Flavonoids and Their Anti-Diabetic Effects: Cellular Mechanisms and Effects to Improve Blood Sugar Levels

Diabetes mellitus (DM) is a prevailing global health metabolic disorder, with an alarming incidence rate and a huge burden on health care providers. DM is characterized by the elevation of blood glucose due either to a defect in insulin synthesis, secretion, binding to receptor, or an increase of insulin resistance. The internal and external factors such as obesity, urbanizations, and genetic mutations could increase the risk of developing DM. Flavonoids are phenolic compounds existing as secondary metabolites in fruits and vegetables as well as fungi. Their structure consists of 15 carbon skeletons and two aromatic rings (A and B) connected by three carbon chains. Flavonoids are furtherly classified into 6 subclasses: flavonols, flavones, flavanones, isoflavones, flavanols, and anthocyanidins. Naturally occurring flavonoids possess anti-diabetic effects. As in vitro and animal model’s studies demonstrate, they have the ability to prevent diabetes and its complications. The aim of this review is to summarize the current knowledge addressing the antidiabetic effects of dietary flavonoids and their underlying molecular mechanisms on selected pathways: Glucose transporter, hepatic enzymes, tyrosine kinase inhibitor, AMPK, PPAR, and NF-κB. Flavonoids improve the pathogenesis of diabetes and its complications through the regulation of glucose metabolism, hepatic enzymes activities, and a lipid profile. Most studies illustrate a positive role of specific dietary flavonoids on diabetes, but the mechanisms of action and the side effects need more clarification. Overall, more research is needed to provide a better understanding of the mechanisms of diabetes treatment using flavonoids.


Diabetes Mellitus
Diabetes mellitus (DM) is one of the epidemics challenging public health problems throughout the world [1]. The prevalence rate of diabetes is increasing exponentially and the World Health Organization predicts that by the year 2030, diabetes is expected to be the seventh leading cause of death worldwide [2,3]. Diabetes mellitus is a metabolic disorder characterized by the elevation of blood glucose due to the defects in insulin action, secretion or both (insulin is insufficient or inefficient) [4]. Type 1, type 2, and gestational diabetes are the three main types of diabetes targeting children, adults, Figure 1. Schematic illustration of seven selected pathways modulated by diabetes. The figure is divided into seven columns and three rows. The column headings represent the pathways, while the rows heading represent: target genes/proteins for each pathway (blue), the overview physiological effect of these genes on pathways (Dark yellow), and changes occur on these pathways modulated by diabetes. Figure 1. Schematic illustration of seven selected pathways modulated by diabetes. The figure is divided into seven columns and three rows. The column headings represent the pathways, while the rows heading represent: target genes/proteins for each pathway (blue), the overview physiological effect of these genes on pathways (Dark yellow), and changes occur on these pathways modulated by diabetes.

Kaempferol
3,4,5,7-Tetrahydroxyflavone is a nontoxic flavonoid that is abundant in grapes, apples, onions, tomatoes, beans, kale, broccoli, potato, tea, and spinach [90]. Kaempferol has neuroprotective, antimicrobial, antioxidant, anti-inflammatory, and anticancer effects [91]. The extracts of kaempferol from Bauhinia forficate leaves reduce hyperglycemia and enhance glucose uptake, mimicking the action of insulin [92]. In vitro studies confirmed that treating with 10 µM of kaempferol enhances cellular viability and represses apoptosis [93]. Kaempferol has several antidiabetic effects, like improving AMP activated cellular protein expression and activation, reducing cellular apoptosis by suppressing caspase 3 activities, and increasing the production and secretion of insulin from β-cells [94] (Figure 3). In addition to this, kaempferol enhances glucose uptake by the cells through protein kinase C and PI3K pathways, and the synthesis of new glucose transporters [95]. The oral administration of kaempferol significantly decreased serum HbA1c levels, fasting blood glucose, and increased insulin resistance. This flavonoid decreased the genetic expression of PPARγ mediated through regulating AMPK activation [96] (Table 1). Moreover, kaempferol improved DM in STZ-induced mice through the promotion of glucose metabolism in skeletal muscle, and the suppression of hepatic gluconeogenesis [97]. In another study, kaempferol attenuated diabetic nephropathy in NRK-52E and RPTEC cells via suppressing RhoA/Rho-kinase mediated pro-inflammatory signaling (i.e., TNF-α, IL-1β, and TGF-β1) [98]. Like kaempferol, resveratrol is a natural phenol found in grapes, blueberries, and peanuts with potent antioxidant and anti-inflammatory activities that could effectively prevent diabetes [99,100].

