1. Introduction
All metabolic processes generate reactive oxygen species (ROS), which can induce oxidative damage to tissue and cell constituents such as lipids, proteins, and nucleic acids [1,2] when left unbalanced. Indeed, the overproduction and accumulation of large amounts of ROS lead to oxidative stress [3,4,5], which represents a major contributor to the risk, onset, and progression of chronic diseases [6,7,8,9,10,11,12]. A key component of oxidative stress implicated in adverse health effects is lipid peroxidation of polyunsaturated fatty acids and the subsequent production of malondialdehyde (MDA) (a major indicator of lipid oxidation) [9]. Therefore, interventions to intensify the body’s antioxidant response to counteract oxidative stress and MDA production are desperately needed.
Natural dietary polyphenolic compounds are potent antioxidants that decrease oxidative stress and protect the body against several oxidative stress-associated diseases [13,14]. Among the common sources of natural dietary polyphenol in foods and beverages is green tea (originate from the leaves of Camellia sinensis), which has high antioxidant properties related to various compounds such as phenolics and flavonoids, and flavonols [15,16,17]. In fact, the major polyphenols with antioxidant properties in green tea are epicatechin (EC), epigallocatechin-3-gallate (EGCG), epicatechin-3-gallate (ECG), epigallocatechin (EGC), and gallocatechin gallate (GCG), which are also called catechins [18]. While some studies have reported that EGCG and other catechins in green tea increase the activity of antioxidant enzymes, decrease indices of oxidative damage and prevent and treat some diseases [19,20,21], there are equivocal effects on markers of oxidative stress in diabetes, metabolic syndrome, β–thalassemia major, and obesity [9,13,22,23,24,25]. Methodological issues such as non-randomization, different outcome measures, short duration of treatment, low study quality, and population heterogeneity have precluded a comprehensive understanding of the extent to which green tea supplementation may exert a beneficial effect on oxidative stress. Further, the potential effectiveness of green tea supplementation in mitigating oxidative stress among adults with pre-existing inflammatory conditions has not been clarified. Thus, we performed a systematic review and meta-analysis of randomized controlled trials (RCTs) to evaluate the relationship between green tea supplementation and oxidative stress markers, as assessed by the MDA and total antioxidant capacity (TAC), in adults with and without inflammatory health conditions.
2. Materials and Methods
Findings from this systematic review and meta-analysis were reported based on the Preferred Reporting Items of Systematic Reviews and Meta-Analysis (PRISMA) guideline [26]. The PICOS model was used to prepare the question: population (all individuals), intervention (green tea), comparison (studies which had control group), outcome (studies that reported two markers of oxidative stress including MDA and TAC), and study design (RCT, Registration code: CRD42021240026) [27].
2.1. Search Strategy
We conducted database searches of PubMed, Scopus, Embase, and ISI web of science from inception to 21 August 2021. All relevant studies written in the English language were included using. Medical subject headings (MeSH) and text words. Search words included:
“green tea” OR “green tea extract” OR “green tea extract AR25” OR “catechin” OR “catechins” OR “EGCG” OR “Camellia sinensis” OR “tea polyphenols” OR “catechinic acid” OR “acid catechinic” OR “sinensis Camellia” OR “Thea sinensis” OR “sinensis Thea” OR “tea polyphenols” AND “oxidative stress” OR malondialdehyde OR MDA OR glutathione OR GSH OR “total antioxidant capacity” OR TAC OR “total antioxidant status” OR TAS AND.
intervention OR “intervention study” OR “intervention studies” OR “controlled trial” OR randomized OR randomized OR random OR randomly OR placebo OR assignment OR “clinical trial” OR trial OR assignment OR “randomized controlled trial” OR “randomized clinical trial” OR RCT OR blinded OR “double blind” OR “double blinded” OR trial OR “clinical trial” OR trials OR “pragmatic clinical trial” OR “cross-over studies” OR “cross-over” OR “cross-over study” OR parallel OR “parallel study” OR “parallel trial”. Reference lists of the included studies were manually checked to identify additional articles. Unpublished documents and grey literature such as thesis, patents, and conference papers were not included.
