Cardiovascular disease (CVD) affects the blood vessels through atherosclerotic damage and is responsible for 17.5 million deaths per year, representing 31% of all deaths worldwide [1
]. Cardiovascular disease risk is predominantly associated with increased or altered lipid concentrations, raised blood pressure, and smoking, which cause atherosclerotic plaque accumulation [3
]. Subsequently, individuals at high risk of developing CVD are often diagnosed with dyslipidemia and/or hypertension [3
]. Dyslipidemia is defined as having a plasma triglyceride (TG) content of greater than 1.7 mmol/L, total cholesterol (TC) content of greater than 5.2 mmol/L, low density lipoprotein cholesterol (LDL-C) content of greater than 3.5 mmol/L, and a high density lipoprotein cholesterol (HDL-C) content of less than 1.0 mmol/L for males and less than 1.2 mmol/L for females [4
]. These biochemical abnormalities are commonly identified in type 2 diabetes mellitus and metabolic syndrome, which are also associated with increased CVD risk [5
]. Hypertension is defined as having a systolic blood pressure at or above 140 mmHg, and/or a diastolic blood pressure at or above 90 mmHg [4
]. It has a global prevalence of approximately 22% in adults [6
] and it is a risk factor for CVD, including coronary heart disease and stroke, as it exacerbates atherosclerotic pathogenesis, clot formation, and narrowing and weakening of the blood vessels, which become more likely to occlude or hemorrhage, causing ischemia [3
]. Other features related to increased CVD risk are hyperglycemia (fasting plasma glucose > 5.6 mmol/L), oxidation of LDL, inflammation, and pro-thrombotic conditions.
CVD is a multifactorial chronic condition and its progression is accelerated through specific behavioral and lifestyle factors such as obesity, sedentary lifestyle, and unhealthy dietary habits [7
]. Obesity is commonly associated with CVD pathogenesis, as it is known to induce insulin resistance, low grade inflammation, endothelial dysfunction, and subsequent atherosclerotic development [8
]. Anthropometric data refers to physical measurements of the body, with height, weight, waist circumference (WC), and body mass index (BMI) being the primary anthropometric outcomes measured in clinical practice and trials, and, as such, they will be used in this systematic review [9
]. Overweight and obesity are defined as an abnormal or excessive fat accumulation in the body, which may have the potential to negatively affect individuals’ health [7
]. These conditions are defined as having a BMI of greater or equal to 25.0 kg/m2
and 30 kg/m2
, respectively [16
Current evidence suggests that a healthy dietary pattern is favorable in protecting against CVD. What still remains controversial is whether it is whole dietary patterns that offer this protection or whether it is specific foods (e.g., a high intake of fiber-rich plant foods, legumes, and fish, and a low intake of processed and saturated-fat-rich foods) and beverages (e.g., wine, coffee, and tea), or their components which are responsible for the protective effect. Most of the data supporting whole diet and food effects are from ecological studies and observational data. Whilst such studies can provide information on hard endpoints of myocardial infarctions and death, due to methodological limitations, such studies only show correlation, not causation. Clinical trials can assess causality, and can examine the health benefits of individual foods or dietary supplements, such as green tea (GT) in reducing the risk for developing CVD. However, often due to limitations in study length, they can usually only reveal effects on biomarkers of CVD risk.