Isorhamnetin
An O-methylated bioactive compound is found commonly in medical plants, like Oenanthe javanica (Chinese celery, Japanese parsley, blume, minari in Korean), Hippophae rhamnoides (known also as sea-buckthorn), and Ginkgo biloba (commonly known as ginkgo) [101]. This flavonoid has anti-obesity and anti-diabetic effects [56]. The oral administration of isorhamnetin for 10 days into a streptozotocin-induced model of diabetes (STZ) at a dose of 10 mg/kg or 20 mg/kg showed an effective reduction in oxidative stress and hyperglycemia. The anti-diabetic effect of isorhamnetin is, not only limited to reducing the blood glucose level, but also it helps in reducing the accumulation of sorbitol level on rat lenses, the sciatic nerve, and red blood cells. [102]. An experimental study proposed that isorhamnetin glycoside has several effects on diabetes, like stimulating insulin secretion, the expression of enzymes involved in lipid metabolism, and the expression of endoplasmic reticulum stress markers [103,104].
Fisetin possess anti-diabetic, anti-inflammatory, and neurotrophic effects [107]. The oral treatment of fisetin in a dose of 10 mg/kg for 30 days decreased Hb1Ac, blood glucose levels, and the expression of the gluconeogenic genes protein level, while it increased the concentration of plasma insulin [108]. In an in vivo study, the results showed that treatment with fisetin significantly reduced the level of NF-κB p65, Hemoglobin A1C (HbA1c), serum nitic oxide (NO), and blood glucose [109]. Fisetin also inhibits high glucose induced cytokine production in monocytes which could prevent diabetes [110]. The anti-diabetic effects of fisetin on hepatic enzymes include enhancing the activities of hexokinase, while reducing the activities of glucose 6 phosphate dehydrogenase (G6PD) and glucose 6-phosphatase (G6Pase) ( Figure 3). Moreover, fisetin improves glucose homeostasis by attenuating carbohydrate metabolism enzymes in STZ diabetic rats [111]. Fisetin has been reported to improve the development of diabetic cardiomyopathy in STZ-induced DM rats by improving hyperglycemia/hyperlipidemia-mediated oxidative stress, the inflammation processes, and the programmed cell death [112]. Preclinical evidence illustrated the therapeutic potential of fisetin in diabetic neuropathy through the modulation of NF-κB and Nrf2 signaling pathways [113].

Morin
A natural flavonoid, morin, is found mostly in traditional medical herbs, like Prunus dulcis, Chlorophora tinctoria L., and fruits such as guava and figs [114]. The oral administration of morin for 30 days in animal models resulted a significant improvement in glucose tolerance, hyperglycemia, and insulin resistance [115]. Diabetic rats were reported to have declined lipid peroxides and antioxidant levels after the treatment with morin [116]. Morin effectively decreased the level of inflammatory cytokines, like IL-6 and TNF-α, which proves its anti-inflammatory effects [117]. In animal models, morin recovered leptin sensitivity and hepatic insulin led to the reduction of liver lipid accumulation and hyperlipidemia [118]. Morin has different effects on hepatic enzymes where it is significantly reduces the activity of G6Pase and Fructose-1,6-diphosphatase (FDPase), while enhancing the activity of hexokinase and G6PD [119] (Figure 3).

