What are the effects of green tea catechins on blood pressure and blood lipid levels in overweight and obese individuals?

Introduction

Obesity is a global problem, with obesity rates increasing worldwide every year. According to the World Health Organization, over 1.9 billion adults aged 18 and older worldwide are overweight, and over 650 million are obese. 39% of adults aged 18 and over (39% of men and 40% of women) are overweight, and 13% of adults (11% of men and 15% of women) are obese. Studies have shown that obesity is a major risk factor for various chronic diseases, including cardiovascular disease, hyperlipidemia, and diabetes. The various diseases caused by obesity increase the burden on healthcare systems. By 2060, the economic impact of overweight and obesity is projected to rise to 3.29% of global GDP. Currently, research on overweight and obese individuals focuses on lifestyle changes, behavioral therapy, pharmacological treatment, and surgical interventions. Dietary supplements also play a role in improving overweight and obesity.

Green tea is one of the most frequently mentioned beverages in the world.  Tea has antioxidant properties and contains trace amounts of protein, carbohydrates, amino acids, lipids, vitamins, and minerals. It also contains various compounds, but polyphenols are primarily responsible for the aroma and beneficial health effects of tea. The main component of green tea polyphenols is catechins, which account for 25%–35% of the dry weight of green tea. Green tea catechins include (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin-3-gallate (ECG), (-)-epigallocatechin (EGC), and (-)-epicatechin (EC). Among these, epigallocatechin-3-gallate (EGCG) is the main active ingredient and the most abundant catechin, accounting for 50%–70% of the total catechins.

EGCG has been extensively studied and is considered an effective nutritional supplement with numerous health benefits, particularly anti-diabetic, anti-angiogenic, and anti-mutagenic properties, including cholesterol-lowering, antibacterial, Alzheimer's disease prevention, and anti-aging effects. A meta-analysis showed that green tea or green tea extract (GTE) supplementation had significant effects on systolic blood pressure (SBP) and diastolic blood pressure (DBP) in overweight and obese individuals compared to placebo. However, another meta-analysis showed that green tea catechin supplementation did not significantly affect blood pressure in overweight and obese individuals after subgroup analysis. In animal experiments, supplementation with green tea extract at a specific dose for three months reduced total cholesterol (TC), triglyceride (TG), and low-density lipoprotein cholesterol (LDL-c) levels in mice. Studies suggest that epigallocatechin-3-gallate (EGCG) achieves this effect by reducing adipocyte differentiation and proliferation during adipogenesis. In human trials, green tea significantly reduced plasma TC and LDL-c levels in overweight or obese individuals. However, another study found that green tea catechin supplementation reduced triglyceride levels in obese individuals but had no effect on other lipid parameters.

In summary, the role of EGCG, the main active compound in green tea, in reducing obesity and improving metabolic status remains controversial. Studies on whether green tea catechins can improve blood pressure and blood lipids in overweight and obese individuals yield inconsistent results. Furthermore, to date, there has been no comprehensive meta-analysis of the effects of green tea catechins on blood pressure and blood lipids in overweight and obese adults. This study aimed to investigate the preventive effects of green tea catechins on blood pressure and lipid abnormalities in overweight and obese adults through a meta-analysis.

Results

1.Search Results and Characteristics of Included Studies

The literature search yielded 3072 citations. 38 were initially excluded due to duplicate publications, 2987 were excluded based on title and abstract, 15 were unavailable, 4 involved mixed interventions, 8 had unavailable data, 9 were non-randomized controlled trials, and 11 were finally included in the study. The search strategy and study selection flowchart are shown in Figure 1.

Figure 1. Flowchart of the selected trials

Table 1 summarizes the basic information of the included studies, including the first author, publication year, sample size, intervention substance, intervention dose, intervention duration, and placebo type. The study included 613 participants, with 313 randomly assigned to the intervention group and 300 to the placebo control group. Analysis of baseline characteristics in all included studies showed no significant differences between the control and intervention groups in terms of age, body weight, BMI, blood pressure, and lipid profile.

