Tea, obesity, and diabetes

Tea consumption may improve biomarkers of insulin sensitivity and risk factors for diabetes

Diabetes mellitus and its sequelae are a major and growing public health problem. The prevalence of diabetes worldwide is 194 million persons, or 5.1% of the population, and is projected to increase to 333 million, or 6.3% of the population, by 2025. Type 2 diabetes accounts for approximately 90-95% of those with diabetes in the United States and other developed countries. Tea is the most widely consumed beverage in the world, second only to water. Tea contains polyphenols and other components that may reduce the risk of developing chronic diseases such as cardiovascular disease and cancer. Some evidence also shows that tea may affect glucose metabolism and insulin signaling, which, as a result, has spurred interest in the health effects of tea consumption on diabetes. Epidemiologic studies suggest some relation between tea consumption and a reduced risk of type 2 diabetes, although the mechanisms for these observations are uncertain. Findings from in vitro and animal models suggest that tea and its components may influence glucose metabolism and diabetes through several mechanisms, such as enhancing insulin sensitivity. Some human clinical studies evaluating tea and its components show improvement in glucoregulatory control and endothelial function. However, further controlled clinical trials are required to gain a better understanding of the long-term effects of tea consumption in persons with diabetes.

Catechin gallates are NADP+-competitive inhibitors of glucose-6-phosphate dehydrogenase and other enzymes that employ NADP+ as a coenzyme

Recent studies have shown that glucose-6-phosphate dehydrogenase (G6PD) is an effectual therapeutic target for metabolic disorders, including obesity and diabetes. In this study, we used in silico and conventional screening approaches to identify putative inhibitors of G6PD and found that gallated catechins (EGCG, GCG, ECG, CG), but not ungallated catechins (ECG, GC, EC, C), were NADP(+)-competitive inhibitors of G6PD and other enzymes that employ NADP(+) as a coenzyme, such as IDH and 6PGD.

Green tea extract protects leptin-deficient, spontaneously obese mice from hepatic steatosis and injury

The incidence of nonalcoholic fatty liver disease (NAFLD) has risen along with the ongoing obesity epidemic. Green tea extract (GTE) inhibits intestinal lipid absorption and may regulate hepatic lipid accumulation. The objective of this study was to determine whether GTE protects against hepatic lipid accumulation during the development of NAFLD in an obese mouse model. Five-wk-old ob/ob (obese) mice and their lean littermates (8 mice x genotype(-1) x dietary treatment(-1)) were fed GTE at 0, 1, or 2% (wt:wt) for 6 wk. The body weights of obese mice and lean littermates fed diets containing GTE were 23-25% and 11-20% lower (P < 0.05) than their respective controls fed no GTE. Histologic evaluation showed a significant reduction in hepatic steatosis in GTE-fed obese mice only and histologic scores were correlated with hepatic lipid concentration (r = 0.84; P < 0.05), which was reduced dose dependently by GTE. GTE protected against hepatic injury as suggested by 30-41% and 22-33% lower serum alanine aminotransferase and aspartate aminotransferase activities, respectively. Hepatic alpha-tocopherol was 36% higher in obese mice than lean mice. GTE tended (P = 0.06) to lower hepatic alpha-tocopherol, which was not fully explained by the GTE-mediated reduction in hepatic lipid. Hepatic ascorbic acid was lower in obese mice than in lean mice (P < 0.05) and was unaltered by GTE. Obese mice had lower serum adiponectin than lean mice and this was not affected by GTE. The results suggest that GTE protects against NAFLD by limiting hepatic lipid accumulation and injury without affecting hepatic antioxidant status and adiponectin-mediated lipid metabolism. Further study is underway to define the events by which GTE protects against obesity-triggered NAFLD.

The effects of epigallocatechin-3-gallate on thermogenesis and fat oxidation in obese men: a pilot study

OBJECTIVES

The development of obesity is characterized by an increase in adipose tissue mass and by concomitant and profound changes in almost all organ functions leading to diseases such as hypertension, diabetes mellitus and coronary heart disease. Recent data from human studies indicate that the consumption of green tea and green tea extracts may help reduce body weight, mainly body fat, by increasing postprandial thermogenesis and fat oxidation. However, human studies investigating the metabolic effects of the most predominant tea catechin, EGCG, alone are absent.