Flavanones
Flavanones are known as di-hydroflavones and they are characterized by an oxidized, saturated carbon ring. Flavanones are widespread in citrus fruits and known for their free radical scavenging ability and antioxidant activity [64].
2.2.1. Hesperidin 5,7,3'-Trihydroxy-4'-methoxyflavanone, a saturated oxidized aglycon, is found abundantly in citrus fruits, such as limes and lemons, tomatoes and cherries [120]. The effects of hesperidin and its glycoside ( Table 2) are not limited to diabetes, but also have vascular, neuroprotective, anti-allergic, anti-inflammatory, anticarcinogenic, and antioxidant effects [121]. A study in db/db C56BL6 mice showed that hesperidin supplementation to the regular diet helps in regulating the activity of gluconeogenesis and glycolytic hepatic enzymes, and in improving hyperglycemia [122]. In db/db mice, the flavonoid has a very effective machinery, like increasing triglyceride fecal excretion and inhibiting lipid metabolizing enzymes which enhances the lipid metabolism activities [123]. Hesperidin effectively lowers blood glucose levels by upregulating GLUT 4 translocation and PPARγ [124]. Hesperidin supplementation showed a decrease in glucose 6 phosphatase (G6Pase) activities in STZ-induced diabetic rats, which diminish glucose exports from the cells by a glucose transporter membrane protein [125] (Figure 3). A dose of 10 g/kg diet of hesperidin treatment decreases glucose levels by altering glucose regulating enzyme activities [126]. The administration of hesperidin and hesperetin together have different effects on lipid and glucose metabolism and show lipid lowering activities [56]. Hesperidin also positively regulates the α-Klotho/FGF-23 pathway in STZ-induced DM rats, which demonstrate positive effects on diabetic toxicity in the liver and kidney [127].
2.2.2. Naringenin 5,7,4'-Trihydroxyflavanone, a saturated oxidized aglycone, is present abundantly in citrus fruits, such as oranges, tomatoes, and grapefruits [128]. It has a wide range of biological activities, such as antioxidant, antidiabetic, anti-inflammatory, anticancer, and anti-mutagenic abilities [129]. Both naringenin and its glycoside ( Table 2) possess anti-diabetic and anti-obesity activities [130]. An in vitro study presented that naringenin mimics the effect of insulin by decreasing pro-liprotein B secretion in liver cells [131]. The administration of this flavonoid (25 mg/kg) into diabetic rats showed a significant inhibition of α-glucosidase activity which delayed carbohydrate absorption, therefore, reducing postprandial blood glucose levels [132]. In type 2 diabetic mice, naringin could upregulate GLUT4, and regulates the expression of hepatic enzymes involved in gluconeogenesis and glycolysis which improves hyperglycemia [133,134]. In an everted rat intestinal sleeve, naringenin was found to inhibit the uptake of glucose by inhibiting sodium-glucose co-transporters in the intestine [135]. The administration of naringenin presented various effects in different diabetic rat models: (i) In STZinduced diabetic rats, the flavonoid decreased the level of plasma glucose; (ii) in fructose-fed insulin resistance rats, naringenin improved insulin sensitivity; in HFD mice, it helped in the reduction of insulin resistance [136][137][138]. Treating diabetic mice with 25 mg/kg for 45 days significantly reduced hyperinsulinemia, hyperglycemia, decreased lipid membrane peroxidation, improved hepatic markers, restored the changes in lipid profile, and enhanced the antioxidants activities [139]. Naringenin anti-diabetic effects in diabetic rats were characterized by anti-oxidant, and anti-apoptotic activities which showed the potential of naringenin to limit neurodegeneration and prevent retinal damage in diabetic retinopathy [140].

Eriodictyol
Eriodictyol, present in lemon fruits, significantly controls obesity and diabetes [141]. Recently, eriodictyol was identified as a novel insulin secretagogue in vitro and in vivo which exerts an exclusive glucose-dependent insulinotropic activity via a cAMP/PKA pathway [142]. Moreover, in diabetic rats, eriodictyol supplementation can effectively suppress oxidative stress [143]. The treatment with eriodictyol upregulated the expression of PPARγ2 and the adipocyte-specific fatty acid binding protein [144]. Furthermore, eriodictyol treatment significantly suppressed diabetes related lipid peroxidation [145] (Figure 3). Recently, eriodictyol was described as a protector of the rat retinal ganglial cells (RGC)-5 from high glucose-induced oxidative stress, inflammation, and cell apoptosis via the activation of Nrf2/HO-1 signaling [146].

