**7. Flavonoids**

Flavonoids (bioflavonoids) are a diverse group of polyphenols (phenyl benzopyrans) which function as phytochemicals [180]. Flavonoids are well-known for their multi-directional biological activities including anti-diabetic efficacy. Experimental evidence has shown that flavonoids exhibits anti-inflammatory [181], anticarcinogenic [182], antiviral [183] and antiallergic properties. These effects are generally associated with free radical scavenging activity of flavonoids. The antioxidant effects of flavonoids are enhanced by the number and position of hydroxyl groups in the molecule. The catechol structure, presence of unsaturation and 4-oxo function in the C-ring also contributes to their radical scavenging activity [184, 185]. Flavonoids may be capable of binding the transition metal ions, which play a role in glycoxidation, thus preventing metal-catalysed formation of hydroxyl radicals or related species from H2O2 [186]. and 4-oxo function in the C-ring also contributes to their radical scavenging activity [184, 185]. Flavonoids may be capable of binding the transition metal ions, which play a role in glycoxidation, thus preventing metal-catalysed formation of hydroxyl radicals or related species from H2O2 [186].

flavonoids exhibits anti-inflammatory [181], anticarcinogenic [182], antiviral [183] and antiallergic properties. These effects are generally associated with free radical scavenging

**Figure 1.** Classes of flavonoids [187]

rats for 8 weeks had cardioprotective effects which was simultaneously associated with an ability of vitamin E to blunt diabetes-induced amplification of myocardial 8-*iso* PGF2 and oxidized GSSG formation [162]. Clinical trials with vitamin E provided evidence that vitamin E may improve cardiovascular function [163, 164]. However, most large studies with vitamin E have not yielded positive benefits for decreasing the development or progression of diabetic

Vitamin C is an antioxidant vitamin which plays an important role in protecting free radicalinduced damage and a decrease in basal vitamin C levels has been documented in type 2 DM. Treatment of diabetic rats with vitamin C significantly decreased renal malondialdehyde, albuminuria, proteinuria, glomerular and tubulointerstitial sclerosis, suggesting the role of vitamin C in suppressing the progression of renal injury in diabetic rats [167]. Vitamin C also improved diabetes-induced endothelial dysfunction in a rat model by enhancing NO bioa‐

The beneficial effects of vitamin C supplementation in humans are controversial. A study reported that vitamin C may improve glycemic control, lowering both fasting blood glucose and glycated haemoglobin (HbA1c) [169]. Chronic oral administration of vitamin C to patients with type 2 diabetes causes a decline in plasma free radicals that is associated with improved whole body glucose disposal [170,171] and improved endothelial function [172]. Recently, another study reported a reduction in the malondialdehyde (MDA) level, a major product of oxidative damage in both fasting and postprandial states of type 2 diabetic patients after vitamin C (1000 mg day-1) supplementation for 6 weeks although no effect was observed on lipid profiles [173]. Some studies have indicated that the intra-arterial infusion of vitamin c restores endothelium-dependent vasodilation in patients with type 1 or type 2 diabetes [174, 175] suggesting that hyperglycemia-induced oxidative stress mediates endothelial dysfunc‐

However, in contrast to these promising results, other studies showed no beneficial effect with vitamin C treatment. Chen and colleagues [176] concluded that a high oral dose of vitamin C therapy was ineffective at improving endothelial dysfunction and insulin resistance in type 2 DM. It is important to note that complete replenishment of vitamin C levels was not achieved in the subjects. This is crucial since high concentrations of vitamin C (>80 μM) has been documented as a requirement for the preservation of NO-dependent endothelial function as vitamin C only competes with NO for superoxide anion at these high concentrations [177-178]. Also, in another study, no beneficial effects of oral vitamin C supplementation (1.5 g daily for 3 weeks) was observed on blood pressure, oxidative stress, and endothelial function in type 2

Flavonoids (bioflavonoids) are a diverse group of polyphenols (phenyl benzopyrans) which function as phytochemicals [180]. Flavonoids are well-known for their multi-directional biological activities including anti-diabetic efficacy. Experimental evidence has shown that

microvascular and cardiovascular pathologies or mortality [165, 166].

vailability [168].

34 Antioxidant-Antidiabetic Agents and Human Health

tion in diabetic patients.

diabetes [179].