2.2. Eligibility Criteria
Studies were included if they met the following criteria: RCT design, population aged >18 years, and reported MDA and/or TAC in both the intervention and placebo groups. Studies with duplicated data, non-RCT design, animal studies, studies that assess the effects of green tea supplementation and other interventions, and those with insufficient data for the outcomes of interest or did not meet inclusion criteria of our meta-analysis were excluded.
2.3. Data Extraction
Two independent researchers extracted data and evaluated any disagreement was resolved through open discussion with a third independent investigator (KhM). First author’s name, year of publication, study location, duration and design, gender, mean age and mean body mass index (BMI) of population, the health status of participants, number of participants in each group, type and dose of green tea, and results (means and standard deviations for MDA and TAC before and after intervention) were extracted from the full text of included studies.
2.4. Quality Assessment
Assessment of study quality each included study was precisely evaluated for quality by the Cochrane scoring system [28] of 7 points based on the following criteria: (1) random sequence generation, (2) allocation concealment, (3) blinding of participants and personnel, (4) blinding of outcome assessment, (5) incomplete outcome data, (6) selective reporting, (7) other biases. Three scores of L, H, and U could be given to each aforementioned item, which is interpreted as low risk, high risk, and unknown risk, respectively. Two studies had low random sequence generation [29,30]. Allocation concealment reported in 15 studies [4,5,10,13,23,29,30,31,32,33,34,35,36,37]. Moreover, 3 trials had a high risk of bias regarding blinding of participants, personnel [9,24,31], and 3 trials had low-risk outcome assessors [22,29,34]. Selective reporting was considered as low risk in one trial [10]. All of the studies showed a low risk of bias based on incomplete outcome data. Other sources of bias were low in 10 trials [5,10,13,22,29,30,32,34,35,37].
2.5. Statistical Analysis
This meta-analysis was performed using STATA software, version 14.0 (Stata Corp. LP, College Station, TX, USA). The effect of green tea supplementation on oxidative stress (MDA and TAC) was estimated by pooling mean and standard deviation (SD) values of the baseline and final of the studies in both intervention and placebo groups. Missing SDs for changes were calculated utilizing the following formula: SD change = square root [(SD baseline2 + SD final2) − (2 × R × SD baseline × SD final)], if the studies did not report mean and SD. We considered a correlation coefficient of 0.9 as the R-value of the mentioned formula [38]. We used the following formula to calculate standard errors (SE) from SD: SE√n. If a study provided medians (interquartile ranges), we converted them to means (SD) as described by Hozo and colleagues [39]. If the study results were reported graphically, we used GetData Graph Digitizer software to estimate mean (SD)
3. Results
3.1. Study Selection
We found 3204 publications in Scopus, PubMed, Embase, and ISI web of science in the initial search. Of these, eight hundred seventy-four articles were duplicated. Thus, a total of 2330 articles were assessed for the title and abstract screening. After screening of title and abstract, 2288 unrelated studies were discarded due to primary evaluation of inclusion criteria: unrelated title (n = 1729), review (n = 123), and animal study (436). Consequently, 42 studies were retrieved for full-text review, 26 of which were excluded due to: lack of sufficient data and the use of two or more interventions at the same time [2,21,22,24,30,34,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61]. Finally, sixteen RCTs with one double-arm study [4,5,9,10,13,22,24,29,37] were eligible for this systematic review and meta-analysis. The flow chart of the literature search is shown in Figure 1.