METHODS

In a randomized double blind, placebo-controlled, cross-over pilot study, six overweight men were given 300 mg EGCG/d for 2d. Fasting and postprandial changes in energy expenditure (EE) and substrate oxidation were assessed.

RESULTS

Resting EE did not differ significantly between EGCG and placebo treatments, although during the first postprandial monitoring phase, respiratory quotient (RQ) values were significantly lower with EGCG compared to the placebo.

CONCLUSIONS

These findings suggest that EGCG alone has the potential to increase fat oxidation in men and may thereby contribute to the anti-obesity effects of green tea. However, more studies with a greater sample size and a broader range of age and BMI are needed to define the optimum dose.

Effects of green tea and EGCG on cardiovascular and metabolic health

Since ancient times green tea has been considered a health-promoting beverage. In recent years, scientists throughout the world have investigated the potential benefits of green tea and its most abundant catechin, epigallocatechin gallate (EGCG). The anti-cancer effects of green tea and EGCG were the focus of early research, and encouraging data from in vitro, animal model, and human studies have emerged. Due to the dominant role of cardiovascular disease and the dramatic rise of obesity and type 2 diabetes mellitus as major and interlinked healthcare problems, green tea and EGCG are increasingly being investigated in these areas. Dose-response relationships observed in several epidemiological studies have indicated that pronounced cardiovascular and metabolic health benefits can be obtained by regular consumption of 5-6 or more cups of green tea per day. Furthermore, intervention studies using similar amounts of green tea, containing 200-300 mg of EGCG, have demonstrated its usefulness for maintaining cardiovascular and metabolic health. Additionally, there are numerous in vivo studies demonstrating that green tea and EGCG exert cardiovascular and metabolic benefits in these model systems. Therefore, green tea and EGCG can be regarded as food components useful for the maintenance of cardiovascular and metabolic health. To prove the effectiveness for disease prevention or treatment, several multi-center, long-term clinical studies investigating the effects of one precisely-defined green tea product on cardiovascular and metabolic endpoints would be necessary. The aim of this manuscript is to provide an overview of the research investigating the effects of green tea and green tea catechins on cardiovascular and metabolic health.

Protective effect of epigallocatechin-3-gallate on ischemia/reperfusion-induced injuries in the heart: STAT1 silencing flavonoid

The beneficial effect of naturally occurring flavonoids in health is believed to be due to their strong antioxidant activity. However, recent laboratory evidence indicates the involvement of a more specific action. Here, we present evidence that, among a number of catechins present in green tea extract, only epigallocatechin-3-gallate (EGCG) exerts a strong inhibitory action on interferon-gamma-elicited activation of signal transducer and activator of transcription 1 (STAT1). Protective action of EGCG in ischemia/reperfusion injury in the heart and the molecular mechanism of action, which has nothing to do with its anti-oxidant capacity are described.

Tea polyphenols for health promotion

People have been consuming brewed tea from the leaves of the Camellia sinensis plant for almost 50 centuries. Although health benefits have been attributed to tea, especially green tea consumption since the beginning of its history, scientific investigations of this beverage and its constituents have been underway for less than three decades. Currently, tea, in the form of green or black tea, next to water, is the most widely consumed beverage in the world. In vitro and animal studies provide strong evidence that polyphenols derived from tea may possess the bioactivity to affect the pathogenesis of several chronic diseases. Among all tea polyphenols, epigallocatechin-3-gallate has been shown to be responsible for much of the health promoting ability of green tea. Tea and tea preparations have been shown to inhibit tumorigenesis in a variety of animal models of carcinogenesis. However, with increasing interest in the health promoting properties of tea and a significant rise in scientific investigation, this review covers recent findings on the medicinal properties and health benefits of tea with special reference to cancer and cardiovascular diseases.

