Effects of Polyphenols in Tea ( Camellia sinensis sp.) on the Modulation of Gut Microbiota in Human Trials and Animal Studies

: A diet high in polyphenols is associated with a diversiﬁed gut microbiome. Tea is the second most consumed beverage in the world, after water. The health beneﬁts of tea might be attributed to the presence of polyphenol compounds such as ﬂavonoids (e.g., catechins and epicatechins), theaﬂavins, and tannins. Although many studies have been conducted on tea, little is known of its effects on the trillions of gut microbiota. Hence, this review aimed to systematically study the effect of tea polyphenols on the stimulation or suppression of gut microbiota in humans and animals. It was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) protocol. Articles were retrieved from PubMed and Scopus databases, and data were extracted from 6 human trials and 15 animal studies. Overall, large variations were observed in terms of microbiota composition between humans and animals. A more consistent pattern of diversiﬁed microbiota was observed in animal studies. Tea alleviated the gut microbiota imbalance caused by high-fat diet-induced obesity, diabetes, and ultraviolet-induced damage. The overall changes in microbiota composition measured by beta diversity analysis showed that tea had shifted the microbiota from the pattern seen in animals that received tea-free intervention. In humans, a prebiotic-like effect was observed toward the gut microbiota, but these results appeared in lower-quality studies. The beta diversity in human microbiota remains intact despite tea intervention; supplementation with different teas affects different types of bacterial taxa in the gut. These studies suggest that tea polyphenols may have a prebiotic effect in disease-induced animals and in a limited number of human interventions. Further intervention is needed to identify the mechanisms of action underlying the effects of tea on gut microbiota.


Introduction
Studies on the relationship between gut microbiota and health have garnered much interest in recent years. The term "gut microbiota" is defined as the microbial ecosystem or community that resides within the human intestinal tract [1]. The gut ecosystem comprises microorganisms, mainly bacteria, and a small number of viruses, protozoa, and eukaryotic organisms such as fungi that are distributed throughout the gastrointestinal tract [2]. As stated by Nahoum et al., 2016, diversified microbiota are a crucial indicator of good health and well-being [3].
Gut microbiota play an important role in human health, and they are considered a "forgotten organ" and "super-organism" that maintains intestinal epithelium integrity [4][5][6]. The human gut contains an estimated 100 trillion microorganisms [7]; in addition, over 1000 different species of microbes colonize the human gut [8]. The dominant groups of bacteria phyla in the gut are Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria [5,9]. Fusobacteria, Cyanobacteria, and Verrucomicrobia phyla are usually less well-represented [10].
To the best of our knowledge, the effects of tea on gut microbiota were studied in cells (in vitro) and in mechanistic studies on animal models (in vivo) [41][42][43][44]. However, studies using cell lines or animal models to study gut microbiota have their own limitations. Casotta et al., 2020 showed that findings from animal models and cell cultures do not represent and are not translatable to humans [45]. The main limitation of in vivo studies is due to the host's tolerance of microbial infections, which varies greatly across different species [46]. In vitro colonic fermentation models are cheaper, are more reproducible, and can be conducted in a shorter time compared with in vivo studies [47]. Pham et al., 2018 showed several limitations of cell studies, including the absence of human or animal cells and low pH, which reduces microbial activity [47].
Furthermore, it remains a challenge to translate findings obtained from cells and animal models to humans [48]. The role of polyphenols in tea in modulating the human gut microbiota is not well understood. This highlights the need and importance of standardizing human studies, and better outcomes could be predicted. It is still too unclear to suggest an effective dose, choice of types (green, oolong, black, or dark tea) or forms (liquid, powder, or extract), and the duration of tea intake needed to increase the diversity of the gut microbiota in humans. Therefore, this systematic review aimed to contribute to the current updated evidence and knowledge on tea polyphenol stimulation or suppression of the diversity in gut bacteria population in humans and animals. The next aim was to determine the effective types of tea (green, black, oolong, or dark tea), dosage, tea forms (liquid, powder, or pure extract), and duration of intake to modulate the gut microbiota.

