Extraction Efficiencies of Green Tea Bioactive Metabolites and their Anti-Diabetic Activity

From ancient time green tea has been used as traditional medicine due to their various health beneficial effects antioxidant, anticancer, antimutagenic, antidiabetic, antimicrobial, neuroprotective and cardiovascular diseases preventing activities [1,2]. These potential health functionalities are mainly associated with green tea derived bioactive compounds such as, various polyphenols, flavonoids, amino acids, caffeine, flavanols like, catechin, epicatechin, epigallocatechin gallate, Gallocatechin gallate, gallocatechol, epicatechin gallate, epigallocatechin, catechin gallate [3] and phenolic acid like gallic acid, protocatechuic acid, vanillic acid, galloylquinic acid, coumaroyl quinin acid, ferulic acid, sialic acid, and galloylated glucose derivatives [2]. Recently, the health benefits in relation to diabetes from natural antioxidant compounds are in focus of scientific investigation due to their less undesirable adverse effects [4]. Diabetic Mellitus (DM), a complex and progressive metabolic disorder, may lead to various health complications and one of the leading cause deaths in the world [5]. Among two types of diabetes, type 2 diabetes mellitus (T2DM), serious chronic diseases accompanied by cellular mediated destruction of pancreatic β cell and attendant several health complications such as cardiovascular diseases and oxidative damages etc. [6]. Epidemiologically, it has been suggested that green tea bioactive metabolites were demonstrated to modify glucose metabolism and reduces the risk of diabetes [5,7-10].


Introduction
From ancient time green tea has been used as traditional medicine due to their various health beneficial effects antioxidant, anticancer, antimutagenic, antidiabetic, antimicrobial, neuroprotective and cardiovascular diseases preventing activities [1,2]. These potential health functionalities are mainly associated with green tea derived bioactive compounds such as, various polyphenols, flavonoids, amino acids, caffeine, flavanols like, catechin, epicatechin, epigallocatechin gallate, Gallocatechin gallate, gallocatechol, epicatechin gallate, epigallocatechin, catechin gallate [3] and phenolic acid like gallic acid, protocatechuic acid, vanillic acid, galloylquinic acid, coumaroyl quinin acid, ferulic acid, sialic acid, and galloylated glucose derivatives [2]. Recently, the health benefits in relation to diabetes from natural antioxidant compounds are in focus of scientific investigation due to their less undesirable adverse effects [4]. Diabetic Mellitus (DM), a complex and progressive metabolic disorder, may lead to various health complications and one of the leading cause deaths in the world [5]. Among two types of diabetes, type 2 diabetes mellitus (T2DM), serious chronic diseases accompanied by cellular mediated destruction of pancreatic β cell and attendant several health complications such as cardiovascular diseases and oxidative damages etc. [6]. Epidemiologically, it has been suggested that green tea bioactive metabolites were demonstrated to modify glucose metabolism and reduces the risk of diabetes [5,[7][8][9][10].
Extraction amounts of the bioactive polyphenols during the preparation of green tea extracts can facilitate the nutritional values and provides health benefits. Brewing conditions, including water temperature, solid to solvent ratio, time and extraction techniques, contribute to the amounts of bioactive compounds within the tea infusion [3].
Despite the considerable differences in the bioactive metabolites of green tea extracts obtained by different extraction techniques and conditions, studies comparing the anti-diabetic activities of different green tea extracts are limited. Therefore, this review will focus on extraction efficiencies of green tea metabolites and their anti-diabetic activities.

Extraction efficiencies of green tea bioactive metabolites
The suitable extraction conditions are influential to increase the extraction yield of green tea polyphenols that will elevate the health functionalities. Many extraction techniques have been used to extract phenolic compounds like hot water extraction, microwave-assisted extraction, supercritical fluid extraction, ultrasonication extraction etc. [3]. Green tea major catechin and phenolic compounds extraction yields are dependent on the extraction parameters, solvent, ratio of solvent to material, extraction temperature and time. In respect to extraction temperature and time, it is advisable to use either a combination of high temperature and short extraction time, or lower temperature and longer extraction time to increase the extraction efficiency and a higher ratio of solvent to material gave better yields [11]. In addition, the application of different exaction techniques could increase the extraction yields of bioactive compounds [3].
The effects of green tea extracts on hyperglycemic activities are described in Table 1. The anti-diabetic activity of 70% green tea extracts (GTE) and black tea extract was conducted in mice model [5]. Authors observed that GTE was more effective in hyperglycemic activity through lowering blood glucose levels and ameliorated glucose intolerance. The possible mechanism for the anti-diabetic activity of GTP was explored by insulin resistance (IR) and β-cell function (B%) using homeostatic model assessment (HOMA) to find out the relationship between hepatic glucose and insulin in the basal state that, reflects the balance between hepatic glucose output and insulin secretion. This result indicated that GTE was potential to suppress the IR whereas, in diabetic group was higher. In contrast, β-cell function was also higher with GTE treatment. This might be due to the presence of 71.5% catechins in green tea extracts. The effects of polyphenols present in aqueous green tea extract were conducted by in diabetic rat induced by alloxan [7]. The result found that green tea extract at a dose of 100mg/kg (b.wt.) was potential to reduce serum glucose level and the daily consumption (15%) at 50, 100mg/kg (b.wt.) showed 29 and 44% reduction. Green tea extracts also increased the liver glycogen, which was decreased due to alloxan treatment and showed antioxidant potentials through increased the superoxide dismutase and glutathione levels.
Hyperglycemic effects of green tea ethanol extract using glucose tolerance methods on rats were studied [8]. From the results, the consumption of 720mg/kg (b.wt.) of green tea ethanol extract could reduce the glucose content in the body at 120, 180 and 240 minutes after application. In contrast, the application of 180 and 360mg/kg (b.wt.) of green tea ethanol extract reduced body glucose content at 180 and 240minutes, respectively.
The effects of green tea water extract (65mg/kg) on serum paraoxonase/aryl esterase activities and lipoprotein ionizability in streptozotocin-induced diabetic rats was studied [11]. The result indicated that the green tea ethanol extract was capable to increase the serum total antioxidant ability. Finally, the author group concluded that green tea ethanol extract had antihyperlipidemic and antioxidative effects and slowed the progression of atherogenesis by reducing oxidation of lipoproteins and preserving paraoxonase activity.