Table 1. Characteristics of Included Studies

Table 1 shows that most intervention groups received green tea catechin extract capsules; one study used a green tea extract supplement, and the control group received lactose or microcrystalline cellulose. Two studies had an intervention duration of less than 12 weeks, while the others lasted 12 weeks or longer. Five studies had an EGCG dose greater than 300 mg, and six studies had a dose of 300 mg or less. All 11 included studies were prospective, randomized, placebo-controlled trials; one was a randomized crossover trial, and one used intention-to-treat analysis.

2.Methodological Quality and Risk of Bias

The quality of the included studies was assessed using the JADAD scale; the median score was 7 (range 4–7), indicating high-quality studies.

The risk of bias in the included studies was assessed using the Cochrane risk of bias assessment tool. The results are shown in Table 2. Overall, all trials (100%) clearly described the randomization process and allocation concealment.

Table 2. Cochrane Risk of Bias Assessment

3.Summary of Anthropometric Outcomes

Of the 11 included studies, 10 reported effects on BMI and waist circumference (WC), three reported the efficacy of green tea catechins as a supplement, and four reported the efficacy of green tea catechins in reducing waist circumference. The results of all studies are summarized in Figure 2, including BMI (a) and WC (b). Green tea catechin supplementation had no significant effect on BMI in overweight and obese individuals (WMD = -0.38, 95% CI: -1.13, 0.38, P = 0.326). Green tea catechins supplementation had a significant effect on waist circumference in overweight and obese individuals (WMD = -1.37, 95% CI: -2.52, -0.22, P = 0.019).

Figure 2. (a) Funnel plot of BMI effect; (b) Funnel plot of waist circumference effect

4.Summary of blood pressure results

Eleven studies reported the effects of green tea catechins supplementation on systolic blood pressure (SBP) and diastolic blood pressure (DBP). All results are summarized in Figure 2, including SBP (c) and DBP (d). The results showed that green tea catechins supplementation had no significant effect on SBP (WMD = -1.84, 95% CI: -3.81, 0.14, P = 0.069) or DBP (WMD = -1.62, 95% CI: -4.10, 0.86, P = 0.201) in overweight and obese individuals.

5.Summary of lipid profile results

Eleven studies reported the effects of green tea catechins supplementation on total cholesterol (TC), triglycerides (TG), LDL-cholesterol (LDL-c), and HDL-cholesterol (HDL-c) levels. Four studies showed that green tea catechins supplementation significantly reduced TC levels, three studies showed significant reductions in TG levels, eight studies showed significant reductions in LDL-c levels, and five studies showed significant increases in HDL-c levels. However, when all data, including meta-analysis studies, were pooled, the lipid profile indicators, as shown in Figure 2, indicated that green tea catechin supplementation had no significant effect on TC (WMD = -0.19, 95% CI: -0.44, 0.06, P = 0.142), LDL-c (WMD = -0.13, 95% CI: -0.30, -0.04, P = 0.129), TG (WMD = -0.18, 95% CI: -0.35, -0.02, P = 0.032), and HDL-c (WMD = 0.07, 95% CI: 0.01, 0.14, P = 0.031) in overweight and obese individuals.

6.Subgroup Analysis and Sensitivity Analysis

Subgroup analyses were performed based on intervention dose (>300 mg vs. ≤300 mg), intervention duration (≥12 weeks vs. <12 weeks), health status (healthy vs. disease), and weight management (yes vs. no). The results are shown in Table 3. Table 4 summarizes the detailed data.

Table 3. Summary of Subgroup Analysis Results

Table 4. Data Summary

Sensitivity analyses of the main outcomes were performed by excluding low-quality or high-risk-of-bias studies. The results of all meta-analyses were not affected by the low quality or high risk of bias of the included studies (see Figure 3).

Figure 3. (a) Sensitivity analysis of BMI; (b) Sensitivity analysis of waist circumference

Conclusion

In overweight and obese individuals, green tea catechin supplementation has some effects on lipid profiles, effectively reducing waist circumference and lowering TG and HDL-c levels in those with dyslipidemia. However, it had no significant effect on BMI, SBP, DBP, TC, or LDL-c levels. This meta-analysis shows that green tea catechin supplementation has moderate benefits for the lipid profile in overweight and obese individuals, particularly in lowering TG levels and raising HDL-c levels.