Table 3.

Phase 2: Tea compounds and the gut microbiome: Evidence from mechanistic studies.

Study (Author, Year, Reference Number)	Study Design	Tea Intervention (type)	Main Findings
Lu et al. (2019) [52]	Obese murine study.	Ripened Pu-erh tea extract.	Ripened Pu-erh tea extract could potentially prevent obesity through rebalancing the gut microbiota.
Xia et al. (2019) [40]	Metagenomic/meta-proteomic using obese rats.	Aqueous raw and ripe Pu-erh tea extracts.	Raw and ripe Pu-erh teas, administration at two doses significantly increased microbial diversity and changed the composition of cecal microbiota by increasing Firmicutes and decreasing Bacteroidetes.
Zhang et al. (2019a) [32]	Animal and human in vitro studies.	(-)-epigallocatechin-3-gallate and green tea.	Microbiota facilitates the formation of the aminated metabolite of green tea polyphenol (-)-epigallocatechin-3-gallate which trap reactive endogenous metabolites.
Zhang et al. (2019b) [53]	Diabetic murine study.	Corn-starch tea.	Corn-starch‒tea diet resulted in reduced blood glucose, increased levels of Coriobacteriaceae, Lactobacillaceae, Prevotellaceae and Bifidobacteriaceae, and decreased Bacteroidaceae, Ruminococcaceae, Helicobacteraceae and Enterobacteriaceae.
Zhou et al. (2019) [26]	Human study.	Green tea polyphenols.	GTP may have anti-obesity actions namely via changes in gut-microbiota metabolism.
Annunziata et al. (2018) [33]	Simulated GI digestion.	Tea polyphenols from green, white and black tea.	Gut microbiota appear to metabolise polyphenols generating metabolites with a greater antioxidant activity.
Chen et al. (2018a) [54]	Normal and obese rats.	Tea polyphenols.	A high-fat high sugar diet appeared to influence the excretion of tea catechins, leading to insufficient metabolism of catechins by the gut microflora.
Chen et al. (2018b) [42]	Murine study.	Fuzhuan brick tea polysaccharides.	Increased the phylogenetic diversity of high-fat diet-induced microbiota. Could help prevent modulation of gut microbiota.
Cheng et al. (2018) [44]	Murine study.	Oolong tea polyphenols.	A large increase in Bacteroidetes with a decrease in Firmicutes was observed having a positive modulatory and prebiotic effect.
Cheng et al. (2017) [39]	Mice model.	(-)-Epigallocatechin 3-O-(3-O-methyl) gallate.	A large increase in Bacteroidetes with concomitant decrease of Firmicutes was observed after the administration of EGCG3 for 8 weeks. Could help to prevent gut dysbiosis.
Henning et al. (2018) [34]	Murine study.	Green and black tea polyphenols.	GTP and BTPs decreased cecum Firmicutes and increased Bacteroidetes.
Wang et al. (2018) [35]	Human flora-associated C57BL/6J mice model.	Green tea polyphenols.	A high-fat diet significantly impacted gut microbiota composition and lipid metabolism which was ameliorated by tea polyphenols.
Gao et al. (2017) [41]	Murine study.	Pu-erh tea.	Post fermented pu-erh tea providing polyphenols and caffeine improved diet-induced metabolic syndrome which was attributed to remodelling of the gut microbiota.
Jung et al. (2017) [36]	Murine microbiome-metabolome analysis.	Green tea supplementation.	Green tea supplementation improved the microbial community diversity by altering states of various endogenous metabolites in mice groups subjected to UVB-exposure.
Foster et al. (2016) [43]	Pyrosequencing using rats.	Fuzhuan tea.	Fuzhuan tea altered intestinal function and was associated with a threefold increase in two Lactobacillus spp.
Liu et al. (2016) [22]	Obese C57BL/6J mice.	Green, oolong and black tea.	Tea infusion consumption substantially increased diversity and altered the structure of gut microbiota.
Wang et al. (2016) [37]	C57BL/6J Human Flora-Associated mice.	Green tea polyphenols.	High-fat diet was associated with a significant reduction in microbial diversity which was alleviated by tea polyphenol ingestion.
Seo et al. (2015) [38]	Murine study.	Fermented green tea extract.	Fermented green tea restored the changes in gut microbiota composition (e.g., the Firmicutes/Bacteroidetes and Bacteroides/Prevotella ratios) closely linked to development of obesity and insulin resistance, induced by high-fat diets.

Key: BTP, Black Tea Polyphenols; GI, gastrointestinal; GTP, Green Tea Polyphenols; TF, theaflavin; TF3G, theaflavin-3-gallate; TF3’G, theaflavin-3’-gallate.