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observed in rats or mice rendered diabetic by stroptozotocin or alloxane.231,234,235 Polyphenols may affect glycemia through different mecha- nisms, including the inhibition of glucose absorption in the gut or of its uptake by peripheral tissues. The hypoglycemic effects of diacetylated anthocyanins at a 10 mg/kg diet dosage were observed with maltose as a glucose source, but not with su- crose or glucose.230 This suggests that these effects are due to an inhibition of α-glucosidase in the gut mucosa. Inhibition of α-amylase and sucrase in rats by catechin at a dose of about 50 mg/kg diet or higher was also observed.233 The inhibition of intestinal glycosidases and glucose transporter by polyphenols has been studied in vitro. An IC50 of 60–200 μM was reported for a rat intestinal α-glucosidase.236 Quercetin (10–20 μM) inhib- ited glucose transport by GLUT2 in a transfected oocyte model and also inhibited glucose absorption in Zucker fa/fa rats.237 At much higher concentrations (1 mM), quercetin 3- O -glucoside, tannic acid, and chlorogenic acid inhibited the Na+-dependent hexose uptake in a rat everted gut sac model, or isolated rat brush border vesicules; catechin, ferulic acid, and caffeic acid were inactive.238,239 Polyphenols could not only inhibit the glucose absorption in the small intestine, but they could also limit their reabsorption in the kidney, as has been shown for phlorizin.240 Several in vitro studies on cultured cells have shown that polyphenols may increase glucose uptake by peripheral tissues. Caffeic acid increases glucose uptake by rat adipocytes and mice myoblasts.227,241 Black and green tea extracts and EGCG also increased glucose uptake by rat epididymal adipocytes, both in the presence or absence of insulin.242 Isoferulic acid increased glucose uptake by soleus muscle isolated from streptozotocin- diabetic rats.228 However, opposite results were also reported for quercetin and genistein, which both inhibited glucose uptake when induced by insulin on rat adipocytes or hydrogen perox- ide on leukemic cell lines.243,244 Genistein, but not daidzein, also inhibited glucose uptake by HL-60 cells and erythrocytes.245 Ac- tive polyphenol concentrations differed widely (1 to 115 μM) according to the studies. It is still difficult to explain these contra- dictory results. They could be explained by the different nature of the polyphenols tested or by the concentrations used in the assays. Polyphenols may exert different actions on peripheral tis- sues that would diminish glycemia. They include the inhibition of gluconeogenesis,228,246,247 adrenergic stimulation of glucose uptake,241 or the stimulation of insulin release by pancreatic β-cells.248 The involvement of such mechanisms is still hypo- thetical. p-Hydroxybenzoic acid, which shows hypoglycemic effects in diabetic rats when submitted to a glucose tolerance test, had no effect on insulinemia and hepatic glycogen.235 On the contrary, some polyphenols might have opposite effects and decrease glucose uptake in peripheral tissues by inhibiting the GLUT1 glucose transporter, as shown on transfected Chinese hamster ovary cells overexpressing this transporter,245 or by in- hibiting the response to insulin.243 In humans, evidence of the effects of polyphenols on glyce- mia or diabetes risk is still very limited. The consumption of 400 mL decaffeinated coffee did not affect glycemia or insuline- mia when ingested with glucose, but it decreased the secretion of glucose-dependent insulinotropic polypeptide and increased that of glucagon-like peptide 1 in a manner consistent with a delayed intestinal glucose absorption.249 The effects of the consumption of polyphenol supplements were also evaluated in diabetic pa- tients. No effect on glycemia was observed in type II diabetic patients after consumption for 2 mo of 50 mg/d of a red orange supplement containing anthocyanins, flavanones, and phenolic acids.250 In another clinical trial, type I diabetic patients in- gested larger doses of diosmin (1800 mg/day) and hesperidin (200 mg/day) as tablets for 3 mo. Although the doses adminis- tered per body weight unit were similar to many of the animal studies previously described, such a supplementation had no effect on glycemia, but it significantly reduced the level of gly- cated hemoglobin (HbA1c).251 Polyphenols could, thus, limit the risk of diabetic complications, as advanced glycation end (AGE) products are known to generate oxidative stress. This effect on AGE products could explain the reduction of renal damage by curcumin observed in streptozotocin-treated rats.252 Epidemiological evidence is also very limited. The consump- tion of coffee (rich in chlorogenic acid) has recently been asso- ciated with a decreased risk of type II diabetes.253 The con- sumption of decaffeinated coffee (20 g/d solids for 14 d, i.e., the equivalent of about 10 cups of coffee per d) lowered fasting glycemia in healthy volunteers.254 The consumption of 400 mL decaffeinated coffee containing 25 g glucose by healthy vol- unteers reduced the postprandial level of glucose-dependent insulinotropic-polypeptide (GIP) and enhanced that of glucagon-like peptide-1 (GLP-1), suggesting that chlorogenic acid decreases the rate of intestinal absorption of glucose.249 Chlorogenic acid could, therefore, counter the known hyper- glycemic effects of caffeine. POLYPHENOLS AND OSTEOPOROSIS Estrogen deficiency in postmenopausal women is an impor- tant cause of osteoporosis, and hormone replacement therapy is often recommended to prevent bone loss. However, many women have been reluctant to follow such a treatment because of the fear of possible side effects and long-term risks. Isoflavones with weak estrogen-like activity have attracted much attention as a possible alternative treatment to prevent osteoporosis.255 Their osteoprotective effects have been evaluated in mice or rats in which an estrogen deficiency has been induced by ovariec- tomy. The supplementation of the diet with genistein, daidzein, or their glycosides during several weeks prevents the loss of bone mineral density and trabecular volume caused by the ovariectomy.256−259 These effects were observed at daily doses of 10–50 mg/kg body weight. The highest doses also induced uterine hypertrophy, but the lowest protective doses did not affect the uterine weight.257,259 Feeding soy proteins with either normal or reduced isoflavone content to ovariectomized rats also sug- gested that the osteoprotective effects of soy proteins were due DIETARY POLYPHENOLS AND DISEASE PREVENTION 295PDF Image | Dietary Polyphenols and the Prevention of Diseases
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