3.2. Study Characteristics
Sixteen RCTs with one double-arm study assessing the effects of green tea supplementation on oxidative stress were identified. Included studies carried out in various countries such as Iran [5,9,22,24,33,36,37], Poland [4,29,31,32], Lithuania [13,31], Taiwan [34,35] and Brazil [10]. Publication dates ranged from 2009 and 2020. The follow-up period ranged from 2 weeks to 36 weeks. The sample size of the included studies ranged from eight to forty-eight participants. Some studies enrolled only females [35] or males [4,5,9,31,34,36], and the rest of included studies involved both genders [10,13,22,24,29,32,33,37], and one of them was unclear [31]. Type of intervention administration were green tea [9,22,24,34,35] and green tea extract [4,5,9,10,13,29,32,34,36,37]. Three studies were conducted on obese individuals [9,29,32], four studies on healthy individuals [4,5,31,34,36], five studies were conducted on diabetics [10,13,24,30,33,37], one study on β–thalassemia major [29] and one study on hypercholesterolemic participants [35]. The studies were performed in participants with different baseline BMI; five studies carried out in participants under 25 kg/m2 [4,5,22,31,34,36], eight studies over than 25 kg/m2 [9,10,24,29,30,32,33,35,37] and one study did not report BMI [13]. All studies were parallel RTCs [4,5,9,10,13,22,24,29,37]. The summary of the characteristics of the included studies is indicated in Table 1. Results from the quality assessment are shown in Table 2.
3.3. Meta-Analysis
3.3.1. Effect of Green Tea Supplementation on TAC
Overall, 16 studies with seventeen arms evaluated the effect of green tea supplementation on TAC. Pooled effect size from random effect model showed a significant increasing effect of green tea supplementation on TAC (WMD: 0.20 mmol/L; 95% CI: 0.09, 0.30, p < 0.001). There was significant heterogeneity between studies (I2 = 98.6%, p < 0.001) (Figure 2A). However, after subgroup analysis we observed significant effect of green tea supplementation on TAC in studies of less (WMD: 0.43 mmol/L; 95% CI: 0.10, 0.75, p = 0.010) or more than 8 weeks (WMD: 0.08 mmol/L; 95% CI: 0.02, 0.13, p = 0.004). Similar significant effects were also detected in green tea extract (WMD: 0.23 mmol/L; 95% CI: 0.03, 0.42, p = 0.019) and brewed green tea (WMD: 0.07 mmol/L; 95% CI: 0.03, 0.10, p < 0.001) and male (WMD: 0.38 mmol/L; 95% CI: 0.07, 0.69, p = 0.003), female (WMD: 0.05 mmol/L; 95% CI: 0.04, 0.05, p = 0.015) and both sexes (WMD: 0.12 mmol/L; 95% CI: 0.08, 0.17, p < 0.001). In addition, sub-group analysis based on dose showed that green tea supplementation for >400 mg/day had significant effect on TAC (WMD: 0.29 mmol/L; 95% CI: 0.09, 0.49, p = 0.004). In addition, subgroup analysis demonstrated that both healthy (WMD: 0.08 mmol/L; 95% CI: 0.02, 0.13, p = 0.004) and unhealthy status (WMD: 0.43 mmol/L; 95% CI: 0.10, 0.75, p = 0.010) significantly affect TAC. Sub-group analysis based on BMI showed that those with BMI = 18.5–24.9 and 25–29.9 had significant effect on TAC respectively (WMD: 0.38 mmol/L; 95% CI: 0.08, 0.68, p = 0.011) (WMD: 0.11 mmol/L; 95% CI: 0.07, 0.15, p < 0.001) (Table 3).
3.3.2. Effect of Green Tea on MDA
Eight clinical trials evaluated the effect of green tea on MDA. Pooled effect size from random effect model showed a nonsignificant decreasing effect of green tea supplementation on MDA (WMD: −0.00 μmol/L; 95% CI, −0.00, 0.00, p = 0.636). There was significant heterogeneity between studies (I2 = 84.1%, p < 0.001) (Figure 2B). In addition, we performed subgroup analysis which did not indicate any changes in MDA following green tea supplementation; however, heterogeneity was reduced in interventions with less than 400 mg/day (I2 = 0.0%, p = 0.530), BMI = 25–29.9 (I2 = 0.0%, p = 0.609) and BMI ≥30 (I2 = 0.0%, p = 0.910) (Table 3).
