4.1. Flavonoids
Polyphenols are diverse group of natural compounds that fall into four distinct classes: flavonoids, phenolic acids, stilbenes, and lignans. The effects of the most common polyphenols on the biomarkers of oxidative stress are summarized in (
Table 1). Flavonoids are most abundant polyphenols which is found in an optimal human diet. This class of compound is further grouped into flavones, flavonols, flavan-3ols, isoflavones, anthocyanidins, and flavanones [
170]. Phenolic acids are divided into two sub-groups called hydroxyl benzoic and hydroxyl cinnamic acids. The anti-inflammatory [
171] and antioxidant properties of flavonoids [
172] have been reviewed (
Table 1 and
Table 2). Though the mechanisms underlying benefits of flavonoids are complex and remain incompletely understood, flavonoids have been shown to reduce glycemia and related complications [
173,
174]. In contrast, others think the bioavailability and effectiveness of flavonoids are relatively low [
170]. So, it was suggested that nanoparticle systems should be used to prolong circulation and flavonoid efficacy, and potentially reduce their non-specific bindings.
Seaweed and tropical papaya have been utilized as traditional remedies, and passed down through generations. Based on animal research, both papaya leaves and seaweed exhibit beneficial effects in diabetes treatment. They protect against diabetes-induced β cell damage, reduce fasting plasma glucose levels, decrease A1C, increase the expression of antioxidative enzyme, and reduce ROS production [
175]. Both seaweed and papaya also contain package of antioxidants (vitamins A, C, and E complexes), other substances such as polysaccharides, phenolic compounds, crucial fatty acids, saponins, fucoidan, and phlorotannin found naturally in other fruits and vegetables. It is unlikely that flavonoids alone can accomplish all diabetes outcomes.
Linarin, a natural flavonoid compound, is capable of counteracting oxidative stress and exerting anti-inflammatory effect in diabetic mice [
176]. Inhibition of aldo-keto reductase (AKR)1B, an NADP(H)-oxidoreductase, by linarin appears to serve as a mechanism for reducing oxidative stress and inflammation in a high-glucose and high-palmitic acid-induced hepatocyte injury model and a type 2 diabetic rat model [
177].
Isorhamnetin, a methylated derivative of quercetin, belongs to the flavonoid group of phenolic compounds [
178]. Isorhamnetin appears to hold great promise against diabetes via aldose reductase inhibition [
179]. The important role the isorhamnetin plays in lowering glucose concentrations, improving oxidative status, reducing inflammation, and adjusting lipid metabolism in both in-vitro and in-vivo models is reviewed [
180] suggesting that isorhamnetin may be a useful compound for treatment of diabetes.
Amomum tsao-ko Crevost et Lemarie (A. tsao-ko), a very common dietary spice, is rich in flavonoids [
181]. A. tsao-ko methanol extracts appear to exert remarkable antioxidant and antidiabetic effects in both
in-vitro and
in-vivo studies. Nevertheless, further investigations are needed to confirm the detailed mechanisms underlying A. tsao-ko-mediated control of diabetes and oxidative stress.
With the importance of apigenin in numerous physiological functions, there has been great interest in its strong antioxidant and anti-inflammatory actions [
182]. It’s action is mediated through neutralizing superoxide, singlet oxygen, and hydroxyl radicals, enhancing the function of the proliferator-activated receptor gamma (PPARγ) signaling, and suppressing CD38 [
183,
184,
185].
Licochalcone A, a flavonoid derived from licorice, has been shown to have potential preventative and therapeutic effects on diabetic nephropathy in both
in-vitro and
in-vivo studies [
186,
187]. The antidiabetic effects of the most recent common polyphenols on diabetes are summarized in
Table 3.
Myricitrin, a flavone isolated from the bark of
Myrica esculenta, has demonstrated a substantial reduction in blood glucose levels in type 2 diabetic mouse and rat models [
188]. It facilitates glucose absorption by skeletal muscles through the activation of IRS-1/PI3K/Akt/GLUT4 signaling, as evidenced by both
in-vitro and
in-vivo studies. Additionally, myricitrin is shown to mitigate oxidative stress by scavenging and neutralizing oxidative radicals and enhancing the body's natural oxidative defense through Nrf-2 activation, in both laboratory and live organism studies [
189,
190].
