Role of Tea Polyphenols in Metabolic Syndrome

*Telma Angelina Faraldo Corrêa, Adriana Campos Rozenbaum and Marcelo Macedo Rogero*

### **Abstract**

Metabolic syndrome (MetS) increases the risk of type 2 diabetes and cardiovascular diseases (CVD). Tea (*Camellia sinensis*), one of the most consumed beverages in the world, is rich in polyphenols, mainly catechins. Tea polyphenols may ameliorate obesity by reducing body weight, increasing energy expenditure and fat oxidation, stimulating lipolysis, and improving thermogenesis. Tea polyphenols also reduce the risks of type 2 diabetes (T2D), hypertension, hyperlipidemia, and inflammation. Results of clinical trials on the effects of the consumption of tea beverage, tea extracts, or isolated tea polyphenols on biomarkers of metabolic syndrome will be reviewed in this study. The effects of tea polyphenols on antioxidant status and low-grade chronic inflammation and the molecular mechanisms involved will also be discussed.

**Keywords:** *Camellia sinensis*, catechins, inflammation, insulin resistance, dyslipidemia, hypertension, obesity

#### **1. Introduction**

Metabolic syndrome (MetS) is a cluster of interrelated prejudicial conditions that leads to type 2 diabetes (T2D) and cardiovascular disease (CVD). These conditions include elevated fasting plasma glucose level (hyperglycemia), abdominal/ visceral obesity, dyslipidemia, and hypertension [1, 2]. The International Diabetes Federation (IDF) estimates that around 20–25% of the global adult population suffer from MetS and are more likely to die from a heart attack or stroke compared with people without MetS [1].

Since there is no specific treatment for MetS, individual characteristics must be taken into consideration. There is a need for long-term studies to determine whether existing and new therapeutic agents benefit patients with MetS, reducing the effects of MetS and preventing the appearance of associated diseases and to evaluate the potential of novel candidates as effective treatment options [3]. Several clinical studies demonstrate that lifestyle modification, especially dietary changes, is an effective strategy to reduce several factors responsible for the development of MetS. Introducing foods rich in dietary phytochemicals, such as polyphenols, into the diet of an individual is an effective lifestyle modification for the prevention of several diseases, including MetS [4, 5].

Polyphenols (phenolic compounds), one of the most relevant families of phytochemicals with health benefits, are biomolecules found in natural products. Several preclinical studies report that some polyphenols exert protective effects in

**Figure 1.**

*Chemical structure of green tea catechins: epicatechin (A), epigallocatechin (B), epicatechin-3-gallate (C), epigallocatechin-3-gallate (D).*

many diseases, including CVD and MetS, both triggered by oxidative stress [6, 7]. These compounds present antioxidant and anti-inflammatory properties and may be able to delay or prevent MetS by decreasing blood pressure, blood glucose levels, and body weight, as well as by improving lipid metabolism [7, 8]. One of the main sources of polyphenols is tea prepared from the processed leaves of *Camellia sinensis*, an herbal plant belonging to the Theaceae family. The chemical composition of tea is characterized by the presence of polyphenols (especially catechins), phenolic acids, amino acids, proteins, and fats. The catechins most commonly found in tea include epigallocatechin-3-gallate (EGCG), epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epicatechin (EC) (**Figure 1**). These compounds constitute up to 30% of the dry leaf weight. A typical green tea beverage, with 2.5 g of tea leaves and 250 mL of hot water, contains 240–320 mg catechins and 60–65% EGCG [7, 9, 10]. Although the bioavailability of tea polyphenols is not clearly known, it depends on the molecular size and the number of phenolic groups. A review demonstrated that the consumption of 2 or 3 cups of tea daily resulted in 0.2–0.3 μM peak plasma levels of tea catechins [9, 11]. Notably, the health-promoting properties of green tea are due to the presence of the catechins mentioned earlier, mainly because of their antioxidant (scavenging of reactive oxygen species, inhibition of the formation of free radicals, and lipid peroxidation) and anti-inflammatory effects [10].

#### **2. Tea, obesity, and inflammation**

Obesity is a major health concern in the developed and developing world. Obesity leads to an inflammatory condition that is directly involved in the etiology of CVD, T2D, and certain types of cancer. Furthermore, the accumulation of adipose tissue in the abdominal region is a significant risk factor for the development of MetS and associated morbidities. It should be noted that inflammation is

#### *Role of Tea Polyphenols in Metabolic Syndrome DOI: http://dx.doi.org/10.5772/intechopen.92888*

a common feature implicated in the pathophysiology of many obesity-associated disorders. The inflammatory response in obese and MetS individuals manifests systemically and is characterized by a chronic low-intensity reaction, unlike classical inflammation [12–16].

The anti-inflammatory and anti-obesity effects of *Camellia sinensis* have been associated with its catechin content, and EGCG is the most abundant and pharmacologically active catechin. Green tea, which is more effective than black tea, has been shown to significantly alleviate MetS symptoms, such as abdominal adiposity indicated by waist circumference in obese subjects. The anti-obesity mechanisms of tea polyphenols are associated with two major mechanisms: (i) decreasing the absorption of lipids and proteins in the intestine by tea constituents, thus reducing calorie intake, and (ii) activating adenosine monophosphate-activated protein kinase (AMPK) by tea polyphenols that are bioavailable in the liver, skeletal muscle, and adipose tissues. The relative importance of these two mechanisms depends on the types of tea and diet consumed by individuals. It should be noted that AMPK activation can reduce gluconeogenesis and fatty acid synthesis, leading to bodyweight reduction and MetS alleviation [17, 18].

Clinical trials [19–21] verified that green tea reduced body weight and other biomarkers linked to MetS (**Table 1**).

Cellular, animal, and human experiments demonstrated that green tea and its major component, EGCG, have anti-inflammatory effects. Moreover, EGCG


*BMI, body mass index; CRP, C-reactive protein; EGCG, epigallocatechin gallate; HDL-C, high-density lipoprotein cholesterol; LDL-C, low-density lipoprotein cholesterol; MetS, metabolic syndrome; TC, total cholesterol; TG, triglycerides; TNF-α, tumor necrosis factor-alpha; ↑, Increase; ↓, reduction.*

#### **Table 1.**

*Clinical trials showing the effects of tea or tea catechins on inflammation, obesity, and lipid profile.*

inhibits the in vitro activation of the transcription factor NF-κB and attenuates the IκB-α degradation induced by tumor necrosis factor-alpha (TNF-α) activation. The anti-inflammatory mechanism of EGCG seems to be associated with a decrease in the activity of the IKK-β protein, involved in the phosphorylation of IκB-α. Because of this effect on the NF-κB signaling pathway, catechins can reduce the gene expression of COX-2. In addition, EGCG demonstrates anti-inflammatory activities in the MAPK pathway by inhibiting the phosphorylation of p38. Catechins also reduce the gene expression of c-Jun N-terminal kinase (JNK) protein and the transcription factor AP-1 [25, 26].

It should be noted that only a limited number of studies on humans provided strong evidence related to the anti-inflammatory activity of green tea. One example is a double-blind, placebo-controlled trial, in which 56 obese, hypertensive subjects received green tea extract or placebo for 3 months [22]. Green tea extract reduced diabetes and inflammation risk, increased total antioxidant status, and improved the lipid profile.