3.3.3. Publication Bias and Sensitivity Analysis
The sensitivity analysis indicated that removing any of the studies would not substantially change the effect of green tea supplementation on oxidative stress (Figure 2). We used Egger’s weighted regression tests to explore the publication bias. There was no evidence of publication bias for studies examining the effect of green tea supplementation on MDA (p = 0.335, Egger’s test) (p = 0.711 Begg test), and TAC (p = 0.093, Egger’s test) (p = 0.893 Begg test) (Figure 3A,B).
3.3.4. Non-Linear Dose-Response between Dose and Duration of Green Tea Supplementation and Antioxidant Status
Non-linear dose-response analysis was conducted between the dose and duration of green tea supplementation for TAC and MDA. Dose-response analysis indicated that green tea supplementation did not affect TAC and MDA concentrations (Figure 4A,B and Figure 5A,B).
3.3.5. Meta-Regression Analysis
We performed meta-regression analysis to evaluate the association between the dose of green tea (mg/day) and the duration of the intervention (weeks) with antioxidant status. The results of the analysis revealed that green tea supplementation significantly changed MDA (r = −2117.18, p = 0.017) based on the dose of the intervention (Figure 6) and did not show a linear relationship between dose and duration with significant changes in TAC (Figure 7 and Figure 8A,B).
4. Discussion
To our knowledge, this is the first meta-analysis to evaluate the effects of green tea or green tea extract supplementation on oxidative stress (assessed TAC and circulating MDA concentrations). We identified 16 RCTs and 17 arms with 760 participants. These studies suggest that green tea and green tea extract supplementation were positively associated with TAC, but no association with MDA was observed in all subgroups. The association was independent of sex, BMI, duration, and green tea dosage.
Increased oxidative stress elevates the risk of chronic diseases, such as cardiovascular and end-stage renal disease, as it is related to the onset and progression of these adverse conditions [11,12,62,63]. A greater antioxidant intake has been previously related to a lower risk of disease mortality [62]. In general, Camellia sinensis and its tea are known as one of the significant sources of antioxidants such as flavonoids. It has also been shown that green tea is especially rich in a very strong antioxidant called epigallocatechin gallate [63,64]. For the first time, our research demonstrated a significant beneficial effect of green tea supplementation on TAC via a systematic review and meta-analysis, indicating the decreasing impact of this compound on oxidative stress development. The existence of catechins such as EGCG and ECG in green tea, which have numerous free hydroxyl groups, have significant beneficial effects on free radical scavenging activity, and therefore, can lessen the product of lipid peroxidation [65]. A growing body of evidence has suggested the possible role of green tea supplementation in enhancing the antioxidant capacity [30,66,67,68,69]. Additionally, catechin improves antioxidant capacity and may result in a significant decrease in lipid peroxidation production, which has been supported by the study of Lin et al. [66].
Several possible mechanisms are responsible for green tea polyphenols on increased plasma TAC. The vigorous antioxidative function of catechins is related to the dihydroxyl or trihydroxyl groups on the B-ring and the meta-5,7-dihydroxyl groups on the A ring. This function can also be heightened by the trihydroxyl structure in the D-ring (gallate) in EGCG and epicatechin gallate. Based on the polyphenolic structure, free radicals can be neutralized by Electron delocalization [67]. Moreover, reactive oxygen and nitric radicals can be excluded by tea catechins [68,69].
The concentration of lipid peroxidation products representing oxidative stress across health and disease has not been adequately referenced [17,18,19,20]. In this present research, the MDA was proposed as an indicator of lipid peroxidation productions [70]. Regardless of the antioxidant activity of catechin, there are no consistent results on its effects on MDA. For instance, Hadi et al. in 2017 suggested that MDA concentrations were substantially subsided in athletes who consumed green tea extract [5]. This lowering effect of green tea on MDA was also confirmed by Noronha et al. 2019, which explains our findings [25]. Although we found that catechin has no significant effect on MDA concentrations in all subgroups, our meta-regression analysis demonstrated that green tea supplementation reduced MDA concentrations based on the dose of the intervention (mg/day).
