Table 2.
Phase 1: Tea compounds and the gut microbiome: Evidence from human trials.
| Study (Author, Year, Location, Reference Number) | Subjects (age, gender) | Study design | Tea Intervention (type) | Tea Intervention (dosage) | Main Findings | JADAD Score |
|---|---|---|---|---|---|---|
| Zhou et al. (2019) United States [26] | Postmenopausal F. | 12-month intervention. | GTP supplement. | GT catechin extract containing 843.0 ± 44.0 mg/day epigallocatechin gallate or placebo capsules for 1 year. | Microbial metabolism of GTP and aromatic amino acids appear to play a role in the health effects of GT consumption in humans. | 5 |
| Yuan, et al. (2018) China [27] | n = 12 healthy subjects, M and F, 27–46 years. | 2-week intervention. | GT beverage. | 400 mL green tea daily. One-week washout and 2-week intervention. | An irreversible, increased Firmicutes to Bacteroidetes ratio was observed along with a reduction of bacterial LPS synthesis in faeces after GTL ingestion. | 0 |
| Janssens et al. (2016) United States [28] | n = 58 Caucasian, M and F, 18–50 years. | 12-week randomised, single blind, placebo-controlled design. | GT capsules. | Capsules with GT extract (containing >0.06 g Epigallocatechin-3-gallate and 0.03–0.05 g caffeine per capsule). Nine capsules were taken daily. |
Significant effects on composition of the gut microbiota were not observed although a reduced bacterial alpha diversity in overweight vs. normal-weight subjects was seen (p = 0.002). | 2 |
| Van Duynhoven et al. (2014) Netherlands [29] | n = 12 healthy men. | 30-hour randomised, open, placebo-controlled, crossover study. | Single bolus of BTE. | 2650 mg of Brook Bond red label extract, dissolved in 250 mL of hot water. | Inter-individual variation in response was greater for gut microbial catabolites than for directly absorbed BTPs. Rapid and sustained circulation of conjugated catabolites suggests these may be relevant to BTE health benefits. |
4 |
| Jin et al. (2012) Japan [30] | n = 10 non habitual green tea drinkers, M and F, 33–70 years. | 17-day trial (intervention for 10 days). | GT beverage. | 1000 mL of GT daily. Drank GT instead of water for 10 days. | There was an overall tendency for the proportion of Bifidobacteria to increase due to GT ingestion. GT consumption may act as a prebiotic and improve the colon environment by increasing the proportion of the Bifidobacterium species. | 1 |
| Del Rio et al. (2010) Italy [31] | n = 20 healthy subjects. | 24-hour feeding trial. | GT beverage. | 400 mL of a RTD GT containing approximately 400 μmol of flavan-3-ols. | Colonic microflora-derived polyhydroxyphenyl-γ-valerolactones were the main urinary catabolites, averaging 10 times greater concentration than flavan-3-ol conjugates. | 0 |
Key: BTE, Black Tea Extract; BTP, Black Tea Polyphenols; F, Female; GT, GTE, Green Tea Extract; Green Tea; GTL, Green Tea Liquid; GTP, Green Tea Polyphenol; LPS, lipopolysaccharide; M, male; RTD, Ready-To-Drink. Source: Jadad, A.R. et al. Control Clin Trials, 1996. 17(1): pp. 1–12.