**7. Flavonoids**

**Figure 1**: Classes of flavonoids [187] The potential beneficial effects of flavonoids in the prevention of diabetes mellitus and its associated complications have been investigated both *in vitro* and *in vivo* studies (Table 3). The inhibitory effect of flavonoids on glycation has been demonstrated and it is suggested that this effect is partly due to their antioxidant properties [188]. Epigallocatechin (EGC) has a beneficial effect in a rat model of diabetic nephropathy via suppressing hyperglycemia, The potential beneficial effects of flavonoids in the prevention of diabetes mellitus and its associated complications have been investigated both *in vitro* and *in vivo* studies (Table 3). The inhibitory effect of flavonoids on glycation has been demonstrated and it is suggested that this effect is partly due to their antioxidant properties [188]. Epigallocatechin (EGC) has a beneficial effect in a rat model of diabetic nephropathy via suppressing hyperglycemia, proteinuria and lipid peroxidation. EGC also reduced renal accumulation of AGE's and their related oxidative stress [189]. Another study demonstrated the *in vitro* inhibitory effect of different flavonoids on pentosidine formation in collagen in the presence of glucose (250 mmol/L). The decreasing inhibitiory activity was observed from myricetin, quercetin, rutin, catechin and kaempferol in a structure and concentration dependent manner [190]. Kim and colleagues [191] also inves‐ tigated the effect of quercetin, isoquercitrin, hyperin and cacticin on formation of AGE's *in vitro*. At a concentration of 50 mΜ, the percentages of inhibition were 1.0, 89.6, 92.0 and 40.5, respectively. The inhibitory effect of hyperin on AGE formation was 6.5 times higher than that of aminoguanidine, a known AGE inhibitor which showed 14.1% inhibition at a concentration of 50 μM.

(ECM) component and glomerular basement membrane thickening in the renal cortex of diabetic rats suggesting its renoprotective effect in experimental diabetic nephropathy [204]. The inhibitory effect of rutin on AGE formation in STZ-induced rats has also been shown [205].

Oxidative Stress and Diabetic Complications: The Role of Antioxidant Vitamins and Flavonoids

Diosmin (DS) (diosmetin 7-*O*-rutinoside) is a natural flavone glycoside which can be obtained by dehydrogenation of the corresponding flavanone glycoside, hesperidin that is abundant in the pericarp of various citrus fruits [206]. Diosmin treatment of streptozotocin-nicotinamide induced diabetic rats, ameliorated oxidative stress in plasma and tissues as evidenced by improved glycemic and antioxidant status along with decreased lipid peroxidation [207]. Experimental evidence showed the potential of rutin, a flavonol to delay glomerulosclerosis of diabetic nephropathy (DN) due to its ability to inhibit cell hypertrophy and the accumulation of ECM mediated by TGF-β1/Smads and ROS signals in mesangial cells cultured by high glucose [208] Quercetin enhances endothelium-derived NO bioavailability, reduced blood glucose levels and oxidative stress in diabetic rats suggesting its beneficial effect in vascular

**Mechanism of action Reference**

[210]

http://dx.doi.org/10.5772/57282

37

[211]

[204]

[212]

[213]

Decreased MDA levels ,

concentration.

TGF-β.

Increased activity of SOD and GSH

Decreased ROS production, DNA damage and apoptosis. Modulation of antioxidant enzymes; GSH, CAT, SOD and GPX.

Lowered blood glucose and improved renal function. Increased total antioxidant capability activities of SOD, CAT and GPX. Lowered ECM accumulation and AGE formation. Decreased renal expression of

Lowered MDA and NO level, increased antioxidant enzyme activity and preservation of islet cells integrity.

Lowered blood glucose and improved renal function. Reduced renal lipid peroxides and increased activity of anti-oxidative enzymes;

SOD and CAT and non-enzymatic

antioxidant GSH.

function [209].

**Selected examples**

Morin

Quercetin

**Target organ, tissue or cells**

Liver

Rutin Kidney, serum, urine

Hepatocytes

Pancreas, serum, erythrocyte.

