	Short physical performance battery	Groningen Frailty Indicator	Fried's definition	Frailty index		Rockwood Frailty Index	Fried’s Frailty Phenotype	Rockwood Index	CFS	CFS	Rockwood Index	Frailty Phenotype
Outcome measurement	α-diversity, β-diversity, family level, genus level	Genus level, species level	α-diversity, β-diversity, phylum level, genus level	α-diversity, genus level, dpecies level		α-diversity, β-diversity, phylum level, family level, genus level	α-diversity, phylum level, genus level	α-diversity, β-diversity	β-diversity, species level	α-diversity, β-diversity, species level	Genus level, species level	β-diversity, phylum level, genus level

CSS: Cross-sectional study: CCS: Case-control study; CHS: Cohort study; CFS: Clinical Frailty Scale.