The effect of green tea on blood pressure and lipid profile: A systematic review and meta-analysis of randomized clinical trials

Abstract

Introduction

Many different dietary supplements are currently marketed for the management of hypertension, but the evidence for effectiveness is mixed. The aim of this systematic review was to evaluate the evidence for or against the effectiveness of green tea (Camellia sinensis) on blood pressure and lipid parameters.

Methods and results

Electronic searches were conducted in Medline, Embase, Amed, Cinahl and the Cochrane Library to identify relevant human randomized clinical trials (RCTs). Hand searches of bibliographies were also conducted. The reporting quality of included studies was assessed using a checklist adapted from the CONSORT Statement. Two reviewers independently determined eligibility, assessed the reporting quality of the included studies, and extracted the data. As many as 474 citations were identified and 20 RCTs comprising 1536 participants were included. There were variations in the designs of the RCTs. A meta-analysis revealed a significant reduction in systolic blood pressure favouring green tea (MD: −1.94 mmHg; 95% CI: −2.95 to −0.93; I² = 8%; p = 0.0002). Similar results were also observed for total cholesterol (MD: −0.13 mmol/l; 95% CI: −0.2 to −0.07; I² = 8%; p < 0.0001) and LDL cholesterol (MD: −0.19 mmol/l; 95% CI: −0.3 to −0.09; I² = 70%; p = 0.0004). Adverse events included rash, elevated blood pressure, and abdominal discomfort.

Conclusion

Green tea intake results in significant reductions in systolic blood pressure, total cholesterol, and LDL cholesterol. The effect size on systolic blood pressure is small, but the effects on total and LDL cholesterol appear moderate. Longer-term independent clinical trials evaluating the effects of green tea are warranted.

Introduction

Hypertension is a leading cause of death, and a major risk factor for cardiovascular disease [1]. Its global prevalence varies worldwide from as low as 3.4% to as high as 72.5% [2]. Although over 90% of hypertensive cases are idiopathic [3], dietary and lifestyle factors are major risk factors associated with its increasing incidence [4], [5]. Many different dietary supplements are currently marketed for the management of hypertension, but the evidence for effectiveness is mixed [6]. One such supplement thought to have an antihypertensive effect is the extract of Camellia sinensis, green tea.

Green tea is one of the most commonly consumed beverages worldwide [7]. The leaves of the plant contain a variety of phytochemicals including phenols and catechins [8]. The polyphenolic compounds in green tea are thought to possess antioxidant properties by virtue of their ability to scavenge for free oxygen and nitrogen radicals [9]. The catechins in green tea comprise epigallocatechin-3-gallate (EGCG), epigallocatechin, epicatechin-3-gallate, and epicatechin. EGCG is reported as the most abundant catechin in green tea, comprising over half of the total catechin content, as well as being the most bioactive component [10], [11].

Green tea catechins have been postulated to stimulate thermogenesis, modify appetite and downregulate the enzymes involved in lipid metabolism [12]; consequently this group of flavonoids are commonly marketed as slimming aids. Invitro studies have shown that the epigallocatechins in green tea have angiotensin converting enzyme inhibitor properties [13], [14], and findings from several animal studies have suggested that green tea lowers blood pressure by suppressing the NADPH oxidase activity and reducing the numbers of reactive oxygen species in the vascular system [15], [16]. Molecular studies have demonstrated that green tea catechins enhance cholesterol 7alpha-hydroxylase gene expression in HepG2 cells [17], [18], a process which is thought to stimulate bile acid production and decrease cholesterol concentration in the hepatocytes. Animal studies have also shown that green tea extracts inhibit intestinal absorption of lipids and also upregulate low-density lipoprotein receptors in the liver [19], [20], [21]; mechanisms which lead to improvements in the blood lipid profile.

Case-control and epidemiologic studies have suggested that green tea intake has a cardioprotective effect [22], [23], but a previous meta-analysis of five studies concluded that green tea had no beneficial effect on blood pressure [24]. However, a recent meta-analysis reported beneficial effects of green tea on blood vessel dilatation [25], and two meta-analyses (both including open-label and blinded trials) reported beneficial effects on lipid profile [26], [27]. Therefore, the purpose of this systematic review was to evaluate the evidence for or against the efficacy of green tea extracts on blood pressure and lipid profile, using published data from blinded-only clinical trials.