On the contrary, several studies have shown no significant association between catechin and the value of MDA. The beneficial effects of green tea catechins in the elimination of ROS have been shown in vitro, although Heijnen et al. supported the idea that it may not be effective in vivo [71]. It has been observed that even with the highest dosage, the number of flavonoids and polyphenol in serum and intracellular are remarkably less than other types of antioxidant concentrations. It is also worth mentioning that some polyphenols, such as catechins, are the result of primitive polyphenols, and thus have poorer antioxidants levels than the parental polyphenols. It seems likely that dietary flavonoid actions as an antioxidant in vivo are not noticeable [72].
The present systematic review and meta-analysis is not without limitations. Most analyses had high levels of heterogeneity, but this was expected because included studies had different types of participants, doses, and intervention durations. In addition, there were not enough studies to evaluate all components of oxidative stress (MDA and TAC) in certain diseases. Moreover, our search was limited to RTCs published in English. Our meta-analytic work has several strengths, including RCTs and diverse participants (both healthy and diseased) from three different continents. Further well-designed RCTs are warranted to corroborate and expand on our outcomes, particularly in regions where green tea supplementation is not popular and in subjects with different diseases that may benefit from a decline in oxidative stress, such as those with cancer.
5. Conclusions
In conclusion, combined data from interventional studies showed green tea supplementation has a beneficial effect on TAC. Additionally, a linear inverse association between the dose of green tea supplementation and a significant change in MDA was reported. Green tea and green tea extract can be proposed as suitable drinks to increase antioxidant status in adults, which may decrease the risk and progression of chronic diseases related to higher oxidative stress levels.
Conceptualization, N.R. and O.A.; methodology, N.R., O.A., L.S., F.G.; software, O.A.; validation, R.B., L.S. and F.G.; formal analysis, O.A.; investigation, R.B.; resources, N.R.; data curation, L.S.; writing—original draft preparation, N.R., L.S., R.B.; writing—review and editing, A.W., R.B., K.C., M.S., N.S. and K.S.; visualization, O.A.; supervision, R.B., K.M.; project administration, K.M. All authors have read and agreed to the published version of the manuscript.
This research received no external funding.
The authors declare no conflict of interest.
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Enlarge this image.Figure 2. Forest plot detailing weighted mean difference and 95% confidence intervals (CIs) for the effect of green tea consumption on; (A) TAC; (B) MDA.
Enlarge this image.Figure 3. Funnel plot for the effect of green tea consumption on; (A) TAC; (B) MDA.
Enlarge this image.Figure 4. Non-linear dose-response relations between dose of green tea (mg/day) and predicted changes in (A) TAC and (B) MDA.
Enlarge this image.Figure 5. Non-linear dose-response relations between duration of intervention (weeks) and predicted changes in (A) TAC and (B) MDA.
Enlarge this image.Figure 6. Random-effects meta-regression plot of the association between dose of green tea (mg/day) and MDA effect size.
Enlarge this image.Figure 7. Random-effects meta-regression plot of the association between dose of green tea (mg/day) and TAC effect size.
Enlarge this image.Figure 8. Random-effects meta-regression plots of the association between duration of intervention (weeks) and effect size of (A) MDA and (B) TAC.