Kidney , serum,

urine

**Classes of Flavonoid and Food sources**

Flavonols (Brussel sprouts, apples, onion, curly kale, leek, beans, cherries, Citrus fruits, Cranberries)

Flavonoids, in addition to their antioxidant effect, possess inhibitory activity on aldose reductase pathway and can serve as a potential multifunctional agent in the prevention of diabetic retinopathy. Goodarzi *et al.* [192] showed that oral administration of quercetin and the flavanone, naringin to streptozotocin-induced diabetic rats significantly reduced aldose reductase activity in the lenses compared to control. Oral administration of two isoflavone compounds, tectorigenin and irigenin also inhibited sorbitol accumulation in the lenses of streptozotocin induced diabetic rats [193].

Activation of PKC contributes to the loss of capillary pericytes and thickening of vascular basement membrane (BM) in diabetic retinopathy [194]. Also PKC mediated alterations in vascular permeability, blood flow, formation and response to angiogenic growth factors contribute to retinal leakage, ischemia, and neovascularisation [195]. Therefore, PKC inhibitors can be targeted for the treatment of diabetic retinopathy. Hesperetin (Hsp), a flavanone found in citrus fruits and a potent antioxidant has retina vasculo-protective properties due to its strong anti-angiogenic effect via inhibiting VEGF and PKC-β pathways [196]. Modulation of endogenous biomarkers and inhibition of diabetes induced neuropathic pain was observed in diabetic rats after naringin (4′,5,7-trihydroxy flavonone 7-rhamnoglucoside) administration [197]. In the same study, a dose dependent decrease in the levels of oxidative-nitrosative stress, inflammatory mediators as well as apoptosis was documented in neural cells. The antioxidant properties of naringin may be a factor in the inhibition of neurodegeneration.

The soy isoflavone genistein (3 and 6mg/kg), administered by a subcutaneous injection to diabetic mice relieved peripheral painful neuropathy by reverting the proinflammatory cytokine and ROS overproduction. It also restored the inducible nitric oxide synthase (iNOS) and eNOS content and increased NO production in thoracic aorta although treatment had no effect on hyperglycemia [198]. The flavonoid luteolin (200 mg/kg), when administered to rats orally, protected against the progression of diabetes-induced cardiac dysfunction by attenua‐ tion of myocardial oxidative stress probably through its antioxidant properties [199].

In a double blind placebo-controlled study, the effects of daflon 500 (made up of flavonoids diosmin (90%) and hesperidin (10%)) was investigated in a group of 28 type 1 diabetic patients. Treatment with these flavonoids resulted in a decrease in HbA1C which is associated with an increase in the level and activities of thiol-containing antioxidants such as glutathione peroxidase [200]. The *in vitro* protective effect of myricetin on protein oxidation and membrane lipid peroxidation of erythrocytes from diabetic patients was reported in a study by Pandey and co-workers [201].

The treatment of diabetic rats with rutin decreased fasting plasma glucose, glycosylated haemoglobin, thiobarbituric acid reactive substances and lipid hydroperoxides while levels of non-enzymatic antioxidants were increased [202]. In another study, rutin supplementation (500 mg tablets) to diabetic patients for 60 days decreased the levels of fasting blood glucose, blood pressure and improved lipid profiles in the diabetic subjects [203]. Rutin reduced blood glucose, ameliorated oxidative stress and inhibited the accumulation of extracellular matrix (ECM) component and glomerular basement membrane thickening in the renal cortex of diabetic rats suggesting its renoprotective effect in experimental diabetic nephropathy [204]. The inhibitory effect of rutin on AGE formation in STZ-induced rats has also been shown [205].

of aminoguanidine, a known AGE inhibitor which showed 14.1% inhibition at a concentration

Flavonoids, in addition to their antioxidant effect, possess inhibitory activity on aldose reductase pathway and can serve as a potential multifunctional agent in the prevention of diabetic retinopathy. Goodarzi *et al.* [192] showed that oral administration of quercetin and the flavanone, naringin to streptozotocin-induced diabetic rats significantly reduced aldose reductase activity in the lenses compared to control. Oral administration of two isoflavone compounds, tectorigenin and irigenin also inhibited sorbitol accumulation in the lenses of

Activation of PKC contributes to the loss of capillary pericytes and thickening of vascular basement membrane (BM) in diabetic retinopathy [194]. Also PKC mediated alterations in vascular permeability, blood flow, formation and response to angiogenic growth factors contribute to retinal leakage, ischemia, and neovascularisation [195]. Therefore, PKC inhibitors can be targeted for the treatment of diabetic retinopathy. Hesperetin (Hsp), a flavanone found in citrus fruits and a potent antioxidant has retina vasculo-protective properties due to its strong anti-angiogenic effect via inhibiting VEGF and PKC-β pathways [196]. Modulation of endogenous biomarkers and inhibition of diabetes induced neuropathic pain was observed in diabetic rats after naringin (4′,5,7-trihydroxy flavonone 7-rhamnoglucoside) administration [197]. In the same study, a dose dependent decrease in the levels of oxidative-nitrosative stress, inflammatory mediators as well as apoptosis was documented in neural cells. The antioxidant