Search Strategy
Studies were selected using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Two search databases, namely PubMed and Scopus, were used to search for articles published between the years 2000 and 2020. The Boolean operator term AND was used to focus and narrow the search, while OR was used to expand the search by linking synonyms. The following key terms were applied during the search: (Tea) AND (Gut Microbiota OR Gut Microbiome OR Gut Microflora):

Study Selection
Two authors independently screened the articles and extracted the data. Jadad scoring was used to assess the risk of bias in human trials. The lowest possible score is 1, while the highest possible score is 5 (indicating the highest-quality human trials) [49]. Studies were qualified for eligibility according to pre-specified inclusion criteria. The inclusion criteria were: (1) English primary research paper published between 2000 and 2020; (2) papers on randomized control trials and in vivo studies; (3) studies with normal or overweight (BMI of 18.5-29.9) subjects, non-smokers and non-drinkers, free from medications or supplements; (4) subjects who have had a low-polyphenol diet before enrolling into intervention; (5) all subjects given Camellia sinensis tea and compared with placebo and/or no treatment; (6) study outcomes measuring gut microbiota diversity, including alpha diversity (richness, evenness, and relative abundance) and beta diversity (overall bacteria composition).

Results
A total of 5671 articles on human trials and in vivo animal studies were retrieved from the preliminary search using Scopus and PubMed. Duplicate articles (n = 2663) were removed and the remaining 3008 articles were screened for the relevant title and abstract. A total of 2963 nonrelevant articles were further excluded, and the remaining 45 articles were screened for full content. Twenty-four articles did not meet the inclusion criteria and were excluded. Among those, fifteen studies were excluded because they used multi-component tea supplements in the intervention. Eight studies focused on urinary metabolites of the gut microbiota rather than the composition of the commensal microorganism; thus, they were excluded. One randomized control trial described only intervention protocols and, hence, was excluded. A total of 6 human trials and 15 animal studies were included in the final qualitative review (n = 21); Figure 1 shows the PRIMA flow diagram. Three human studies showed a "high" risk of bias, as assessed by the Jadad score (Table 1). the highest possible score is 5 (indicating the highest-quality human trials) [49]. Studies were qualified for eligibility according to pre-specified inclusion criteria. The inclusion criteria were: (1) English primary research paper published between 2000 and 2020; (2) papers on randomized control trials and in vivo studies; (3) studies with normal or overweight (BMI of 18.5-29.9) subjects, non-smokers and non-drinkers, free from medications or supplements; (4) subjects who have had a low-polyphenol diet before enrolling into intervention; (5) all subjects given Camellia sinensis tea and compared with placebo and/or no treatment; (6) study outcomes measuring gut microbiota diversity, including alpha diversity (richness, evenness, and relative abundance) and beta diversity (overall bacteria composition).

Results
A total of 5671 articles on human trials and in vivo animal studies were retrieved from the preliminary search using Scopus and PubMed. Duplicate articles (n = 2663) were removed and the remaining 3008 articles were screened for the relevant title and abstract. A total of 2963 nonrelevant articles were further excluded, and the remaining 45 articles were screened for full content. Twenty-four articles did not meet the inclusion criteria and were excluded. Among those, fifteen studies were excluded because they used multi-component tea supplements in the intervention. Eight studies focused on urinary metabolites of the gut microbiota rather than the composition of the commensal microorganism; thus, they were excluded. One randomized control trial described only intervention protocols and, hence, was excluded. A total of 6 human trials and 15 animal studies were included in the final qualitative review (n = 21); Figure 1 shows the PRIMA flow diagram. Three human studies showed a "high" risk of bias, as assessed by the Jadad score (Table 1).

