3. Prevention of insulin resistance and diabetes by green tea polyphenols
Insulin resistance is an early marker of type 2 diabetes (T2D) and development of insulin resistance is associated with obesity . It has been suggested that a major contributor to the development of insulin resistance is an overabundance of free fatty acids in the plasma . Insulin resistance results in the interruption of insulin signaling in responsive tissues, which leads to hyperinsulinemia and ultimately T2D. Over the long term, the body is unable to produce enough insulin to overcome insulin resistance and the pancreas may reduce or stop insulin production . In addition insulin resistance can result in decreased levels of lipoprotein lipase in peripheral tissues (i.e. adipose tissue and muscle). By contrast type I diabetes (T1D) begins early in life and occurs as a result of autoimmune destruction of pancreatic β-cells resulting in insulin deficiency . It is predicted that the number of people with both diagnosed and undiagnosed diabetes will increase from 23.7 million in 2009 to 44.1 million in 2034 . A number of studies have been conducted to examine the effects of green tea in animal models of both T1D and T2D (Table 2.).
Islam and Choi  showed that green tea improved glucose tolerance in streptozotocin (STZ)-treated rats. One week after STZ injection, diabetic rats (non-fasting blood glucose ≥ 300 mg/dL) were given low (0.5%) and high (2.0%) doses of green tea extract as the sole source of drinking fluid for 4 weeks. Green tea treatment significantly decreased blood glucose following glucose challenge in diabetic rats treated with low dose of green tea extract. Interestingly, the high dose green tea extract group actually had elevated blood glucose. The reason for this lack of dose-response was not discussed, but could indicate some level of toxicity at this higher dose.
Roghani et. al., examined the beneficial effects of EGCG on chemically induced T1D . Male Wistar albino rats were injected with STZ (60 mg/kg BW), after one week the rats with serum glucose higher than 250 mg/dL were treated with EGCG (25 mg/kg, i.g. daily) for 8 weeks . EGCG treatment decreased serum glucose levels compared to diabetic controls and baseline values. In addition, the EGCG treatment caused a 30.9% decrease in malondialdehyde levels and a 21.3% increase in superoxide dismutase (SOD) in aortic rings compared to the diabetic control demonstrating the potential antioxidant effects of EGCG in this model.
Another study has reported that EC can also reduce the toxicity of STZ in rat pancreatic islets . Rats were randomly divided into four groups: control, EC (30 mg/kg)-treated, STZ (60 mg/kg)-treated, and EC (30 mg/kg) plus STZ (60 mg/kg)-treated. Rats given EC twice a day for 6 days followed by a single injection of STZ showed no hyperglycemic effects. In vitro EC (0.8mM) treatment protected islet cells rom STZ and potentiated compared to STZ (5mM)-treated islet cells. Although these in vitro results are interesting, the dose of EC used in the study is much higher than what is physiologically-achievable by tea consumption.
In addition to its effects on hyperglycemia, green tea has also been examined for its effects on diabetes-related co-pathologies including cataracts. Due to hyperglycemia and increased oxidative stress induced by diabetes, nonenzymatic glycation of proteins can occur and subsequent advanced glycation end-product can form leading to structural and functional changes of proteins and sorbitol accumulation. Male Sprague-Dawley rats injected with STZ and treated with green tea extract (1.25%) for 3 months had reduced formation of diabetic cataracts and significantly decreased glucose, glycated lysine and sorbitol levels in the lens . Levels of oxidative stress markers were also reduced.
Although the effects of green tea on T1D are interesting, the recent increases in the incidence of obesity make understanding the effects of green tea against T2D very important. Green tea and tea polyphenols have been studied using both diet-induced and genetic models. Ramadan et al.  examined the effect of green tea aqueous extract on male Wistar albino rats fed a cholesterol-rich diet. Treatment with 50 mg/kg - 100 mg/kg BW for 4 weeks lowered body weight (28% - 49%), serum glucose (20% - 31%), total lipid (26% - 31%), triacylglycerides (22% - 40%), phospholipid levels (32% - 38%), ALT (49% - 56%), aspartate aminotransferase (58% - 62%) and alkaline phosphatase activities (16% - 22%).