Characteristic of included studies in meta-analysis.
| Author | Publication Year | Country | Study Design | Participant | Sex | Trial Duration |
Means Age | Means BMI | Intervention | Sample Size | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| IG | CG | IG | CG | Treatment Group | Green Tea Dose (mg) | Control Group | IG | CG | |||||||
| Azizbeigi et al. 2009 | 2009 | Iran | RCT | Obese men | M | 8 | 23.9 | 22.8 | 31.8 | 30.8 | Green tea extract | 500 mg | 500 mg sucrose | 10 | 10 |
| Jowko et al. 2011 | 2011 | Poland | DB/R/PC | Healthy individuals | M | 4 | 21.5 | 21.2 | 23.85 | 23.31 | Green tea extract | 640 mg of polyphenols | Maltodextrin | 17 | 18 |
| Bogdanski et al. 2012 | 2012 | Poland | DB/R/PC | Obese, hypertensive patients | F/M | 12 | 49.2 | 51.5 | 32.5 | 33.9 | Green tea extract | 379 mg | Pure microcrystalline cellulose | 28 | 28 |
| Suliburska et al. 2012 | 2012 | Poland | DB/R/PC | Obese patients | F/M | 12 | 48.56 | 52.26 | 32.07 | 33.45 | Green tea extract | 379 mg | Pure microcrystalline cellulose | 23 | 23 |
| Mousavi et al. 2013 (A) | 2013 | Iran | RCT | T2DM patients | F/M | 8 | 54.6 | 52 | 27.4 | 28.1 | Green tea | 4 cup = 10,000 mg | - | 26 | 14 |
| Mousavi et al. 2013 (B) | 2013 | Iran | RCT | T2DM patients | F/M | 8 | 56.2 | 52 | 28.1 | 28.1 | Green tea | 2 cup = 5000 mg | - | 25 | 14 |
| Lasaite et al. 2014 (A) | 2014 | Lithuania | DB/R/PC | T2DM patients | F/M | 36 | 57.2 | 56.8 | NR | NR | Green tea extract | 400 | Microcrystalline cellulose | 17 | 14 |
| Lasaite et al. 2014 (B) | 2014 | Lithuania | DB/R/PC | T2DM patients | F/M | 36 | 57.2 | 56.8 | NR | NR | Green tea extract | 600 | Microcrystalline cellulose | 17 | 14 |
| Spadiene et al. 2014 (A) | 2014 | Lithuania | DB/R/PC | T2DM patients | nr | 36 | 62.18 | 62.18 | 35.23 | 34.98 | Green tea extract | 400 | Microcrystalline cellulose | 20 | 25 |
| Spadiene et al. 2014 (B) | 2014 | Lithuania | DB/R/PC | T2DM patients | nr | 36 | 62.18 | 62.18 | 35.23 | 34.98 | Green tea extract | 600 | Microcrystalline cellulose | 20 | 25 |
| Mozaffari-Khosravi et al. 2014 | 2014 | Iran | RCT | Type 2 Diabetes Mellitus | F/M | 4 | 52.2 | 52.1 | 28 | 28.3 | Green tea | 450 mL | Sour tea | 48 | 46 |
| Kuo et al. 2015 | 2015 | Taiwan | DB/PC | Healthy individuals | M | 4 | 20 | 20 | 21.95 | 23.55 | Green tea extract | 250 mg | Starch capsule | 10 | 10 |
| Hadi et al. 2016 | 2016 | Iran | DB/R/PC | Soccer players | M | 3 | 22.6 | 22.82 | Green tea extract | 450 mg | 450 mg of maltodextrin | 18 | 18 | ||