Green Tea and Gut Microbiota
Green tea is processed swiftly using fresh leaves to prevent fermentation [28]. Thus, the polyphenol content is higher in green tea compared with other types of tea [28]. Tables 2 and 3 summarize the findings on four human trials and five animal studies on the modulation effects of green tea and the gut microbiota.
No changes were observed in the gut microbiota from a high-quality clinical trial administering four decaffeinated green tea capsules daily containing 1315 ± 115.0 mg of catechins in post-menopausal women for one year [50]. Another trial observed the same results, except for the fact that overweight subjects showed a lower microbiota diversity compared with normal-body-weight subjects before the intervention [52]. Yuan et al., 2018 found that tea reversed the gut microbiota patterns seen in patients with colorectal cancer [53,[56][57][58][59]. However, it must be noted that Yuan et al., 2018 showed a low Jadad score for study quality [53]. The same study found increased levels of bacteria responsible for producing short-chain fatty acids (the main energy source of cells in gut lumen) after receiving 400 mL of green tea beverage per day (approximately two cups daily) for two weeks [53]. Jin et al., 2012 found an increase in probiotic Bifidobacteria when the subjects replaced their water with green tea liquids for ten days [55].
The effects of green tea in animal models were consistent. Mice were given different stressors to cause dysbiosis (imbalances) in their gut microbiota. Zhang et al., 2020 supplemented the diet of diabetic-induced mice with green tea for one month [60]. Diabetes had shifted all diversity measures of the microbiota, and incorporating tea in the diet lowered the indexes to levels almost similar to those in normal mice [60]. Wang et al., 2018 administered tea as drinking water along with a high-fat diet in human flora-associated mice for eight weeks [61]. Tea reversed all changes induced by obesity, hence increasing the overall microbial diversity [61]. Wang et al., 2016 supplemented green tea in a high-fat diet and showed an increased abundance of beneficial lactic acid bacteria (Lactobacillus sp.) [62]. Jung et al., 2017 exposed the mice to chronic ultraviolet rays, which subsequently changed the dominant phylum of microbiota [63]. Receiving tea extract for 10 weeks completely reversed the changes induced in the mice by the ultraviolet rays [63]. Seo et al., 2015 found a significant reduction in biomarkers of obesity and insulin resistance (ratio of Firmicutes to Bacteroidetes phyla and ratio of Bacteroidetes to Prevotella phyla) in the high-fat diet group after intubating tea extracts orally for eight weeks [64].

Oolong Tea and Gut Microbiota
Oolong tea is also known as "semi-fermented" or "partially oxidized" tea. Catechins in oolong tea are oxidized into theaflavins, thearubigins, and theabrownins during partial fermentation, hence producing a slightly darker color than green tea [71]. Oolong tea was supplemented in two murine studies (Table 3). Studies by Cheng et al. investigated the effects of oolong tea extracts in mice induced with human flora and given a highfat diet [41,43]. Tea increased gut microbiota diversity after four to eight weeks of tea supplementation [41,43].

Black Tea and Gut Microbiota
Black tea is a "fully fermented" tea and is characterized by a darker color and astringent taste due to a higher concentration of theaflavins, thearubigins, and theabrownins compared to other types of tea [71,72]. Polyphenol oxidase is a heat-labile enzyme present in black tea [72]. The activity of this enzyme is reduced by steam-heating during the fermentation of black tea, consequently reducing its antioxidant properties compared to green tea [72,73]. In this review, one human study demonstrated the effect of black tea on the gut microbiota (Table 3). Black tea infusion was given to hypocholesterolemic volunteers in a double-blind, randomized crossover feeding trial for six weeks [51]. However, no significant changes were observed in the gut microbiota [51].

Pu-erh Tea and Gut Microbiota
Pu-erh tea is a traditional Chinese tea. There are two types of Pu-erh tea, namely raw (unfermented) and ripe (after microbial fermentation) [74]. In this review, one human trial and four murine studies were done on Pu-erh tea (Tables 2 and 3). Huang et al., 2019 investigated the cholesterol-lowering activity of ripe Pu-erh tea in humans and animals [54]. In this study, male human subjects received 600 mL of tea infusion (approximately three cups) daily for four weeks, while the mice were provided with a daily dose of 450 mg of tea extracts per kg body weight in a high-fat diet for 26 weeks [54]. Hyper-cholesterolenriching bacterial genera were significantly reduced compared to high-fat diet numbers in human and animal studies [54]. Three murine studies demonstrated the effects of raw and ripe Pu-erh tea in restoring the altered gut microbiota caused by a high-fat diet. Lu et al., 2019 and Xia et al., 2019 showed that Pu-erh tea at a dose between 0.1 to 0.4 g of tea extracts for five to eight weeks effectively increased gut microbiota diversity [65,66]. Gao et al., 2017 found that ripe Pu-erh tea extract and Pu-erh tea polyphenol components increased gut microbiota diversity in the high-fat diet group [75].