Flavones
The structure of flavones is comprised of an unsaturated carbon ring at C2-3 and a ketone group at C4, but they lack the hydroxylation at carbon 3 if compared to flavonols. They are widely synthesized in flowers, leaves, and fruits [64].
2.3.4. Chrysin 5,7-Dihydroxyflavone is found abundantly in honey, fruits, bee pollen, propolis, and medical plants, such as Passiflora caerulea L. and Tilia tomentosa [165]. This flavonoid is an analog to apigenin but with lower bioavailability due to rapid excretion and metabolism [166]. Chrysin treatment in STZ-induced rats reported an elevation of glucose, MDA, TG, TC, LDL-C and a reduction of HDL-C, total protein, SOD, CAT, and GST [167]. The treatment with chrysin demonstrated an improvement in renal pathology and suppressed collagen-IV protein expressions in renal tissue [168]. In HFD/STZ-induced diabetic rats, chrysin significantly prevented the development of diabetic neuropathy (DN) due to the reduced level of pro-inflammatory cytokines in the serum [169]. Chrysin treatment decreases lipid peroxidation, glucose levels and increases insulin levels in diabetic rats [170]. The data suggest that chrysin has anti-diabetic and antihypertensive effects [171].
2.3.5. Wogonin 5,7-Dihydroxy-8-methoxyflavone is a flavonoid extracted from the root of Scutellaria baicalensis and it has been used as a traditional medicine in East Asian countries [172]. It is associated with anti-inflammatory, neuroprotective, anti-viral, anti-bacterial, and antioxidants effects [173]. Wogonin has several beneficial effects on insulin sensitivity, blood glucose, and lipid metabolism through the activation of AMPK and PPARα [174]. The pretreatment with wogonin attenuated vascular inflammatory effects seen in high glucose induced (HG) vascular inflammation [175]. A preclinical study found that the anti-oxidative and anti-inflammatory of wogonin could attenuate diabetic cardiomyopathy [176].

Diosmin
A naturally occurring flavonoid glycoside was first isolated from Scrophularia nodosa L. in 1925. It can be isolated either from several plant sources or by the dehydrogenation of the flavanone glycoside, hesperidin [177]. The diosmin administration to type 1 diabetic patients showed a decrease in HbA1c and an increase in glutathione peroxidase (GPx) ( Table 3) [178]. Rats treated orally with diosmin for 45 days showed a significant decrease in plasma glucose levels, G6Pase, FDPase and an increase in G6PD and hexokinase ( Figure 3) [179]. The diosmin treatment at a dose of 50 and 100 mg/kg for one month improved oxidative stress and hyperglycemia in diabetic rats [180]. Diosmin activates imidazoline receptors which increase adrenal β-endorphin secretion and, thereby improving metabolic homeostasis and the alleviation of serum glucose and lipids in STZ-induced type 1 diabetic rats [181].
2.3.7. Baicalein 5,6,7-Dihydroxyflavone aglycone is isolated from the roots of Scutellaria baicalensis and fruits of Oroxylum indicum L. [182]. It exhibits extensive ant-inflammatory, anti-neuro-degenerative, and anti-cardiovascular effects [183]. The administration of 0.25 g or 0.5 g of baicalein to high fat diet (HFD) induced mice displayed a significant improvement in glucose tolerance, insulin levels, and hyperglycemia [184]. Diabetic rats treated with baicalein showed a substantial decrease in fasting blood glucose levels, HbA1c, food intake, and body weight [185]. Moreover, the treatment with baicalein was reported to decrease the TNF levels, advanced glycation end-products (AGEs), and NF-κB activation ( Figure 3) [186]. The baicalein mechanism of action could upregulate AMPK signaling pathways, which can attenuate insulin resistance and inhibit inflammation [187]. The treatment of baicalein in HepG2 cells (0.001 umol/L and 0.01 umol/L) enhanced glucose uptake and glycolysis and suppressed [188]. In addition, baicalein prevents oxidative stress and inflammation in diabetic cardiomyopathy rats through the regulation of PI3K/AKT signaling [189]. Baicalein alleviates hepatic inflammation in diabetic db/db mice via the modulation of HMGB1/TLR4/NF-κB signaling [190].

Isoflavones
Isoflavones are found mostly in legumes, soybeans, and some microbes [64]. Genistein and daidzein are the major source of isoflavones. They have shown to have an anti-diabetic effect by stimulating insulin secretion from the pancreatic beta cells [56].
2.4.1. Genistein 5,7,4'-Trihysroxyisoflavone, a naturally occurring soy isoflavone, is present numerously in soy, soybean products, and Chinese plants [191]. Genistein exerts the anti-diabetic effects by enhancing plasma lipids [192]. Genistein supplementation in type 1 diabetes animals led to the improvement of insulin levels and glucose metabolism [193]. An in vivo study found that genistein improved hyperglycemia through promoting cAMP/PKA signaling pathways [194]. The administration of genistein to rats fed with a fructose rich diet showed a protective role on renal malfunction through the modulation of insulin resistance [195]. The supplementation of genistein (0.02% in diet) in non-obese diabetic (NOD) rats showed the onset of diabetes was prevented and glucose homeostasis was improved through the preservation of β cell functions [196]. The beneficial effects were observed in non-generic mouse models ingested with 250 mg/kg of genistein like reduction in the fasting glucose level and β cell mass [197]. In STZ-induced mice, genistein improved glucose tolerance, hyperglycemia, and the level of circulating insulin [198]. Genistein demonstrated an inhibitory effect on tyrosine kinase which dysregulates glucose homeostasis ( Figure 3) [199]. The administration of genistein to mice reduced body weight and improved glucose and lipid metabolism [200]. A transcriptome analysis revealed that genistein could affect the regulation of the hypothalamic circadian rhythms which could provide a novel target for the therapy of diabetes and obesity. Moreover, genistein has a protective effect against inflammation, neuropathic pain, and oxidative stress [201].