| Soeizi et al. 2017 | 2017 | Iran | SB/R/PC | β–Thalassemia Major | F/M | 8 | 23.1 | 24.2 | 20.9 | 19.42 | Green tea | 2500 mg | Warm water | 26 | 26 |
| Ribeiro Vaz et al. 2018 | 2018 | Brazil | DB/R/PC | type 1 DM or type 2 DM | F/M | 20 | 46.48 | 52.29 | 27.58 | 25.6 | Green tea extract | 1120 mg | cellulose capsules | 27 | 28 |
| Venkatakrishnan et al. 2018 | 2018 | Taiwan | DB/R/PC | Hypercholesterolemic subjects | F | 12 | 45 | 45 | 31.39 | 28.81 | Green tea | 600 mL (780.6 mg of catechin) | Tea flavor with very less concentration of catechin and caffeine | 20 | 20 |
| Sadowska-Krepa et al. 2019 | 2019 | Poland | RCT | Male students of the physical education | M | 6 | 22 | 23.1 | 23.73 | 24.72 | Green tea extract | 250 mg | Microcrystalline cellulose, magnesium |
8 | 8 |
| Sobhani et al. 2020 | 2020 | Iran | RCT | Healthy individuals | M | 2 | 26.12 | 24 | 23.85 | 24.62 | Green tea extract | 640 mg of polyphenols | Maltodextrin | 8 | 7 |
| Bazyar et al. 2020 | 2020 | Iran | RCT | T2DM patients | F/M | 8 | 51.75 | 52.61 | 29.46 | 29.28 | Green tea extract | 600 mg of EGCG | 600 mg of wheat flour | 22 | 22 |
Abbreviations: IG, intervention group; CG, control group; DB, double-blinded; SB, single-blinded; PC, placebo-controlled; CO, controlled; R, randomized; NR, not reported; F, female; M, male; RCT, randomized clinical trial.
Quality assessment of the studies.
| Study | Random Sequence Generation | Allocation Concealment | Blinding of Participants and Personnel | Blinding of Outcome Assessment | Incomplete Outcome Data | Selective Outcome Reporting | Other Sources of Bias |
|---|---|---|---|---|---|---|---|
| Mousavi et al. (2013) | U | H | H | U | L | L | U |
| Lasaite et al. |
U | L | L | U | L | L | L |
| Spadiene et al. (2014) | U | L | L | U | L | L | L |
| Jówkoa et al. |
U | L | L | U | U | L | U |
| Sadowska-Krępa et al. |
U | L | H | U | L | L | U |
| Bogdanski et al. (2012) | U | L | L | U | L | L | L |
| Hadi et al. |
U | L | L | U | L | L | L |
| Mozaffari-Khosravi et al. |
U | H | U | U | U | L | U |
| Azizbeigi et al. (2019) | U | U | H | U | U | L | U |
| Ribeiro Vaza et al. |
U | L | L | U | H | L | L |
| Suliburska et al. (2012) | L | L | L | L | L | L | L |
| Kuo et al. |
U | L | L | L | L | L | L |
| Soeizi et al. (2017) | L | L | L | L | L | L | L |
| Venkatakrishnan et al. (2018) | U | L | L | U | L | L | L |
| Sobhani et al. |
U | L | L | U | L | L | U |
| Bazyar et al. |
U | L | L | U | L | L | L |
Abbreviations: U; unclear risk of bias, L; low risk of bias, H; high risk of bias.
Subgroup analyses of green tea supplementation on oxidative stress biomarkers.