Wolfram et al. examined the effects of EGCG in two rodent genetic models of T2D . Zucker Diabetic Fatty (ZDF) rats are an inbred rat model with ineffective leptin receptors and prone to T2D, glucose intolerance and obesity. Similarly, BKS.Cg-m +/+ Leprdb/J (db/db) mice have a leptin receptor mutation, are obese and have elevated insulin and blood glucose levels. Db/db mice treated with0.25– 1% dietary EGCG for 7 weeks, had dose-dependently improved oral glucose tolerance and decreased fasting glucose. EGCG-treated ZDF rats showed similar decreases in fasting blood glucose. In addition, mechanistic experiments showed that EGCG dose dependently increased glucokinase expression and decreased the expression of phosphoenolpyruvate carboxykinase (PEPCK) in the liver of db/db mice. The former enzyme enhances glycolysis and glucose uptake in the liver, whereas the latter enzyme functions in gluconeogenesis. EGCG treatment also increased expression of acyl-CoA oxidase-1 (ACO-1) and carnitine palmitoyl transferase-1 (CPT-1) in liver and adipose tissue of the db/db mice. Both enzymes are involved in fatty acid catabolism. These alterations in glucose and lipid metabolism may explain the observed improvement in glucose homeostasis.
Serisier et al.  showed that green tea can affect insulin sensitivity in dogs. Obese and insulin-resistant beagle dogs were treated with oral green tea extract (80 mg/kg BW per day) just before the daily meal for 12 weeks. Insulin sensitivity index was markedly increased by green tea supplementation (60% increase). Moreover, the homeostasis model for insulin resistance (HOMA-IR) was decreased by 20% after green tea treatment. Gene expression analysis in visceral and subcutaneous adipose tissue showed that expression of PPAR γ, LPL, adiponectin, and GLUT4 mRNA were dramatically elevated after 12 weeks of green tea supplementation (3-fold, 10-fold, 6-fold and 3-fold, respectively). In skeletal muscle, green tea supplementation also markedly increased the expression of PPAR α and LPL mRNA (2-fold and 3-fold) but did not increase GLUT4 mRNA. Green tea did increase however GLUT4 translocation to the plasma membrane in muscle cells.
Studies in non-diabetic animals have also supported the potential beneficial effects of green tea against diabetes. Wu et al.  studied the effect of green tea supplementation on insulin sensitivity in Sprague-Dawley rats given 0.5% green tea as the sole source of drinking fluid. At 4, 6, and 12 weeks, the oral glucose tolerance test (OGTT) was conducted in fasting rats. Green tea supplementation was found to decrease plasma glucose levels and plasma insulin levels at week 4 and 6 but not only plasma insulin at week 12. The lack of effect at week 12 on blood glucose is interesting and deserved furthere study. In addition to improved glucose and insulin homeostasis, the green tea group also had lower fasting plasma triacylglyceride and free fatty acids than the control rats.
Many studies have focused on regulation of the expression of genes involved in glucose uptake and insulin signaling to help explain the mechanisms of antidiabetic activities of green tea polyphenol. Wistar rats given high-fructose diet and 0.1 – 0.2% dietary green tea for 6 weeks had increased GLUT4 mRNA both in the liver and in muscle . An increase of insulin receptor substrate (IRS)2 mRNA levels in the liver and IRS1 in the muscle of rats treated green tea extract was also observed. Previous studies found that the lack of functional insulin receptor substrate (IRS) is the key molecular lesion in hepatic insulin . Qin et al.  examined the effects of green tea polyphenols in the same model and found that green tea polyphenols (200 mg/kg BW per day) significantly decreased blood glucose and plasma insulin, triglyceride, total cholesterol, LDL-cholesterol and free fatty acids. They also found a significant reduction in blood glucose, plasma insulin, plasma retinol-binding protein 4, and plasma soluble CD36 (sCD36), which is reported to be a novel marker of insulin resistance and inflammation. Green tea polyphenols also increased the cardiac mRNA levels of IRS1 and IRS2. Additionally cardiac mRNA expression of GLUT1, GLUT4 and glycogen synthase 1 were increased by green tea treatment, whereas glycogen synthase kinase 3β was decreased. Green tea polyphenols also decreased inflammatory factors including TNFα, IL-1β, and IL-6 and increased zinc-finger protein 36 (an anti-inflammatory marker).