Epigallocatechin Gallate, a Green Tea Polyphenol, Mediates NO-dependent Vasodilation Using Signaling Pathways in Vascular Endothelium Requiring Reactive Oxygen Species and Fyn

Green tea consumption is associated with reduced cardiovascular mortality in some epidemiological studies. Epigallocatechin gallate (EGCG), a bioactive polyphenol in green tea, mimics metabolic actions of insulin to inhibit gluconeogenesis in hepatocytes. Because signaling pathways regulating metabolic and vasodilator actions of insulin are shared in common, we hypothesized that EGCG may also have vasodilator actions to stimulate production of nitric oxide (NO) from endothelial cells. Acute intra-arterial administration of EGCG to mesenteric vascular beds isolated ex vivo from WKY rats caused dose-dependent vasorelaxation. This was inhibitable by L-NAME (NO synthase inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), or PP2 (Src family kinase inhibitor). Treatment of bovine aortic endothelial cells (BAEC) with EGCG (50 microm) acutely stimulated production of NO (assessed with NO-specific fluorescent dye DAF-2) that was inhibitable by l-NAME, wortmannin, or PP2. Stimulation of BAEC with EGCG also resulted in dose- and time-dependent phosphorylation of eNOS that was inhibitable by wortmannin or PP2 (but not by MEK inhibitor PD98059). Specific knockdown of Fyn (but not Src) with small interfering RNA inhibited both EGCG-stimulated phosphorylation of Akt and eNOS as well as production of NO in BAEC. Treatment of BAEC with EGCG generated intracellular H(2)O(2) (assessed with H(2)O(2)-specific fluorescent dye CM-H(2)DCF-DA), whereas treatment with N-acetylcysteine inhibited EGCG-stimulated phosphorylation of Fyn, Akt, and eNOS. We conclude that EGCG has endothelial-dependent vasodilator actions mediated by intracellular signaling pathways requiring reactive oxygen species and Fyn that lead to activation of phosphatidylinositol 3-kinase, Akt, and eNOS. This mechanism may explain, in part, beneficial vascular and metabolic health effects of green tea consumption.

Proposed mechanisms of (−)-epigallocatechin-3-gallate for anti-obesity

Green tea catechins (GTCs) are polyphenolic flavonoids formerly called vitamin P. GTCs, especially (-)-epigallocatechin-3-gallate (EGCG), lower the incidence of cancers, collagen-induced arthritis, oxidative stress-induced neurodegenerative diseases, and streptozotocin-induced diabetes. Also, inhibition of adipogenesis by green tea and green tea extract has been demonstrated in cell lines, animal models, and humans. The obesity-preventive effects of green tea and its main constituent EGCG are widely supported by results from epidemiological, cell culture, animal, and clinical studies in the last decade. Studies with adipocyte cell lines and animal models have demonstrated that EGCG inhibits extracellular signal-related kinases (ERK), activates AMP-activated protein kinase (AMPK), modulates adipocyte marker proteins, and down-regulates lipogenic enzymes as well as other potential targets. Also, the catechin components of green tea have been shown to possess anti-carcinogenic properties possibly related to their anti-oxidant activity. In addition, it was shown that dietary supplementation with EGCG could potentially contribute to nutritional strategies for the prevention and treatment of type 2 diabetes mellitus. In this review, the biological activities and multiple mechanisms of EGCG in cell lines, animal models, and clinical observations are explained.

Silencing of the human microsomal glucose-6-phosphate translocase induces glioma cell death: Potential new anticancer target for curcumin

G6P translocase (G6PT) is thought to play a crucial role in transducing intracellular signaling events in brain tumor-derived cancer cells. In this report, we investigated the contribution of G6PT to the control of U-87 brain tumor-derived glioma cell survival using small interfering RNA (siRNA)-mediated suppression of G6PT. Three siRNA constructs were generated and found to suppress up to 91% G6PT gene expression. Flow cytometry analysis of propidium iodide/annexin-V-stained cells indicated that silencing the G6PT gene induced necrosis and late apoptosis. The anticancer agent curcumin, also inhibited G6PT gene expression by more than 90% and triggered U-87 glioma cells death. Overexpression of recombinant G6PT rescued the cells from curcumin-induced cell death. Targeting G6PT expression may provide a new mechanistic rationale for the action of chemopreventive drugs and lead to the development of new anti-cancer strategies.