| NO | WMD (95%CI) | p | P Heterogeneity | I2 | |
|---|---|---|---|---|---|
| Subgroup analyses of green tea supplementation on TAC concentrations. | |||||
| Overall effect | 16 | 0.20 (0.09, 0.30) | <0.001 | <0.001 | 98.6% |
| Trial duration (week) | |||||
| <8 | 5 | 0.43 (0.10, 0.75) | 0.010 | <0.001 | 99.6% |
| ≥8 | 11 | 0.08 (0.02, 0.13) | 0.004 | <0.001 | 80.1% |
| Type of green tea | |||||
| Green tea extract | 12 | 0.23 (0.03, 0.42) | 0.019 | <0.001 | 98.9% |
| Brewed green tea | 4 | 0.07 (0.03, 0.10)_ | <0.001 | 0.169 | 40.5% |
| Sex | |||||
| Both | 8 | 0.12 (0.08, 0.17) | <0.001 | 0.173 | 32.0% |
| Male | 6 | 0.38 (0.07, 0.69) | 0.003 | <0.001 | 99.5% |
| Female | 1 | 0.05 (0.04, 0.05) | 0.015 | - | - |
| Intervention dose (mg/d) | |||||
| ≤400 | 6 | 0.06 (−0.01, 0.13) | 0.132 | <0.001 | 87.1% |
| >400 | 10 | 0.29 (0.09, 0.49) | 0.004 | <0.001 | 99.1% |
| Health status | |||||
| Healthy | 11 | 0.08 (0.02, 0.13) | 0.004 | <0.001 | 80.1% |
| Unhealthy | 5 | 0.43 (0.10, 0.75) | 0.010 | <0.001 | 99.6% |
| Baseline BMI (kg/m2) | |||||
| 18.5–24.9 | 6 | 0.38 (0.08, 0.68) | 0.011 | <0.001 | 99.5% |
| 25–29.9 | 4 | 0.11 (0.07, 0.15) | <0.001 | 0.387 | 1.1% |
| ≥30 | 5 | 0.06 (−0.03, 0.16) | 0.176 | <0.001 | 88.2% |
| Subgroup analyses of green tea supplementation on MDA concentrations | |||||
| Overall effect | 8 | −0.00 (−0.00, 0.00) | 0.636 | <0.001 | 84.1% |
| Trial duration (week) | |||||
| <8 | 3 | −0.08 (−0.21, 0.04) | 0.185 | 0.002 | 84.2% |
| ≥8 | 5 | −0.00 (−0.00, 0.00) | 0.860 | <0.001 | 86.3% |
| Type of green tea | |||||
| Green tea extract | 4 | −0.17 (−0.43, 0.09) | 0.202 | 0.004 | 77.1% |
| Brewed green tea | 4 | −0.00 (−0.00, 0.00) | 0.860 | <0.001 | 89.5% |
| Sex | |||||
| Both | 4 | −0.00 (−0.00, 0.00) | 0.860 | <0.001 | 89.5% |
| Male | 3 | −0.18 (−0.51, 0.15) | 0.499 | 0.002 | 84.3% |
| Intervention dose | |||||
| ≤400 | 2 | −0.01 (−0.02, 0.00) | 0.215 | 0.530 | 0.0% |
| >400 | 6 | −0.00 (−0.00, 0.00) | 0.821 | <0.001 | 88.1% |
| Health status | |||||
| Healthy | 6 | −0.00 (−0.00, 0.00) | 0.853 | <0.001 | 82.9% |
| Unhealthy | 2 | −0.20 (−0.61, 0.20) | 0.331 | <0.001 | 92.1% |
| BMI | |||||
| 18.5–24.9 | 3 | −0.46 (−0.98, 0.05) | 0.081 | <0.001 | 95.0% |
| 25–29.9 | 3 | −0.00 (−0.00, 0.00) | 0.493 | 0.609 | 0.0% |
| ≥30 | 2 | −0.12 (−0.45, 0.20) | 0.451 | 0.910 | 0.0% |
Abbreviations: CI, confidence interval; WMD, weighted mean differences.
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; Bagheri, Reza 3 ; Gholami, Fatemeh 1 ; Soveid, Neda 1 ; Casazza, Krista 4 ; Wong, Alexei 5 ; Suzuki, Katsuhiko 6
; Mirzaei, Khadijeh 1 1 Department of Community Nutrition, School of Nutritional Sciences and Dietetics, Tehran University of Medical Sciences, Tehran 14176-13151, Iran;
2 Cancer Research Center, Shahid Beheshti University of Medical Sciences, Tehran 14167-53955, Iran;
3 Department of Exercise Physiology, University of Isfahan, Isfahan 81746-73441, Iran;
4 Marieb College of Health and Human Services, Florida Gulf Coast University, Fort Myers, FL 33965, USA;
5 Department of Health and Human Performance, Marymount University, Arlington, VA 22207, USA;
6 Faculty of Sport Sciences, Waseda University, 2-579-15 Mikajima, Tokorozawa 359-1192, Japan