Polyphenols, including flavonoids, phenolic acids, proanthocyanidins and resveratrol, are a large and heterogeneous group of phytochemicals in plant-based foods, such as tea, coffee, wine, cocoa, cereal grains, soy, fruits and berries. Growing evidence indicates that various dietary polyphenols may influence carbohydrate metabolism at many levels. In animal models and a limited number of human studies carried out so far, polyphenols and foods or beverages rich in polyphenols have attenuated postprandial glycemic responses and fasting hyperglycemia, and improved acute insulin secretion and insulin sensitivity. The possible mechanisms include inhibition of carbohydrate digestion and glucose absorption in the intestine, stimulation of insulin secretion from the pancreatic β–cells, modulation of glucose release from the liver, activation of insulin receptors and glucose uptake in the insulin-sensitive tissues, and modulation of intracellular signalling pathways and gene expression. The positive effects of polyphenols on glucose homeostasis observed in a large number of in vitro and animal models are supported by epidemiological evidence on polyphenol-rich diets. To confirm the implications of polyphenol consumption for prevention of insulin resistance, metabolic syndrome and eventually type 2 diabetes, human trials with well-defined diets, controlled study designs and clinically relevant end-points together with holistic approaches e.g., systems biology profiling technologies are needed.

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Impact of dietary polyphenols on carbohydrate metabolism.

Polyphenolic phytochemicals are ubiquitous in plants, in which they function in various protective roles A 'recommended' human diet contains significant quantities of polyphenolics, as they have long been assumed to be 'antioxidants' that scavenge excessive, damaging, free radicals arising from normal metabolic processes There is recent evidence that polyphenolics also have 'indirect' antioxidant effects through induction of endogenous protective enzymes There is also increasing evidence for many potential benefits through polyphenolic-mediated regulation of cellular processes such as inflammation Inductive or signalling effects may occur at concentrations much lower than required for effective radical scavenging Over the last 2-3 years, there have been many exciting new developments in the elucidation of the in vivo mechanisms of the health benefits of polyphenolics We summarise the current knowledge of the intake, bio-availability and metabolism of polyphenolics, their antioxidant effects, regulatory effects on signalling pathways, neuro-protective effects and regulatory effects on energy metabolism and gut health

Polyphenolic phytochemicals--just antioxidants or much more?

α-Glucosidase and α-amylase inhibitory activities of Nepalese medicinal herb Pakhanbhed (Bergenia ciliata, Haw.)

Three flavonoids from tartary buckwheat bran, namely, quercetin (Que), isoquercetin (Iso) and rutin (Rut), have been evaluated as alpha-glucosidase inhibitors by fluorescence spectroscopy and enzymatic kinetics and have also been compared with the market diabetes healer, acarbose The results indicated that Que, Iso and Rut could bind alpha-glucosidase to form a new complex, which exhibited a strong static fluorescence quenching via nonradiation energy transfer, and an obvious blue shift of maximum fluorescence The sequence of binding constants (K(A)) was Que > Iso > Rut, and the number of binding sites was one for all of the three cases The thermodynamic parameters were obtained by calculations based on data of binding constants They revealed that the main driving force of the above-mentioned interaction was hydrophobic Enzymatic kinetics measurements showed that all of the three compounds were effective inhibitors against alpha-glucosidase Inhibitory modes all belonged to a mixed type of noncompetitive and anticompetitive The sequence of affinity (1/K(i)) was in accordance with the results of binding constants (K(A)) The concentrations which gave 50% inhibition (IC(50)) were 0017 mmol*L(-1), 0185 mmol*L(-1) and 0196 mmol*L(-1), compared with acarbose's IC(50) (0091 mmol*L(-1)); the dose of acarbose was almost five times of that of Que and half of that of Iso and Rut Our results explained why the inhibition on alpha-glucosidase of tartary buckwheat bran extractive substance (mainly Rut) was much weaker than that of its hydrolysis product (a mixture of Que, Iso and Rut) This work would be significant for the development of more powerful antidiabetes drugs and efficacious utilization of tartary buckwheat, which has been proved as an acknowledged food in the diet of diabetic patients

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Comparative Evaluation of Quercetin, Isoquercetin and Rutin as Inhibitors of α-Glucosidase

http://www.teavigoinfo.com/pdf/study-26.pdf

The potential role of green tea catechins in the prevention of the metabolic syndrome – A review