Fuzhuan Tea and Gut Microbiota
Fuzhuan brick tea is a type of dark tea known as fungal fermented tea [76]. The polyphenol content in Fuzhuan tea is lower compared to green tea, due to the process of microbial fermentation occurring in dark tea production [77,78]. A series of reactions, including degradation, oxidation, condensation, structural modification, methylation, and glycosylation, are catalyzed by microbial exo-enzymes or occur as a result of microbial metabolism, leading to the development of dark tea quality [79][80][81]. Studies by Chen et al., 2018 and Foster et al., 2016 incorporated two different dosages of Fuzhuan tea extracts in mice receiving a high-fat diet (Table 3) [42,68]. Daily supplementation of Fuzhuan tea extracts at doses of between 200 to 400 mg for eight weeks was able to reverse the altered dominant phyla bacteria in the gut and also increase the levels of Lactobacillus and Bifidobacteriaceae [42,68].

Multiple Types of Tea and Gut Mmicrobiota
Two murine studies compared the modulating effects of multiple teas on the gut microbiota that were exposed to a high-fat diet (Table 3) [69,70]. Henning et al., 2017 showed that the supplementation of 0.5 g of decaffeinated green and black tea extract daily for four weeks increased the level of phylum Bacteroidetes while suppressing phyla Firmicutes and Actinobacteria [69]. The ratio of Firmicutes to Bacteroidetes was also reduced [69]. Liu et al., 2016 monitored the effects after feeding 100 mL of either green, oolong, or black tea liquid daily for 13 weeks, and they noted a reversed trend in the growth of bacteria, compared to those with only a high-fat diet [70].