Tea (all forms) is a traditionally consumed beverage originating from ancient China, and has been popular for over 4000 years [17
]. It is the second most commonly consumed drink after water globally, well ahead of other beverages such as coffee, beer, wine, and soft drinks [17
]. It is estimated that between 2.5 to 3 million kg of tea is produced annually worldwide. Accounting for 20% of the total tea production [20
], GT is consumed predominantly in Asian countries and Morocco, but its popularity has also increased in Western countries over the last three decades [17
]. In the United States, the sales of GT were approximately $230 million, revealing its popularity [22
]. Favorable effects on human health were first detailed in traditional Chinese medicine, which encouraged the use of GT for body ailments, such as pain, for improved digestion, detoxification and body energy levels and to generally lower mortality rates from disease [17
]. The beneficial effects of GT consumption in reducing CVD risk is mainly attributed to the green tea catechins (GTCs), some of the many constituents present in green tea [17
The GTCs are plant secondary metabolites, with a very strong antioxidant capacity, produced in the leaves of the tea plant Camellia sinensis
]. The approximate leaf dry weight composition of GT includes 30%–40% polyphenols, 26% fibers, 15% proteins, 2%–7% lipids, 5% vitamins and minerals, 3%–4% methylxanthines (mainly caffeine), and 1%–2% pigments. Catechins account for 80% to 90% of total flavonoids in green tea, with epigallocatechin gallate (EGCG) being the most abundant catechin (48%–55%), followed by epigallocatechin (EGC) (9%–12%), epicatechin gallate (ECG) (9%–12%), and epicatechin (EC) (5%–7%) [19
]. One cup of GT (2.5 g of green tea leaves/200 mL of water) contains 150–200 mg of catechins, including 70–90 mg of EGCG [17
]. However, the relative catechin content of GT is dependent on a number of factors, including the degree of fermentation in the production stage and how the infusion is prepared prior to consumption [1
]. The main structural feature of GTCs (Figure 1
) is a 2-phenylchromane skeleton, with substituted hydroxyl groups at the 3, 5, 7, 3′, and 4′ positions [23
]. Much of the proposed beneficial effect of GTCs is thought to be caused by the activity of these catechol hydroxyl groups, which serve as hydrogen donors and metal chelators [24
]. Therefore, the proposed health benefits are primarily attributed to the functional properties of these groups. Additionally, observed improvements in blood glucose control, hypocholesterolemic effect, and blood pressure reduction can also be related to this structural anomaly, as well as various mechanisms of action within the biological system [19
To date, only one study examining the effect of tea on CVD risk (hypertension, hypercholesterolemia, and dyslipidemia) has looked at both black and green tea [27
]. This systematic review revealed that clinical trials investigating the dietary intervention of GTCs supplementation in high risk CVD individuals demonstrated a small decrease in diastolic and systolic blood pressure, and a potential for lowering total cholesterol after mid- to long-term consumption (three to six months) [27
]. As there is a rising demand for tea globally, it could be argued that health effects from GT consumption in humans may have major beneficial implicationsand lead to additional improvements in health status. Although substantial evidence from in vitro
and animal studies indicates that GT preparations inhibit development of CVD [20
], the possible protective role of GT consumption against CVD in humans remains inconclusive. To date, a number of studies have examined the association between green tea consumption and mortality, but there are methodological issues—including, but not limited to, sample size—that have led to inconsistent results.
Despite the variability of effect, with some studies finding conflicting outcomes [28
], the majority of the current literature appears to support the positive therapeutic effect derived from GT consumption [1
]. Furthermore, as tea consumption is high globally, it could be proposed that even modest health effects in humans may have major beneficial implications for global population health and reduction in CVD risk [21
]. This is in alignment with recently published European data, regarding the prevention strategies against CVD, reporting that despite the decrease in CVD mortality within the last decade, there are significant differences between countries. Moreover, those who have persistent higher CVD mortality rates have paid less attention to various strategies concerning CVD risk factors and biomarkers [36
Therefore, the aim of this systematic review was to determine and combine the effects of GTCs supplementation on CVD risk markers such as anthropometric aspects, blood pressure and related biomarkers. A secondary objective was to elucidate potential physiological mechanisms through which GTCs act and discuss its possible therapeutic benefits in the field of CVD prevention and risk reduction.