Biochanin A is a well-known isoflavone for its anti-inflammatory, antihyperlipidemic [
191], antioxidant [
192], and anti-cancer health benefits [
193]. Biochanin A appears to have anti-diabetic effect beyond its function as antioxidant. In animal model of T2DM, it causes increased insulin sensitivity [
193], reduced glucose tolerance [
193], and a reduction in glycohemoglobin A1C formation. Biochanin A maintains steady blood glucose concentrations within a normal range in a T2DM rat model [
194]. A meta-analysis shows that increased transforming growth factor-β (TGF-β) level was associated with a high risk of nephropathy [
195]. All 4 subtypes of protease-activated receptors (PAR1-4) are mainly expressed in the renal epithelial, endothelial and podocyte cells [
196]. PAR-2 inhibition improves autophagy and prevents fibrosis and inflammation [
196]. Surprisingly, biochanin A reduces diabetic nephropathy via suppression of TGF-β1 and PAR-2 genes expression [
191,
192].
Formononetin is an isoflavone from the group of phytoestrogens that induces cell apoptosis via the intrinsic apoptosis pathway, which causes the permeabilization of the mitochondrial outer membrane [
197,
198]. Formononetin has a wide range of biological activities, including eliciting antioxidant [
199] (
Table 1) and antidiabetic properties
in-vitro and
in-vivo (
Table 3). Sirtuin 1 (SIRT1), a nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylase, protects cells from ROS. In addition, SIRT1 regulates hepatic lipid metabolism by increasing AMP-activated protein kinase leading to the inhibition of hepatic lipogenic pathways in favor of fatty acid oxidation [
200]. Formononetin reduces hyperglycemia by increasing SIRT1 expression in pancreatic cells [
201]. Sirtuin-induced fatty acid oxidation is important, because it lowers cytoplasmic lipid accumulation. However, elevated fatty acid oxidation could interfere with glucose metabolism in the muscle [
202]. In addition, formononetin is considered as a strong apoptotic inducer [
198]. Several other mechanisms of action of formononetin have been described [
199,
203,
204]. It is suggested to be used as adjunct therapy for diabetic neuropathy and nephropathy. Clearly, further research investigation is needed in order to gain a better understanding of how precisely formononetin improves multiple aspects of metabolic syndrome, including diabetes.
Hesperetin, a derivative of hesperidin, is a bioflavonoid compound found in citrus fruits [
205]. Like formononetin, hesperetin has been shown to improve diabetes (
Table 3) by regulating SIRT1, alleviating inflammation [
205], combating oxidative stress [
206], and reducing insulin resistance [
206].
Naringenin, a flavonoid compound found in propolis, exhibits potent anti-hyperglycemic and anti-hyperlipidemic properties in diabetic rat models [
207]. The actions of naringenin include improving hyperglycemia, insulinemia, insulin sensitivity, pancreatic cell performance, and lipid profile [
184,
207].
Like other flavonoids, kaempferol exerts both antioxidant (
Table 1) and anti-inflammatory effects (
Table 2). Kaempferol produces anticancer effects through inhibition of epidermal growth factor receptor (EGFR) -dependent Src proto-oncogene, nonreceptor trosin kinase (SRC), ERK1/2, and AKT serine/threonine kinase (AKT) pathways [
208], and cleavage of poly(ADP-ribose) polymerase (PARP) [
209], an endogenous substrate of caspase [
210]. Thus, kaempferol is an antiproliferative, anti-metastatic, and apoptotic drug. Kaempferol exerts antidiabetic effect (
Table 3) through targeting multiple pathways, including improving glycolysis, glucose uptake, glycogen synthesis, AMPK activity, and Glut4 expression [
211]. Additionally several in-vivo studies reported antidiabetic effect of kaempferol through decreasing plasma glucose level [
212], increasing plasma insulin level [
212], decreasing glucose synthesis [
213], and increasing glucagon-like peptide 1 (GLP-1) and insulin release [
214]. Readers are invited to a recent review on all pharmacological mechanisms of kaempferol in diabetes [
215].
Galangin, a flavonoid, exerts an antioxidant (
Table 1) and anti-inflammatory activity [
216,
217], whereas it elicits a potent antitumor activity in diverse cancers [
218]. It is a potent inhibitor of dipeptidyl peptidase-4 (DPP-4) [
219], an integral membrane protein expressed on cells. While DPP-4 cleaves a large number of bioactive molecules, its major physiological substrate is the incretin hormone, glucagon-like peptide-1 (GLP-1), which is responsible for the maintenance of normal glucose homeostasis [
220]. Thus, galangin is an anti-hyperglycemic agent. Although the mechanism of action is not yet completely understood, a recent evidence suggests that galangin improves lipid profile and plasma insulin level [
221].
Fisetin, a bioactive flavonol molecule, has antiproliferative [
222], apoptotic [
223], and antioxidant [
224] activities (
Table 1). Fisetin shows antidiabetic effect, which is mediated by two different mechanisms [
225]. Firstly, fisetin inhibits gluconeogenesis by inhibiting the transport of pyruvate into the mitochondria and reducing the cytosolic NADH/NAD(+) potential redox [
225]. Secondly, fisetin inhibits glycogen breakdown leading to a reduction in blood glucose level [
226,
227]. Fisetin represents a promising therapeutic strategy that may synergize with other antidiabetic therapies.
