Xu R, Yang K, Ding J, Chen G. Effect of green tea supplementation on blood pressure. A systematic review and meta-analysis of randomized controlled trials. Medicine. February 2020;99:(6)e19047. doi: 10.1097/MD.0000000000019047.
Green tea (Camellia sinensis, Theaceae) leaf contains catechins, which have purported cardioprotective properties in vitro and in vivo. However, clinical trials evaluating the association between tea consumption and blood pressure reduction have equivocal conclusions. Hence, the purpose of this study was to conduct a systematic review and meta-analysis of randomized, placebo-controlled trials (RCTs) to quantitatively assess the effect of green tea on blood pressure reduction.
Pubmed, Embase, and the Cochrane Library were searched from inception through August 2019 with key words green tea, green tea extract, tea component(s), tea solid(s), catechin, catechins, EGCG, Camellia sinensis, tea polyphenols, blood pressure, hypertension, hypertensive, hypotension, hypotensive, endothelium, endothelial function, and endothelial dysfunction. Included articles met the following criteria: (1) evaluated adults (>18 years) who ingested green tea beverages or extract for ≥ two weeks; (2) RCTs with either a parallel or crossover design; (3) baseline and endpoint values included systolic and/or diastolic blood pressure with standard deviations, standard errors, or 95% confidence intervals for each study group; (4) only monotherapy was permitted; (5) food intake regimens were consistent between groups, and the only difference between the control and treatment group was green tea consumption; (6) each group had >10 subjects; and (7) studies were English-language only.
Excluded articles met the following criteria: (1) enrolled children or pregnant women; (2) were review, meta-analysis, or abstract-only reports; and (3) there were no details about the epigallocatechin gallate (EGCG) or polyphenol content of the green tea. Reference lists of included articles were also screened. The Jadad score was used to assess quality of the studies. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed. Data were extracted and a meta-analysis was conducted.
A total of 1132 articles was located, and 24 studies (with 25 comparator groups) met all inclusion criteria. Studies were conducted in the United States (two studies), Poland (two studies), the United Kingdom (three studies), the Netherlands (one study), Taiwan (five studies), Japan (nine studies), Australia (one study), Iran (one study), and Brazil (one study). In total, there were 1608 participants (859 treated with green tea and 838 treated with placebo), with each study including 22 to 240 participants aged 22-74 years who were either normotensive or hypertensive. The reported total of green tea catechin doses ranged from 208 to 1344 mg/day for 3-16 weeks. Eight studies evaluated green tea beverage, and 17 studies evaluated green tea extract capsules. Nine studies evaluated decaffeinated products, 12 studies evaluated caffeinated products, and four studies did not specify caffeine content. Specific products evaluated were not reported. Twenty studies were double-blinded RCTs, one study was single-blinded, and three studies were open-label. According to Jadad scoring, 12 studies were judged has having high quality, and 12 studies had low quality.
The meta-analysis revealed that green tea significantly lowered systolic blood pressure (SBP) (P=0.02) and diastolic blood pressure (DBP) (P=0.004) compared with placebo. Subgroup analyses were conducted to determine the effect of the following parameters on outcome: study duration, dose-effect relationship, health status, ethnic group, coffee (Coffea arabica, Rubiaceae) content, catechin dosage, study design, and type of intervention (green tea beverage or capsule). The subgroup analyses showed that the effects of green tea on both SBP and DBP were greater in (1) participants from Western countries, (2) those who were treated with a low catechin dosage (< 615 mg/dL), (3) parallel trials, (4) participants with high-normal or hypertensive blood pressure, and (5) those with cardiovascular risk factors. When stratified by Jadad scores, studies with low Jadad scores had a significant reduction in SBP, while studies with high Jadad scores had significant reductions in DBP. Caffeine content did not alter the results.
The authors conclude that in these top 24 studies green tea consumption significantly lowers both SBP and DBP, and the effect was greater in participants with high-normal blood pressure, hypertension, or other cardiovascular disease risk factors. The authors hypothesize that the benefits of green tea on blood pressure may be via inhibition of oxidation, vascular inflammation, and thrombogenesis, and by improving endothelial dysfunction. Caffeine, which is naturally occurring in tea, has the potential to increase blood pressure by increasing arterial stiffness. The catechins in green tea may counteract the effect of caffeine on blood pressure. A limitation of this study is that the optimal dose of catechin was not evaluated, and a dose-response was not calculated. Another limitation is that all of the studies had a relatively short duration (three to 16 weeks), so long-term effects of green tea on blood pressure could not be evaluated. This is an unusual paper as most research suggests cardio-benefits arise from black tea containing theorubigens, not usually from green tea containing catechins. The authors declare no conflicts of interest.
—Heather S. Oliff, PhD