Daidzein
7,4'-Dihydroxyisoflavone is a phytoestrogen mainly isolated from nuts, fruits, and soybeans [202]. Daidzein exerts an anti-diabetic effect by enhancing lipid and glucose metabolism [203]. Daidzein has promising therapeutic potential on impaired glucose, lipid metabolism, and vascular inflammation associated with T2DM [204]. Moreover, daidzein treatment in gastrocnemius muscle is effective in decreasing blood glucose, total cholesterol, and AMPK phosphorylation ( Figure 3) [201]. Pure synthetic daidzein administered to hamsters significantly lowered plasma total cholesterol levels and blood glucose compared to the control group [205].

Anthocyanins
A water soluble, unoxidized, unsaturated flavonoid, anthocyanin, is present abundantly in flowers and fruits. The dietary consumption of this flavonoid is higher compared to other flavonoids. Several studies, both in animal models and cell lines, suggested that anthocyanins have anti-diabetic activities [56].

Delphinidin
A flavonoid, delphinidin, is profusely found in pigmented vegetables and fruits, like berries, sweet potato, red cabbage, tomato, eggplant, carrots, red onion, and grapes [210]. It possesses anti-inflammatory, antioxidant, anti-mutagenic, and anti-angiogenic activities [211]. In an in vivo study, delphinidin showed to prevent endothelial cell function injuries associated with diabetes [212]. The administration of 100 mg/kg delphinidin to diabetic mice showed a decrease in HbA1c glycation and the rate of albumin [213]. Delphinidin and cyanidin were reported to reduce inflammation and regulate redox signaling pathways by ameliorating insulin resistance in high fat-fed mice [214]. The antidiabetic effects of delphinidin are due to their ability to reduce glucose uptake in mice jejunal tissue and human intestinal cells lines through free fatty acid receptor 1 (also named GPR40) [215].
Reduce serum glucose level and fasting blood glucose level Decrease the level of caspase 3 activity in β-cells Inhibit cellular apoptosis by improving anti-apoptotic Akt activities Improve cAMP signaling and insulin synthesis and secretion Improve glucose uptake by soleus muscles Reduce lipid peroxidation Decrease PPARγ expression through AMPK activity Rats Streptozotocin (STZ)-induced diabetic rats High fat diet mice Pancreatic βcells [90,91]

Isorhamnetin
Oenanthe javanica, Hippophae rhamnoides, and Ginkgo biloba L.         Reduce hyperglycemia and lipid droplets accumulation in the liver Increase vascular permeability and the expression of cell adhesion molecules Activate NF-κB and AMPK pathways Activate PPARα which has a beneficial effect on lipid metabolism db/db mice 3T3-L1 cells [173,175] Scutellaria lateriflora L, and Scutellaria baicalensis Georgi

Citrus fruites, and
In Intestine: Baicalin will be converted into Baicalein and then absorbed rapidly. In the circulation: Baicalein will be converted to Baicalin (A) Antihyperglycmeic effect: (B) Hypolipemic effect Reduce the level of level of hemoglobin A1C (HbA1c) Suppress the activation of NF-κB Improve glucose tolerance and insulin secretion from pancreatic cells Improve viability of clonal β-cells which improves the production of NADH and NADPH Protect against β cells apoptosis Increase hexokinase activity in liver Activate MAPKs signaling pathway which reduce the effect of insulin resistance by phosphorylating Akt and IRS-1 and dephosphorylate NF-κB Suppress fatty acid synthesis Obese diabetic mice Type 2 diabetic rats CA1 hippocampal neurons [187,190]