The present systematic review protocol was planned, conducted, and reported in adherence with the PRISMA 2009 guidelines [37
]. An electronic literature search was conducted using four databases: PubMed, The Cochrane Library, Web of Science and Scopus. Free text keywords were used to conduct the search. Medical subject headings (MeSH) were considered in the development of the search terms (Section 2.1
). Titles, abstracts and methods were screened for relevance. Articles deemed relevant were selected for further consideration (Figure 2
). The search strategy was piloted across each database to improve the effectiveness of the final search. The search was limited to human subjects, RCTs, and peer reviewed original research articles published in English between 1990, until the end of October 2015. It was beyond the scope of this review to include and examine sources from ‘grey
2.1. Search Terminology
The search terms used included: “green tea catechins”, “green tea extract”, “epigallocatechin gallate”, “epigallocatechin”, “epicatechin gallate”, “epicatechin” AND “cardiovascular disease”, “atherosclerosis” and “total cholesterol”. These were combined using the Boolean operators AND as stated above, and OR between other terms. A hand search was undertaken of reference lists, with the intent to assure quality of the chosen articles.
2.2. Selection Criteria and Data Extraction
Studies were eligible for inclusion if the following applied: (1) they were RCTs, had a minimum sample size of ten healthy and/or diseased participants (including single or both genders); (2) they involved examination of any dietary supplement, consisting of GTCs or an individual catechin in the form of tea, compared against a placebo (3) adult participants aged ≥ 18 years old; (4) reported markers of CVD (TG, TC, HDL-C, LDL-C, or BP (blood pressure)) at baseline and at the end of intervention. Animal trials and studies examining the effects of GT extract given in any other form than tea were excluded. Additionally, if multiple publications referred to the same results, the findings of the latest publication were included. More details on the outcomes of the exclusion criteria are further described in Section 3.1
All papers identified from the initial electronic search process were imported into an EndNote library (version X7, Thomson Reuters), and duplicates were removed. Studies were selected based on the eligibility criteria as outlined above. Three investigators (S.O.L., E.N.G., and N.N.) independently screened the titles and abstracts of articles for eligibility to be included in the systematic review. If consensus was reached, ineligible articles were excluded and eligible articles were moved to the next stage (full-text review) in the process. If consensus was not reached, the article was moved to the next stage, in which the full text of the selected articles was evaluated to determine the eligibility for inclusion in the systematic review. Disagreements were resolved by discussion among the reviewers until a consensus was reached. In total, 38 articles were selected for full-text review and only seven articles fitted the inclusion criteria (Figure 2
) and were included in this systematic review. The reason for excluding each study was recorded. At this stage, the reference lists of included studies were scanned, and if any relevant studies were identified, the full text was retrieved and reviewed for inclusion by all reviewers. Data extraction of included studies was completed by two independent reviewers (S.O.L. and E.N.G.) using a data extraction template. The template included the following: author; title; journal; year of publication; study setting; study design; study population; sample size; participant demographic characteristics; method used to assess markers of CVD.
The primary outcomes included in this systematic review on the effect of GTCs supplementation on CVD were responses of several biomarkers associated with CVD risk factors, including immunoradiometric assay (RIA) findings, TC, LDL-C, HDL-C TG glucose metabolism indices, such as fasting glucose, fasting insulin levels and inflammatory biomarkers, including C-reactive protein (CRP) and Tumor Necrosis Factor-α (TNF-α). Other investigated outcomes were blood pressure responses, including systolic (SBP) and diastolic blood pressure (DBP) and anthropometrical measurements including height, weight (wt), BMI, and WC.
2.4. Data Analysis
Each study was analyzed based on its merit and outlined inclusion criteria. These studies were appraised for their design, interventions, sample size, and the age and gender of the participants, outcome, and compliance measures. Study results were recorded as baseline and post intervention outcome values. Due to the heterogeneity of the study designs, settings, interventions, and outcomes, it was not possible to undertake a meaningful meta-analysis in this systematic review.
Preventive approaches to health and lifestyle related diseases have emerged as an area of increasing interest in recent years, primarily in response to economic and societal burden attributed to ageing populations. Interest in nutritional approaches including the use of dietary supplements such as the widely available, accessible, and affordable GT as a potentially cardio- and vasculoprotective agent has grown rapidly amongst consumer groups and the scientific community globally.