The soy isoflavone genistein (3 and 6mg/kg), administered by a subcutaneous injection to diabetic mice relieved peripheral painful neuropathy by reverting the proinflammatory cytokine and ROS overproduction. It also restored the inducible nitric oxide synthase (iNOS) and eNOS content and increased NO production in thoracic aorta although treatment had no effect on hyperglycemia [198]. The flavonoid luteolin (200 mg/kg), when administered to rats orally, protected against the progression of diabetes-induced cardiac dysfunction by attenua‐

In a double blind placebo-controlled study, the effects of daflon 500 (made up of flavonoids diosmin (90%) and hesperidin (10%)) was investigated in a group of 28 type 1 diabetic patients. Treatment with these flavonoids resulted in a decrease in HbA1C which is associated with an increase in the level and activities of thiol-containing antioxidants such as glutathione peroxidase [200]. The *in vitro* protective effect of myricetin on protein oxidation and membrane lipid peroxidation of erythrocytes from diabetic patients was reported in a study by Pandey

The treatment of diabetic rats with rutin decreased fasting plasma glucose, glycosylated haemoglobin, thiobarbituric acid reactive substances and lipid hydroperoxides while levels of non-enzymatic antioxidants were increased [202]. In another study, rutin supplementation (500 mg tablets) to diabetic patients for 60 days decreased the levels of fasting blood glucose, blood pressure and improved lipid profiles in the diabetic subjects [203]. Rutin reduced blood glucose, ameliorated oxidative stress and inhibited the accumulation of extracellular matrix

tion of myocardial oxidative stress probably through its antioxidant properties [199].

properties of naringin may be a factor in the inhibition of neurodegeneration.

of 50 μM.

streptozotocin induced diabetic rats [193].

36 Antioxidant-Antidiabetic Agents and Human Health

and co-workers [201].

Diosmin (DS) (diosmetin 7-*O*-rutinoside) is a natural flavone glycoside which can be obtained by dehydrogenation of the corresponding flavanone glycoside, hesperidin that is abundant in the pericarp of various citrus fruits [206]. Diosmin treatment of streptozotocin-nicotinamide induced diabetic rats, ameliorated oxidative stress in plasma and tissues as evidenced by improved glycemic and antioxidant status along with decreased lipid peroxidation [207]. Experimental evidence showed the potential of rutin, a flavonol to delay glomerulosclerosis of diabetic nephropathy (DN) due to its ability to inhibit cell hypertrophy and the accumulation of ECM mediated by TGF-β1/Smads and ROS signals in mesangial cells cultured by high glucose [208] Quercetin enhances endothelium-derived NO bioavailability, reduced blood glucose levels and oxidative stress in diabetic rats suggesting its beneficial effect in vascular function [209].



**Classes of Flavonoid and Food sources**

Flavones

Flavan-3-ols (Red wine and red grapes, green and black tea)

Isoflavones (Soy foods and legumes)

**Selected examples**

> Chrysin and luteolin

> Epicatechin

Genistein Kidney

Daidzein Aorta

**Table 3.** Beneficial effects of some flavonoids in diabetes mellitus

E2-related factor-2, Cox-2: Cyclooxygenase-2, TAOC: Total antioxidative capability

**Target organ, tissue or cells**

Serum and aorta

Pancreatic Islets, plasma, haemoglobin

Apigenin Serum and liver

Catechin Thoracic aorta

Increased .

in the liver.

IL-10, IL-12.

**Mechanism of action Reference**

[221]

39

http://dx.doi.org/10.5772/57282

[222]

[223]

[224]

[225]

[226]

Aortic relaxation, decreased blood pressure, decreased lipidemia and serum AGE's.