Diosmin
Citrus fruites, and Scrophularia nodosa L. Reduce hyperglycemia and lipid droplets accumulation in the liver Increase vascular permeability and the expression of cell adhesion molecules Activate NF-κB and AMPK pathways Activate PPARα which has a beneficial effect on lipid metabolism db/db mice 3T3-L1 cells [173,175]    HepG2 cells Differentiated3 T3-L1 cells [147,149]  Reduce hyperglycemia and lipid droplets accumulation in the liver Increase vascular permeability and the expression of cell adhesion molecules Activate NF-κB and AMPK pathways Activate PPARα which has a beneficial effect on lipid metabolism db/db mice 3T3-L1 cells [173,175] Citrus fruites, and Scrophularia nodosa L. Reduce the level of hemoglobin A1C (HbA1c) due to increase in glutathione peroxidase (GPx) Decrease G6Pase, PEPCK, and fructose-1,6-bisphosphatase enzymes Reduce plasma glucose and increase plasma insulin by activating anti-oxidant enzymes Reduce hyperglycemia by inducing β-endorphin Increase hexokinase and glucose-6-phosphate dehydrogenase activity Reduce lipid peroxidation Streptozotocin nicotinamide induced diabetic rats [179,180]

Apigenin
Onion, oranges, tea, parsley, chamomile, Reduce hyperglycemia and lipid droplets accumulation in the liver Increase vascular permeability and the expression of cell adhesion molecules Activate NF-κB and AMPK pathways Activate PPARα which has a beneficial effect on lipid metabolism db/db mice 3T3-L1 cells [173,175] Onion, oranges, tea, parsley, chamomile, Hypericum perforatum L, wheat sprouts  In Intestine: Baicalin will be converted into Baicalein and then absorbed rapidly. In the circulation: Baicalein will be converted to Baicalin (A) Antihyperglycmeic effect: (B) Hypolipemic effect Reduce the level of level of hemoglobin A1C (HbA1c) Suppress the activation of NF-κB Improve glucose tolerance and insulin secretion from pancreatic cells Improve viability of clonal β-cells which improves the production of NADH and NADPH Protect against β cells apoptosis Increase hexokinase activity in liver Activate MAPKs signaling pathway which reduce the effect of insulin resistance by phosphorylating Akt and IRS-1 and dephosphorylate NF-κB Suppress fatty acid synthesis Obese diabetic mice Type 2 diabetic rats CA1 hippocampal neurons [187,190]

Luteolin
Parsley, broccoli, onoins leaves, celery, cabbages, apple skins, carrots, and Reduce cAMP response element binding protein and histone acetyl transferase activity of CBP/p300 (NF-κB coactivator) Reduce apoptosis Up-regulate the espression of synaptic protein which target brain cells Improve insulin secretion by supressing Maf A through NF-κB signiling pathway Activate PPAR-γ which targets adiponectin, leptin and GLUT4 genes Obese mice Streptozotocin induced diabetic rats Diabetic rats Endothelium cells Human monocytes cells [155,157]

Apigenin
Onion, oranges, tea, parsley, chamomile, Reduce hyperglycemia and lipid droplets accumulation in the liver Increase vascular permeability and the expression of cell adhesion molecules Activate NF-κB and AMPK pathways Activate PPARα which has a beneficial effect on lipid metabolism db/db mice 3T3-L1 cells [173,175] Poncirus trifoliate L, citrus fruit rinds, and mandarin orange Metabolization is medicated by CYP1A1 and CYP1A2 to produce:   In Intestine: Baicalin will be converted into Baicalein and then absorbed rapidly. In the circulation: Baicalein will be converted to Baicalin (A) Antihyperglycmeic effect: (B) Hypolipemic effect Reduce the level of level of hemoglobin A1C (HbA1c) Suppress the activation of NF-κB Improve glucose tolerance and insulin secretion from pancreatic cells Improve viability of clonal β-cells which improves the production of NADH and NADPH Protect against β cells apoptosis Increase hexokinase activity in liver Activate MAPKs signaling pathway which reduce the effect of insulin resistance by phosphorylating Akt and IRS-1 and dephosphorylate NF-κB Suppress fatty acid synthesis Obese diabetic mice Type 2 diabetic rats CA1 hippocampal neurons [187,190]