Polyphenolic compounds contained in GT can offer protective effects against CVD by inducing antioxidative, lipid-lowering, anti-hypertensive, anti-obesity, anti-thrombogenic, and anti-inflammatory effects [38
]. Importantly, the systemic absorption of GTCs incorporated within the GTE and even individual catechins, such as EGCG, is affected by several factors, including the type and quantity of the food consumed at the same time [39
] and the exposure to biological fluids (gastric, pancreatic, and biliary fluids) prior to reaching the site of absorption. These factors can significantly influence the absorption and bioavailability of the GTCs in human trials. The studies outlined in this review have demonstrated some of the putative cardiovascular protective properties that GTCs may have on the lowering of body weight, arterial blood pressure, and plasma abnormalities in biomarkers commonly seen in individuals who are prone to developing a serious CVD-related illness in the future. Beneficial effects were observed when GTE was supplemented at levels between 300 and 1500 mg/day (total GTCs 208–1344 mg/day), roughly in line with previous literature, which had suggested that 540 mg of pure EGCG (between 5 and 10 cups of green tea [39
]) was the optimal daily requirement for prevention of CVD risk factors [40
]. Furthermore, the duration of the studies was also considered in the present systematic review, as significant sustained lowering of the CVD risk biomarkers were more likely to be seen in studies conducted over at least 12 weeks of supplementation [9
]. Although the current evidence base appears to be varied, the studies reviewed identified some benefits of GTCs on lowering the CVD risk.
The study by Chen et al.
] proposed that daily supplementation of GTCs significantly decreased body weight, BMI, and WC, which are variables readily associated with CVD pathogenesis and relevant comorbidities. However, the utility of this data is compromised by the limited statistical approach which did not compare the effect of the placebo with the changes seen with GTE dietary supplementation. Nonetheless, the results appeared to be consistent with previous literature that made this observation [12
]. A study by Maki et al.
] proposed that weight loss by GTCs supplementation is induced by influences on energy expenditure, fat oxidation promotion, modification of appetite, and a decreased nutrient absorption. Moreover, GTCs are thought to inhibit the enzyme catechol O
-methyltransferase and its degradative effects on noradrenaline (NA), thereby prolonging the effect of NA in the synaptic cleft and increasing energy expenditure through this pathway [42
]. Additionally, findings in previous studies proposed that this specific mechanism has the potential to stimulate lipolysis in peripheral adipose and skeletal muscle tissues, thereby upregulating hepatic lipid metabolism that causes improvement in the fatty acid oxidation rate and metabolism [43
]. However, anthropometric benefits were not universally observed across all GTE supplementation trials in this review [9
] (Table 1
). This inconsistency of results could potentially be attributed to differences in study designs and choice of participants. Another study by Chen et al.
] recorded changes over a period of 12 weeks (wt: p
= 0.025; BMI: p
= 0.018; WC: p
= 0.023). Consequentially, this further indicates that duration of studies is as important as the dosage of the active ingredient when considering the intervention design, which constitutes an important parameter when addressing changes in anthropometry. Thus, it could be argued that the two-week follow-up period is not sufficiently long enough for this outcome.
The study by Bogdanski et al.
] demonstrated that the daily consumption of a high-dose of GTE (379 mg/day) over three months significantly decreased SBP and DBP (p
= 0.04 and p
< 0.001, respectively) in overweight females. Hypotensive effects have also been observed in other trials [9
], with the suggested mechanism of action being vasodilation in response to the stimulation and amplified production of nitric oxide by the vascular endothelium. This is in agreement with the data from a study in an animal model, the stroke-prone, spontaneously hypertensive rats, in which GTCs were shown to significantly lower SBP and DBP [45
]. The authors suggested that a reduction in the formation of reactive oxygen species in the vasculature and an enhanced endothelium relaxation of the aorta in these rats could be the cause of the hypotensive effects of the GTCs. A previous meta-analysis by Taubert et al.
] indicated that green and black tea had no significant effect on BP, whilst another by Liu et al
] determined that GT could result in significant reductions of SBP and DBP. However, Taubert et al.