NO generation

Oxidative Stress and Diabetic Complications: The Role of Antioxidant Vitamins and Flavonoids

Increased insulin levels and decreased hyperglycemia. Normalized LPO and endogeneous antioxidants, CAT, SOD, GSH

Decreased hyperglycemia NADPH oxidase activity and ROS production. Increased insulin level, lowered blood pressure and

Increased anti-inflammatory cytokines,

Improved glucose tolerance and insulin

Activation of antioxidant enzymes and defense against oxidative damage via increase expression of Nrf2, a transactivator

Maintenance of endothelium dependent relaxation and attenuation of oxidative stress via decrease MDA levels and increase

Decreased MDA level and expression of PKC and pro-inflammatory proteins such as NF-

improved aortic relaxation.

levels and lowered HbA1C.

KB, MCP-1 and Cox-2.

of antioxidant genes.

SOD activity

Abbreviations: Find all citations in this journal (defECM: Extracellular matrix, MDA: Malondialdehyde, NO: Nitric oxide, NOS: Nitric oxide synthase, NADPH; nicotinamide adenine dinucleotide phosphate, LPO: Lipid peroxidation, Nrf2: NF-

The inherent antioxidative properties of some common antidiabetic drugs such as aminogua‐ nidine, statins, thiazolidinediones, glibenclamide and repaglinide also provides an additional support to the involvement of oxidative stress in diabetes and therefore suggest that the use

of antioxidants as therapeutic agents in diabetes is a promising approach [227-231].

Oxidative Stress and Diabetic Complications: The Role of Antioxidant Vitamins and Flavonoids http://dx.doi.org/10.5772/57282 39


Abbreviations: Find all citations in this journal (defECM: Extracellular matrix, MDA: Malondialdehyde, NO: Nitric oxide, NOS: Nitric oxide synthase, NADPH; nicotinamide adenine dinucleotide phosphate, LPO: Lipid peroxidation, Nrf2: NF-E2-related factor-2, Cox-2: Cyclooxygenase-2, TAOC: Total antioxidative capability

**Table 3.** Beneficial effects of some flavonoids in diabetes mellitus

**Classes of Flavonoid and Food sources**

Flavanones (Citus peel, Orange juice, grape fruit juice, lemon juice)

Flavanolols (Milk thistle, red

onion, Siberian larch tree)

Flavones (Parsley, pepper celery, broccoli capsicum)

**Selected examples**

38 Antioxidant-Antidiabetic Agents and Human Health

Hesperidin & Naringin

Naringenin

Luteolin

**Target organ, tissue or cells**

Liver, serum

Hesperidin Retina, plasma

Pancreas, heamoglobin, serum, plasma

Kidney, liver serum, urine,

Kidney

Aortic ring

Diosmin Liver and kidney

VIT-E.

**Mechanism of action Reference**

[214]

[215]

[216]

[217]

[219]

[220]

[207]

Boost antioxidant system by increasing activities of SOD, GR, GPx, CAT and levels of non-enzymatic antioxidants; GSH, VIT-C and

Decreased lipid peroxidation product, MDA and proinflammatory markers, TNF-α, IL-6.

Decreased aldose reductase activity and levels of AGE's, VEGF, ICAM-1, TNF-α, IL-1β and MDA while increasing SOD activity.

Lowered fasting blood glucose, decreased hyperglycemia, glycated haemoglobin, MDA

Improved glycemic control and elevated insulin level, reduced plasma levels of kidney dysfunction markers. Lowered renal activity

Decreased activity of SOD, MDA content and expression of Heme Oxygenase-1

Aortic Vasorelaxation, decrease in ROS production, increased activity of SOD, NOS

Decreased TBARS and hydroperoxides. Increased activity of enzymatic

antioxidants ;SOD, CAT, GPx, GST and GR and non-enzymatic antioxidants; GSH,

GPX, CAT. Decreased high blood glucose [218]

and markers of hepatic damage. Increased levels of insulin and enzymatic and non-enzymatic antioxidants.

and expression of NF-KB and proinflammatory cytokine and chemokine,

suppression of PKC activity.

Silymarin Kidney Increased expression and activity of SOD,

(HO-1) protein.

and level of NO.

Vitamin C and Vitamin E.

The inherent antioxidative properties of some common antidiabetic drugs such as aminogua‐ nidine, statins, thiazolidinediones, glibenclamide and repaglinide also provides an additional support to the involvement of oxidative stress in diabetes and therefore suggest that the use of antioxidants as therapeutic agents in diabetes is a promising approach [227-231].