Luteolin
Parsley, broccoli, onoins leaves, celery, cabbages, apple skins, carrots, and Reduce cAMP response element binding protein and histone acetyl transferase activity of CBP/p300 (NF-κB coactivator) Reduce apoptosis Up-regulate the espression of synaptic protein which target brain cells Improve insulin secretion by supressing Maf A through NF-κB signiling pathway Activate PPAR-γ which targets adiponectin, leptin and GLUT4 genes Obese mice Streptozotocin induced diabetic rats Diabetic rats Endothelium cells Human monocytes cells [155,157]

Diosmin
Citrus fruites, and Scrophularia nodosa L. Reduce the level of pro-inflammatory cytokines that helps in the prevention of diabetic neuropathy Reduce blood glucose Improve renal pathology with the suppression of TGF-β, collagen-IV, and fibronectin Improve insulin level Reduce lipid peroxidation INS-1E cells [167,169] passiflora caerulea (L,), honey, Tilia tomentosa Moench, and Pelargonium crispum (Berg.) Reduce the level of pro-inflammatory cytokines that helps in the prevention of diabetic neuropathy Reduce blood glucose Improve renal pathology with the suppression of TGF-β, collagen-IV, and fibronectin Improve insulin level Reduce lipid peroxidation INS-1E cells [167,169] [195,199] [195,199] [195,199]

Challenges Using Flavonoids
Flavonoids have been proven to be strong candidates to reduce the pathogenesis of diabetes and its complications. The modulatory anti-diabetic effects of flavonoids reduce apoptosis and insulin resistance and enhance insulin secretion and GLUT 4 translocation.

Estimated Consumption Level of Flavonoids
Flavonoids derived from vegetables and fruits are consumed in low quantities. Moreover, the content of vegetables and fruits contain not only flavonoids, but also a mixture of secondary plant metabolites. Therefore, it is difficult to stimulate this mixture into a simple purified dietary supplement [220,221]. Efforts have been made to establish an optimal human dietary consumption level of flavonoids worldwide, but the estimate methods used were poorly established [222]. A U.S. study on 805 men aged 65-84 years reported that the estimate intake of flavonoids from quercetin, myricetin, kaempferol, apigenin, and leuteolin was 26 mg/d and the major sources of intake were in apples, tea, and onions [223]. Another study conducted in the Netherlands reported a two-times higher the level of flavonoids consumed in adults compared to the U.S. study (50 g/day) [224]. In addition, two Dutch studies reported the estimated consumption level of flavonoids to be 23 mg/day and 26 mg/day respectively [225,226]. These differences observed in the consumption level of flavonoids depend on dietary habits, geographical location, socioeconomic status, food processing and preparation method, solubility of flavonoids, and the ethnicity of the population. For example, in Japan, soy containing food is highly consumed and, as a result the intake of isoflavone is higher than other flavonoids subclasses [106]. A study reported that orange juice contains 81-200mg/L of soluble flavanones compared to 206-644 mg/L seen in the cloud which clearly suggest that processing and storage affects the concentration of flavonoids [227].
To date, no recommended dosage of flavonoids has been reported due to the heterogeneity of their molecular structure and the limited information about their bioavailability. Major advances in understanding flavonoids bioavailability have been made, but the challenge to overcome problems, such as cellular permeability, solubility, excretion, and metabolic alternation, are still lacking. Research groups are trying to enhance flavonoids bioavailability by targeting absorption sites, improving metabolic stability and intestinal absorption [50].

Possible Side Effects of Flavonoids Consumption
Flavonoids in bacterial and mammalian experimental studies using Ames test indicated possible genotoxicity and mutagenicity of flavonoids if consumed at higher concentrations (ranges from 12.1 nmol to 225.0 nmol) [228]. Furthermore, it may alter amino acid, drug metabolism and the activity of key metabolizing enzymes [229]. Quercetin, a predominant flavonol in the human diet, showed a mutagenic effect by altering base-pair substitution and frame-shift mutation [230]. The isolated nuclei from liver rats treated with morin and naringenin showed an increase in reactive oxygen species, like hydroxyl radicals that lead to DNA degradation [231]. Additionally, flavonoids exert a cytotoxic activity as a topoisomerase II inhibitor. Genistein and quercetin are identified as topoisomerase II inhibitors, even at low concentrations (10 µM), where they accumulate cleavable complexes seen in patients with secondary leukemia [232]. Genistein, naringenin, kaempferol, and daidzein were reported to inhibit thyroxine synthesis by irreversibly inhibiting thyroid peroxidation [233].
Although no data are available to state the long-term side effects of increased flavonoid consumption, following an Asian diet that contains 68 mg of flavonol and 20-240 mg of isoflavone could improve thyroid function, reduce breast cancer mortality, and should not cause adverse health effects [234].
The concentrations needed for most flavonoids to generate mutagenic and cytotoxic side effects are unlikely to occur through dietary sources, but with supplementation, it could result in an increased toxic level. For instance, the recommended dosage of quercetin supplements is between 500 mg/day and 1000 mg/day, which is 20 times higher with what could be consumed in a vegetarian diet [235].