] 2007 findings were less conclusive, with a lower total subject pool (n
= 173) for a meta-analysis and the average study duration was also short (two weeks). Liu et al.’s
] 2014 review concluded that GT consumption could decrease BP; however, they suggested that optimum results only occurred after long term (≥12 weeks) intervention. Despite these conclusions, improved BP status was not reflected in all the studies chosen for the present systematic review, with no significant changes observed even in trials that ran for more than 16 weeks [12
]. Consequently, these findings warrant further investigation, with particular attention paid to the study design, the sample size, and the participants’ inclusion criteria.
The study by Hsu et al.
] found significant reductions in biochemical markers associated with atherosclerosis, metabolic syndrome, and CVD pathogenesis. This finding was in accordance with other reports in the literature, where GTCs have been shown to prolong lag time, inhibit formation of oxidized cholesterol and decrease the linoleic acid and arachidonic acid concentrations [47
], and to have hypocholesterolemic and hypolipidemic effects [10
]. A study by Yang and Koo [48
] using an animal model proposed that the hypocholesterolemic effect of GTCs was suggestive of increased fecal bile and cholesterol excretion in studied rodents. This is not the only potential mechanism, since others have also suggested the cholesterol-lowering effects of GT, including reducing capacity of hepatic cholesterol concentration and upregulation of hepatic LDL receptors [49
]. The hypolipidemic effect of GTCs consumption is attributed to reduced intestinal absorption and decreased digestibility of cholesterol and dietary fats [52
]. Additionally, EGCG has been identified to possess the potential to inhibit pancreatic lipase, contributing to the development of natural lipase inhibitors to prevent human obesity and dyslipidemia [55
]. In particular, significant LDL-C reduction [9
] is a remarkable finding, as LDL-C promotes chronic inflammation of arterial blood vessels by white blood cells [56
]. In addition, GTCs are expected to prevent this vascular inflammation by suppressing leukocyte adhesion ability to the endothelium, thereby indicating the potential to inhibit progression of atherosclerotic lesions and thrombogenesis [38
] However, despite these findings, the reviewed studies showed varying degrees of hypocholesterolemic and hypolipidemic activity in participants, with some studies indicating insignificant change for both parameters in the group supplemented with GTCs [11
]. This finding is possibly attributed to the differing health conditions and statuses of the participants recruited, differences in study designs [1
], and the effect of food consumption on catechins systemic absorption [39
Studies by Hsu et al.
] and Liu et al.
] identified statistically significant changes in within-group analyses, despite having no significant differences in variable outcomes between the GTCs supplemented and placebo groups, whereas others, including Chen et al.
] and Diepvens et al.
], either did not undertake between-group analysis or did not fully report the data. These changes were noted in a reduction in WC [13
], and significant changes in TC, HDL, and TG [14
], respectively. Hsu et al.
] suggested that the significant within-group reduction in WC is possibly attributed to changes in body composition or weight redistribution, although no significant changes in BMI were observed. Liu et al.
] proposed that increased circulating levels of the satiety-related hormone glucagon-like peptide 1 (GLP-1) was the cause of the noted beneficial biochemical outcomes found in the within-group analysis. Some authors have previously indicated a significant interaction between circulating GLP-1, HDL, and TG, but not anthropometry [57
], which is consistent in trials with rabbits [50
A limitation faced when analyzing articles for this systematic review was that there were some discrepancies in the study design of the seven studies, including inadequate statistical approaches and reporting different participant inclusion criteria. The length of trial periods ranged from twelve to sixteen weeks, and the dose of GTCs given to study participants ranged from 46 to 1500 mg/day. Furthermore, the participants recruited included healthy volunteers, obese, and individuals with type 2 diabetes. These listed factors are critical parameters in determining the results of the trials and therefore, absolute comparison between them could not be performed under the context of this systematic review. Additionally, all of the selected articles did not examine the long-term preventive effects of GTCs supplementation on CVD risk and hard endpoints (>12 months). Moreover, there was a near universal lack of adjustment for potential confounders within the reviewed studies, which is an important drawback when addressing a multifactorial disease such as CVD. These limitations highlight the need for further scientifically rigorous trials with bigger samples using robust statistical approaches and longer follow-up periods in order to observe long-term association of GTCs consumption and CVD risk and pathogenesis.