Discussion
Gut microbiota are known for their large variations in terms of taxonomy and functionality [12]. Each individual has a unique gut microbiota profile that differs from another's [12]. Genetic and environmental factors directly influence gut microbiota composition [81]. In terms of genetics, the gut microbiota can be shaped according to birth gestational age, type of birth delivery, methods of milk-feeding, and weaning period [12]. The composition of gut microbiota also differs greatly due to many lifestyle-associated factors, including dietary choices, physical activity, body mass index (BMI), age, food additives and contaminants, and antibiotic consumption, which indirectly shape the gut microbiota composition [24,82].
This review showed that Camellia sinensis could modulate the gut microbiota. Overall, 3 human studies and 15 animal studies from a total of 21 included in the review showed a significant increase in diversity of the gut microbiota. Most animal studies were able to reverse the disrupted microbiota changes due to stressors such as diabetes, obesity, and ultraviolet ray damage. The beta diversity measured in murine studies showed an overall shift in the mice gut microbiota profile after tea supplementation. This indicates that the modulatory effects of tea were attributable to its ability in mediating specific imbalances in the gut. Three out of six human trials showed diversified microbiota as a result of incorporating tea [53][54][55]. An increase in the richness, evenness, and relative abundance of beneficial bacteria and a reduction in nonbeneficial bacteria were observed in the studies [53][54][55].
Green tea was the main type of tea used in this review. An average of two to five cups of green tea per day for 10 days and up to two weeks was associated with increased beneficial probiotic Bifidobacteria and their colon cancer-preventative properties in humans [53,55]. Colonic microbiota have the ability to metabolize tea polyphenols into short-chain fatty acids (SCFA) and phenolic acids, before being metabolized in the liver or being excreted [83]. A previous in vitro study showed that black tea prepared in bread had no impact on short-chain fatty acid (SCFA) production [84]. One portion of the bread containing 30% of polyphenols could be obtained from a cup of black tea [84].
In a low-quality human trial, green tea increased clusters of bacteria specializing in producing short-chain fatty acids (SCFA), namely Lachinospiraceae, Ruminococcaceae, Dorea, Roseburia, Feacalibacterium, Eubacterium, Blautia, and Coprococcus [53]. Short-chain fatty acids are a primary energy source for colonic epithelium cells, as they maintain intestinal homeostasis through anti-inflammatory actions [85,86]. With elevated fecal SCFA concentrations, SCFA-producing bacteria may promote reduced inflammation in the gut [85,86]. This might be important in the preventative steps against colorectal cancer, as inflammatory bowel disease patients showed reduced levels of dominant SCFAs-producing bacteria in several studies [87][88][89][90][91]. However, further study is needed to determine whether green tea could possibly modulate the gut microbiota in cancer patients.
Daily Pu-erh tea intakes of 600 mL (around three cups) for four weeks reduced the proliferation of hypercholesterol-enriching bacteria (Bacilli, Clostridia, Lactobacillus, Bacillus, Streptococcus, and Lactococcus) [54]. These bacteria are involved in bile acid metabolism, i.e., to generate bile salt hydrolase (BSH) enzymes that reduce cholesterol level [54]. Obese human subjects showed a higher Firmicutes/Bacteroidetes ratio after supplementation with polyphenols, and this has been proposed as a reason for weight loss [92,93]. A previous study showed that body weight and dramatic dietary patterns might affect the gut microbiota composition [94,95]. There was no substantial difference in bacterial composition after green tea supplementation in normal human subjects, and this could be due to their "optimum" state of energy balance [52]. However, more human trials are needed to confirm this.
Previous studies have shown that obese animals and humans have higher Firmicutes/Bacteroidetes ratios and higher Firmicutes compared with normal-weight individuals, proposing this ratio as a potential biomarker of obesity [96][97][98][99][100]. However, few studies have proved that a high-fat diet decreased both bacteria levels [67,68]. Tea supplementation increased the Firmicutes: Bacteroidetes ratio and Firmicutes compared with the high-fat group alone [67,68]. A recent human trial showed a higher Firmicutes:Bacteroidetes ratio and higher Firmicutes in normal-weight subjects after tea supplementation [53].
Meta-analyses failed to observe a clear correlation between the ratios of these two phyla and obesity, suggesting the complexity of how the gut microbiome modulates obesity [101]. Although the gut microbiota could contribute to the development of obesity, the evidence suggesting an association between obesity and alterations of the Firmicutes: Bacteroidetes ratio and Firmicutes is not convincing [82]. Thus, tea certainly has effects on the relative species abundance of the gut microbiota, although interpretations of the findings are still lacking [43].
In general, this review showed that low doses of tea might increase the gut microbiota diversity in a short period of time, compared with higher tea doses given for a longer period. A longer period of consumption with higher doses diminished the effects observed during a short period of supplementation. This suggests that the human gut microbiota are resilient toward longer and higher doses of tea supplementation. Human microbiota are stable upon reaching adulthood, and the composition of the gut microbiota remains relatively unaffected by acute perturbations, as its plasticity-like characteristics allow it to return rapidly to its initial composition [102,103]. This review showed a high variability in terms of different types of tea, food matrix, doses, and duration of tea supplementation. Each study used a different type of approach, i.e., richness, evenness, relative abundance, and β diversity.

Conclusions
Tea could increase alpha and beta diversities of the gut microbiota in animals, regardless of tea type, forms, dosage, and duration of intake. However, few effects were observed in humans due to a higher inter-variation in gut microorganisms between individuals. However, the exact mechanism of how tea affects trillions of microbiota in the gut is still poorly understood. More vigorous studies and trials on tea and gut microbiota are needed to understand the effects. While new evidence is needed, Camellia sinensis should be considered as a source of polyphenols in the diet. However, given the differences within and between human and animal studies, there is no specific dose and duration of tea that could be recommended for a healthy gut microbiota.

Conflicts of Interest:
The authors declare no conflict of interest.