Could Flavonoid Combinations have synergistic effects?
While the amounts of flavonoids consumed is crucial to establish positive effects but also to avoid negative effects, the tables list some flavonoids that trigger multiple selected pathways improving the pathogenesis of diabetes (Figure 3, Tables 1-4). The better activity can be defined by the number of diabetes related pathways which are improved through the consumption of different flavonoids. The administration of baicalein triggers four pathways: The suppression in the NF-κB pathway and fatty acid synthesis; the activation in hexokinase activity in the liver; and the protection against cell apoptosis. Quercetin prompts the activity of three different pathways: It improves GLUT 4 translocation; inhibits tyrosine kinase activity; and reduces lipid peroxidation. β-cells apoptosis could be prevented by the administration of cyanidin or kaempferol, or baicalein. The consumption of rutin or cyanidin inhibits α-glucosidase and α-amylase which reduce carbohydrate absorption in the small intestine (Table 4).
Could their positive effects on diabetes be further improved by ingesting a combination of different flavonoids which complement each other by triggering additional pathways? For example, the administration of baicalein and quercetin initiates the positive effects on diabetes in six major pathways: The glucose transporter; hepatic enzymes; beta cells apoptosis; PPARs; AMPK; tyrosine kinase; and NF-κB pathways. As a result of this hypothesized combination, the over activation of these pathways may be prevented, while the needed action to improve diabetes may be achieved. At this time, these are no more than suggestions which need to be proven by research. To date, little is known about flavonoids to flavonoids interactions [235]. In addition, some flavonoids showed an opposite effect on the same pathway and both lead to the improvement of diabetes. For example, fisetin has an inhibitory effect, while morin has a stimulatory effect on glucose 6 phosphate dehydrogenase and the literature states that they both improve diabetes (Figure 1). Extensive studies are required to understand the reasons behind this action-is it because of different binding sites, bioavailability, tissue exposure, absorption, or circulating concentration of these compounds. A similar pattern with different flavonoids was observed with PPAR and NF-κB pathways (Tables 1-4).

Flavonoids and Metformin
Metformin is an oral anti-diabetic drug derived from the French lilac suitable for the treatment of diabetes with a well-known safety profile [236]. Comparing the effect of metformin and anthocyanins extracted from blueberries on blood glucose levels revealed that blueberry extract (595 mg/g total anthocyanins) led to a 33% to 51% reduction in blood glucose compared to 27% reduction seen with metformin [237]. Another study reported that quercetin stimulates an insulin dependent AMPK pathway, which is analogues to metformin activity [15]. In addition, a study which measured the effect of the co-treatment of metformin and flavone on breast cancer patients showed a significant inhibition in cell viability and an increase in apoptosis [238].

Flavonoids for the Treatment of Cancer
Phytochemicals are successfully used in the treatment of various cancers by modulating apoptotic pathways through: Reactive oxygen species (ROS) elevation; DNA damage induction; and apoptotic protein activation as clearly discussed in Abotaleb et al., 2018 [50]. Although flavonoids target and improve both the intrinsic and extrinsic apoptotic protein in cancer, they solely target the intrinsic pathway ( Figure 3). Quercetin has similar beneficial effects on cancer and diabetes by inhibiting PI3K pathway [239]. The administration of luteolin targeting the NF-κB pathway showed an inhibitory effect on cancer, while a stimulatory effect on diabetes [240].

Final Thoughts
Flavonoids, abundantly found in fruits and vegetables, have mostly beneficial effects on diabetes. Eating vegetables and fruits could help to lower blood sugar levels and to decrease the chance to develop diabetes. Generally, it is possible that their combination with other phytochemicals could enhance the anti-diabetic effects, but more research is needed to support this promising way to reduce blood sugar levels.

Conflicts of Interest:
The authors declare no conflicts of interest.