Myricetin, a flavonoid, is widely distributed in different types of fruits, herbs, and tea [
228]. Recent studies have reported myricetin’s mechanism of action in diabetes, such as inhibiting DPP4 [
229], an inactivator of GLP-1, or serving as a GLP-1 receptor agonist [
230]. Surprisingly, myricetin appears to normalize intestinal flora of type 2 diabetic mice [
231].
Anthocyanins, polyphenolic compounds of the flavonoid group, regulate digestive enzymes (α-amylase and α-glucosidase), GLUT-4, GLP-1, G6Pase, PEPCK, or PPARγ. These compounds also manage blood glucose levels by normalizing insulin secretion and insulin resistance [
232]. Additional significant mechanisms involve the protection of pancreatic β cells, through their anti-inflammatory and antioxidant properties [
233]. It is unlikely that anthocyanins alone can regulate various classes of enzymes. Further investigations are required to confirm the clinical utility of the anthocyanins for the treatment diabetes, and studies should be standardized and quantified to draw universal conclusions regarding their true use as suggested by the authors [
232].
The extract of
Delonix regia shows hypoglycemic, antioxidant, and hypolipidemic properties [
234]. Of note, the antidiabetic effect of this extract was found to be similar to that of glibenclamide, a well-known antidiabetic agent, which lowers blood glucose levels by stimulating the amount of insulin produced by the pancreas. However, the constituents in Delonix regia extracts that exert antioxidant, hypoglycemic, and adverse effects remain uncharacterized.
Mulberry (
Morus alba L.) leaves, used extensively as an effective traditional Chinese medicine for blood glucose management, have been reported to have a beneficial protection on skeletal muscle function [
235]. It appears that the flavonoids present in mulberry leaves markedly ameliorate skeletal muscle insulin resistance and enhance mitochondrial function in diabetic mice through the AMPK-PGC-1α signaling pathway [
235,
236,
237]. Flavonoids of mulberry leaves show hypoglycemic effects via inhibiting the TGF-β1 pathway [
238] and increasing antioxidase activity. Although very interesting, more randomized controlled trials should be required to warrant the findings of this study.
Vaccarin (VAC), a potent flavonoid glycoside extracted from Vaccariae Semen, has been discovered to improve blood glucose levels and insulin resistance, reduces oxidative stress, and enhances endothelium-dependent vasodilation in T2DM mouse models [
239,
240], (
Table 1). Thus,
Vaccariae Semen may serve as an inhibitor of ROS and RNS, which are instigators of several illnesses as mentioned earlier (
Table 2). It was further shown that vaccarin improves glucose metabolism and vascular endothelial function through the inhibition of the ROS/AMPK/miRNA-34a/eNOS signaling cascade [
239]. The protective effects of vaccarin against ROS and RNS as a dietary supplement could be used as adjuvant therapy for T2DM.
A recent research has shown that the flavonoid extract of fenugreek restores antioxidant enzyme activities of both SOD and catalase, and causes a decrease in malondialdehyde content [
241]. This extract also alleviates hyperglycemia in streptozotocin (STZ)-induced T2DM mouse model.
Taxifolin, a recognized flavonoid, is found to be most active towards inhibition of α-amylase. Thus, taxifolin has the ability to manage post-meal hyperglycemia [
242]. It also prevents diabetic cardiomyopathy via inhibition of oxidative stress [
243]. This function, in conjunction with its anti-inflammatory and antioxidant properties, can contribute to the treatment of Diabetes Mellitus.
A flavonoid-rich fraction derived from
Trichilia emetica exhibits a prominent radical scavenging and antidiabetic activities [
244]. Binding of free ferrous to oxygen yields ferric iron and superoxide, thus generating hydrogen peroxide. Formed hydrogen peroxide reacts with ferrous iron and gives rise to hydroxyl radical formation.
Trichilia emetica flavonoid-rich fractions are found to be capable of chelating ferrous ions. Although this plant could potentially be a valuable agent exhibiting strong antioxidant activity for controlling high blood sugar levels, the constituents in
Trichilia emetica extracts remain uncharacterized.
2,3-dihydroxybenzoic acid (DHBA), a phenol, is a weak iron chelator and radical scavenger [
245] (
Table 2). This colonic-derived flavonoid metabolite is found to regulate glucose uptake and production in renal tubular NRK-52E cells [
246]. Additional studies are needed to confirm whether or not this phenol offers a treatment benefit in a reliable large animal diabetes model.