To clarify the postprandial glucose suppression effect of flavonoids, the inhibitory effects of catechins and theaflavins against alpha-glucosidase (AGH) were examined in this study. It was initially demonstrated that theaflavins and catechins preferentially inhibited maltase rather than sucrase in an immobilized AGH inhibitory assay system. For the maltase inhibitory effects of theaflavins, the effects were observed in descending order of potency of theaflavin (TF)-3-O-gallate (Gal) > TF-3,3'-di-O-Gal > TF-3'-O-Gal > TF. This suggests that the AGH inhibition induced by theaflavins is closely associated with the presence of a free hydroxyl group at the 3'-position of TF as well as the esterification of TF with a mono-Gal group. In addition, the R-configuration at the 3'-position of TF-3-O-Gal showed a higher inhibitory activity than the S-configuration. As a result of a single oral administration of maltose (2 g/kg) in rats, a significant reduction in blood glucose level was observed at a dose of 10 mg/kg of TF-3-O-Gal, demonstrating for the first time that TF-3-O-Gal can suppress glucose production from maltose through inhibition of AGH in the gut.

α-Glucosidase Inhibitory Profile of Catechins and Theaflavins

BACKGROUND: A new fermented tea produced by tea-rolling processing of loquat (Eriobotrya japonica) leaf with green tea leaf (denoted as LG tea) showed a potent antihyperglycaemic effect in maltose-loaded rats. The aim of this study, therefore, was to identify α-glucosidase inhibitors in the antihyperglycaemic tea product.



RESULTS: LG tea had a threefold higher maltase-inhibitory activity (IC50 0.065 mg dried extract mL−1) than either the constituent loquat leaf or green tea alone. In addition, LG tea favourably inhibited maltase action rather than sucrase action. As a result of bio-guided high-performance liquid chromatography separations of LG tea, theasinensin A, theasinensin B, strictinin and 1,6-digalloylglucose were newly identified as maltase inhibitors with IC50 values of 142, 225, 398 and 337 µmol L−1 respectively, along with previously identified catechins and theaflavins.



CONCLUSION: Judging from the magnitude of the α-glucosidase-inhibitory contribution of each isolated compound to the overall inhibition of LG tea, catechins were the main candidates responsible for α-glucosidase or maltase inhibition in LG tea, followed by theaflavins, theasinensins, strictinin and 1,6-digalloylglucose. Copyright © 2010 Society of Chemical Industry

Identification of α-glucosidase inhibitors from a new fermented tea obtained by tea-rolling processing of loquat (Eriobotrya japonica) and green tea leaves.

BACKGROUND: In the field of food science, much interest has been focused on the development of alternative medicinal foods with the ability to regulate excess blood glucose level (BGL) rise. The authors have successfully developed a new fermented tea product (LG tea) by co-fermentation of loquat (Eriobotrya japonica) leaf and summer-harvested green tea leaf. The objective of this study was to examine the acute suppression effect of LG tea on BGL rise in disaccharide-loaded Sprague-Dawley (SD) rats and to evaluate its possible usage as an antidiabetic functional food material.



RESULTS: As a result of single oral administration of hot water extract of LG tea (50 mg kg−1) to maltose-loaded SD rats, BGL at 30 min was significantly decreased by 23.8% (P < 0.01) compared with the control. A corresponding reduction in serum insulin secretion was also observed. The ED50 value of LG tea (50.7 mg kg−1) was estimated to be about 16-fold higher than that of the therapeutic drug acarbose (3.1 mg kg−1).



CONCLUSION: No significant change in BGL was observed when sucrose or glucose was administered, suggesting that the suppression effect of LG tea was achieved by maltase inhibition, not by sucrase inhibition or glucose transport inhibition at the intestinal membrane. Copyright © 2010 Society of Chemical Industry

Suppression of blood glucose level by a new fermented tea obtained by tea‐rolling processing of loquat (Eriobotrya japonica) and green tea leaves in disaccharide‐loaded Sprague‐Dawley rats

Asami Toshima

Papers

α-Glucosidase Inhibitory Profile of Catechins and Theaflavins

Identification of α-glucosidase inhibitors from a new fermented tea obtained by tea-rolling processing of loquat (Eriobotrya japonica) and green tea leaves.

Suppression of blood glucose level by a new fermented tea obtained by tea‐rolling processing of loquat (Eriobotrya japonica) and green tea leaves in disaccharide‐loaded Sprague‐Dawley rats